REMOTE SENSING OF SHELF SEA HYDRODYNAMICS
FURTHER TITLES IN THISSERIES 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18...
83 downloads
705 Views
15MB Size
Report
This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form
REMOTE SENSING OF SHELF SEA HYDRODYNAMICS
FURTHER TITLES IN THISSERIES 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 20. 21. 22. 24. 25. 26. 27. 28. 30. 31. 32. 33. 34.
35. 36. 37.
V. VACQUIER, Geomagnetism.in Marine Geology W.J. WALLACE, The Development of the Chlorinity/Salinity Concept i n Oceanography E. LISITZIN, Sea-Level Changes R.H. PARKER, The Study of Benthic Communities J.C.J. NIHOUL (Editor), Modelling of Marine Systems 0.1, MAMAYEV, Temperature-Salinity Analysis of World Ocean Waters E.J. FERGUSON WOOD and R.E. JOHANNES (Editors), Tropical Marine Pollution E. STEEMANN NIELSEN, Marine Photosynthesis N.G. JERLOV, Marine Optics G.P. GLASBY (Editor), Marine Manganese Deposits V.M. KAMENKOVICH, Fundamentals of Ocean Dynamics R.A. GEYER (Editor), Submersibles and Their Use in Oceanography and Ocean Engineering J.W. CARUTHERS, Fundamentals of Marine Acoustics P.H. LeBLOND and L.A. MYSAK, Waves i n the Ocean C.C. VON DER BORCH (Editor), Synthesis of Deep-sea Drilling Results in the Indian Ocean P. DEHLINGER, Marine Gravity F.T. BANNER, M.B. COLLINS and K.S. MASSIE (Editors), North-West European Shelf Seas: The Sea-Bed and the Sea i n Motion J.C.J. NIHOUL (Editor), Marine Forecasting H.-G. RAMMING and Z. KOWALIK, Numerical Modelling of Marine Hydrodynamics R.A. GEYER (Editor), Marine Environmental Pollution J.C.J. NIHOUL (Editor), Marine Turbulence A. VOlPlO (Editor), The Baltic Sea E.K. DUURSMA and R. DAWSON (Editors), Marine Organic Chemistry J.C.J. NIHOUL (Editor), Ecohydrodynamics R. HEKINIAN, Petrology of the Ocean Floor J.C.J. NIHOUL (Editor), Hydrodynamics of Semi-Enclosed Seas B. JOHNS (Editor), Physical Oceanography of Coastal and Shelf Seas J.C.J. NIHOUL (Editor), Hydrodynamics o f the Equatorial Ocean W. LANGERAAR, Surveying and Charting of the Seas
Elsevier Oceanography Series, 38
REMOTE SENSING OF SHELF SEA HYDR0DYNA M ICS PROCEEDINGS OF THE 15th INTERNATIONAL LIEGE COLLOQUIUM ON OCEAN HYDRODYNAMICS
Edited by JACQUES C.J. NIHOUL Professor of Ocean Hydrodynamics, University of L i2ge Lisge, Belgium
ELSEVl ER Amsterdam - Oxford
-
New York - Tokyo 1984
ELSEVIER SCIENCE PUBLISHERS B.V. 1, Molenwerf, P.O. Box 21 1, 1000 AE Amsterdam, The Netherlands Distribution for the United Stares and Canada:
ELSEVIER SCIENCE PUBLISHING COMPANY INC. 52, Vanderbilt Avenue New York, N.Y. 10017, U S A .
Library of Congress Cataloging in Publication Data
International Lisge Colloquium on Ocean Hydrodynamics (15th : 1983) Remote sensing of shelf sea hydrodynamics. (Elsevier oceanography series ; 38) Bibliography: p. 1. Ocean circulation--Remote sensing--Congresses. 2. Ocean currents--Remote sensing--Congresses. 3. Continental shelf--Remote sensing--Congresses. I. Nihoul, Jacqaes :. J . 11. Title. 111. Series. GCZ8.5.156 1933 551.47 84-1b72 ISBN 0-444-42314-1 (U.S. )
ISBN-0-44442314-1 (Vol. 38) ISBN 0-44441623-4 (Series) 0 Elsevier Science Publishers B.V., 1984 All rights reserved. No part of this publication may be reproduced, stored i n a retrieval system or transmitted i n any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V., P.O. Box 330, 1000 AH Amsterdam, The Netherlands
Printed in The Netherlands
V
FOREWORD
The International Liege Colloquia cn Ocean Hydrodynamics are organized annually. Their topics differ from one year to another and try to address, as much as possible, recent problems and incentive new subjects in physical oceanography. Assembling a group of active and eminent scientists from different countries and often different disciplines, they provide a forum for discussion and foster a mutually beneficial exchange of information opening on to a survey of major recent discoveries, essential mechanisms, impelling question-marks and valuable recommendations for future research. The Scientific Organizing Committee and all the participants wish to express their gratitude to the Belgian Minister of Education, the National Science Foundation of Belgium, the University of Liege, the Intergovernmental Oceanographic Commission and the Division of Marine Sciences (UNESCO) and the Office of Naval Research for their most valuable support. The editor is indebted to Dr. Jamart for his help in editing the proceedings. Jacques C.J. NIHOUL
This Page Intentionally Left Blank
VII
LIST OF PARTICIPANTS BALLESTER, A., Prof. Dr., Instituto Investigaciones Pesqueras, Barcelona, Spain BOHM, E., Dr., Dipartamento di Fisica, Universita Roma, Italy. BOUKARY, S., Mr., University of Niamey, Niger. CARSTENS, T., Prof. Dr., Norwegian Hydrodynamic Laboratories, River and Harbour Laboratory, Trondheim, Norway. CHABERT D'HIERES, G., Eng,, Universite Scientifique et Medicale de Grenoble, Institut de Mgcanique, Grenoble, France. CLEMENT, F., Mr., MCcanique des Fluides Geophysiques, Universitg de LiPge, Belgium. CREPON, M., Dr., Laboratoire d'Oc6anographie Physique, Museum d'Histoire Naturelle, Paris, France. DANIELS, J . W . , Mr., Department of Oceanography, University of Southampton, U.K. DISTECHE, A., Prof. Dr., Laboratoire d'Oceanologie, Universitg de LiSge, Belgium. DJENIDI, S., Eng., Mgcanique des Fluides Ggophysiques, Universite de LiPge, Belgium. DUPOUY, C., Miss, Laboratoire d'Optique AtmosphCrique, Universit6 des Sciences et Techniques de Lille, France. DYKE, P.P.G., Department of Mathematics and Computer Studies, Sunderland Polytechnic, U.K. GASPAR, Ph., Mr., Institut d'Astronomie et de GCophysique, Universit6 Catholique de Louvain, Belgium. GIDHAGEN, L., Mr., Swedish Meteorological and Hydrological Institute, NorrkGping, Sweden. GILLOT, R.H., Dr., Joint Research Centre, Commission of the European Communities, Ispra, Italy. GOFFART, A . , Miss, Laboratoire de Biologie Marine, Universit6 de Ligge, Belgium. GORDON, C.M., Mr., Naval Research Laboratory, Washington, U.S.A. GOWER, J.F.R., Dr., Institute of Ocean Sciences, Sidney, Canadp. GRILLI, S . , Hydraulique GBn6rale et Mecanique des Fluides, Universit6 de Liege, Belgium. GROSJEAM, P., Mr., MBcanique des Fluides GBophysiques, UniversitB de L S g e , Belgium. HECQ, J.H., Dr., Laboratoire de Biologie Marine, Universit6 de Liege, Belgium.
VIII
JACOBS, W., Mr., Institut fcr Geophysik und Meteorologie der Universitat K61n, Germany. JAMART, B., Dr., Unit6 de Gestion, Modele Mathgmatique Mer du Nord et Estuaire de l'Escaut, Cellule de Liege, Belgium. LEBON, G., Prof., Dr., Thermodynamique des Phenomenes Irrgversibles Universite de Liege, Belgium. LE CANN, B., Mr., Laboratoire d'Oc6anographie Physique, Universitg de Bretagne Occidentale, Brest, France. LIN, S., Mr., The Second Institute of Oceanography, Hangchow, Zhejiang, People's Republic of China. LOFFET, A., Eng., Belfotop Eurosense, Wemmel, Belgium. LYGRE, A., Mr., Continental Shelf Institute, Trondheim, Norway. MARUYASU, T., Prof., Dr., The Science University of Tokyo, Noda, Chiba, Japan. MASSIN, J.M., Dr., Ministsre de l'Environnement, Direction de la Prevention des Pollutions, Neuilly, France. MONREAL, M.A., Mrs., Consejo Nacional de Ciencia y Tecnologia (Conacyt), Mexico. MORCOS, S., Dr., Division des Sciences de la Mer, UNESCO, Paris, France. MURALIKRISHNA, I.V., Dr., National Remote Sensing Agency, Balanagar, India. NEVES, R., Mr., Instituto Superior Tecnico, Lisboa, Portugal. NIHOUL, J.C.J., Prof., Dr., Mecanique des Fluides Geophysiques, Universite de Liege, Belgium. NISHIMURA, T., Dr., The Science University of Tokyo, Noda, Chiba, Japan. ONISHI, S., Prof., Dr., The Science University of Tokyo, Noda, Chiba, Japan. PIAU, P., Eng. Institut Franqais du Petrole, Rueil-Malmaison, France PINGREE, R.D., Dr., Marine Biological Association, Plymouth, U . K . POULAIN, P.M., Mr., Mecanique des Fluides Geophysiques, Universite de Liege, Belgium. RONDAY, F.C., Dr., M6canique des Fluides Geophysiques, Universite de Liege, Belgium. SALAS DE LEON, D., Mr., Consejo Nacional de Ciencia y Tecnologia (Conacyt), Mexico.
IX SALUSTI, S.E., Dr., Istituto di Fisica, Universita Roma, Italy. SENCER, Y., Eng., Mithatpasa cad, Ankara, Turkey. SMITZ, J., Eng., Mecanique des Fluides Ggophysiques, Universit6 de Ligge, Belgium. TANAKA, S., Dr., Remote Sensing Technology Center, Tokyo, Japan. VENN, J.F., Mr., Mathematics Department, City of London Polytechnic, U.K. VAN DER RIJST, H., D r . ,
Elsevier Publ. Company, Amsterdam,
Holland. WITTING, J.M., Dr., Naval Research Laboratory, Computational Physics, Washington, U.S.A. YENTSCH, C.S., Prof., Dr., Bigelow Laboratory for Ocean Sciences, Maine, U.S.A.
This Page Intentionally Left Blank
XI CONTENTS J.F.R. GOWER
:
Water Colour imaging from space
. . . . . . .
J.C.J. NIHOUL : Contribution of remote sensing to modelling
........................
I.V. MURALIKRISHNA : Optimal remote sensing of marine environment . . . . . . . . . . . . . . . . . . . . .
1
25
..
37
E. BOHM and E. SALUSTI : Satellite and field observations of currents on the Eastern Sicilian Shelf
51
T. NISHIMURA, Y. HATAKEYAMA, S . TANAKA and T. MARUYASU: Kinetic study of self-propelled marine vortices based on remotely sensed data
69
. . . . . . . .
. . . . . . . . . . . . . . . . .
S. ONISHl : Study of vortex structure in water surface jets by means of remote sensing
. . . . . . . . . . . . .
T. SUGIMURA, S. TANAKA and Y. HATAKEYAMA
:
107
Surface
temperature and current vectors in the Sea of Japan from NOAA-7/AVHRR data . . . . . . . . . . . . . .
. . .
133
R.V. OZMIDOV and V.I. ZATZ : Study of mesoscale processes in the shelf zone of the Black Sea using remote techniques
149
C. GORDON, D. GREENWALT and J. WITTING : Surface-wave expression of bathymetry over a sand ridge . . . .
159
.......................
. . .
J.M. WITTING : Wave-Current interactions. A powerful mechanism for an alteration of the waves on the sea surface by subsurface bathymetry
187
P.P.G. DYKE : Remote Sensing of oil
............ slick behaviour . . . .
205
A. LYGRE : An intercomparison of GEOS-3 altimeter and ground truth data off the Norveqian coast
217
T. CARSTENS, T.A. McCLIMANS and J.H. NILSEN imagery of boundary currents
235
. . . . . . . . . . . :
Satellite
..............
P. PIAU and C. BLANCHET : Turbulence distribution off USHANT ISLAND measured by the OSUREM HF Rad?r
......
257
XI1
L. LOTH and M. CREPON
A quasi geostrophic model of the circulation of the Mediterranean Sea :
. . . . . . . . . .
R.D. PINGREE : Some applications of remote sensing to studies in the Bay of Biscay, Celtic Sea and English Channel
. . . . . . . . . . . . . . . . . . . .
277
287
S. LIN, G.A. BORSTAD and J.F.R. GOWER : Remote Sensing of
Chlorophyll in the red spectral region
. . . . . . . . .
C.S. YENTSCH : Satellite representation of features of ocean circulation indicated by CZCS colorimetry
. .. .
317
337
1
WATER COLOUR IMAGING FROM SPACE J.F.R. GOWER Institute of Ocean Sciences, P . O .
Box 6000, Sidney, B.C.,
Canada
V 0 L 4B2
ABSTR~CT
Water colour images from the Coastal Zone Color Scanner on the NIMBUS 7 satellite can now show physical and biological processes in the ocean with greater clarity than has ever been possible before. Examples are presented here of turbulent flow patterns in the Gulf Stream affected by the New England seamounts, coastal upwelling off South Africa, the surface pattern formed by the Alaskan Stream, and regions of high phytoplankton concentration on the continental shelf of Argentina. The processing steps now being used to obtain these results are described, with references to more detailed treatments. Possibilities for future improvements in this type of remote sensing measurement are discussed, with particular reference to the possibility of mapping naturally stimulated phytoplankton pigment fluorescence from space.
INTRODUCTION The colour of the sea has been used by sailors for centuries as a check on navigation and as an a d to locat ng productive waters for fishing. Several seas round the world are named after their colours, the most commonly cited example being the Red Sea, named
after
its
sporadic
Trichodesmium ( = Oscillatoria).
blooms
of
the
phytoplankton
Currents bring together water
masses with more subtle colour differences.
The Kuroshio ( "dark
water") is named for this difference, and the colour change at the edge of the Gulf Stream can also be distinguished by eye from a ship. Near the coasts the colour changes can be due to resuspension of bottom sediments silt-laden water.
in shallow water or to river discharge of
Water from the Yangtse Kiang river in China,
for example, gives the Yellow Sea
its name.
Colour changes
further from shore must be due to the growth of phytoplankton where conditions of nutrients and sunlight are favorable.
Such
2
growth can cause colour changes from blue through blue-green to green and in extreme cases to yellow, brown or red. Patches and streaks of
strongly discoloured water were widely
early travelers.
reported by
Darwin (1845) for example, cites references and
describes passing through large areas off South America where "the colour of the water as seen at some distance was like that of a river which has flowed through a red clay district; but under the shade of the vessel's side it was quite as dark as chocolate.
The
line where the red and blue water joined was distinctly defined. The weather for some days previously had been calm and the ocean abounded to an unusual degree with living creatures." Darwin here points out two important connections--with calm weather, allowing near surface stratification, stability and high growth rates, and with the other
"living creatures" in the ocean who
depend on
phytoplankton as the first link in their food chain. Early observers were greatly intrigued by the fronts and narrow bands exhibited by the patches of visibly discoloured water.
In
fact only minute elements of the f u l l patterns can be seen from a ship.
The satellite images presented below show more of the full
complexity of s&ucture due to current streams, and mesoscale eddy fields influenced by larger scale water movements. The water colour images are from the Coastal Zone Color Scanner on
NASA's
techniques
7
NIMBUS used
in
satellite. deriving
Processing these
images
and
correction
are
discussed.
Opportunities for future developments are suggested, including the possibility of mapping naturally stimulated phytoplankton pigment fluorescence from satellites. SATELLITE WATER COLOUR IMAGERY Early weather satellites provided visible and thermal images of clouds, to show the locations of weather systems by day or night. The thermal imagery could also faintly distinguish the sea surface temperature structure associated with major boundary currents such as the Gulf Stream. (Scanning
Improvement of these sensors, through the SR
Radiometer)
to
the
VHRR
(Very
High
Resolution
Radiometer ) to the present AVHRR (Advanced Very High Resolution Radiometer) now gives clear and sharp images, such as Fig. 1, which can resolve temperature and spatial differences as small as 0.2OC
and
1 km
respectively.
This
image
shows
the
thermal
patterns of the surface skin of the ocean, associated with the northeastward flow of the Gulf Stream off the coast of N o r t h
3
Fig. 1. Thermal infrared image (TIROS N AVHRR) showing the Gulf Stream at 19.22 GMT on May 7, 1979. America on May 7, 1979. clouds appear white.
Warmer water is dark, and cold, high
Eddies shed by the stream can be seen to
the north and south of the main current. The degree of detail and the geometrical fidelity of these images have made them a major tool of physical oceanography.
By
contrast, the associated visible images (Fig. 2 ) have been useful only indirectly, providing additional information on the presence of low cloud in daytime passes, though in some cases ocean information can also be deduced from sunglint patterns (La Violette et al, 1980). the water
In Fig. 2 the only contrast visible over
are the white patches due to cloud, with a
faint
brightening at the lower left due to sunglint. The amount of light upwelling from beneath the sea surface gives only about 1% of the signal from sunlit clouds, so that variations in this quantity should indeed be hard to detect on an image designed for cloud mapping. Thermal contrast due to the Gulf Stream, on the other hand, can easily amount to 5 % of the full scale signal, making this an easier target for satellite remote sensing.
4
Fig. 2.Visible image recorded at the same time as Fig. 1. A specialized ocean colour sensor can, however, do much better
than Figure 2 would suggest.
Sensitivity can be increased and
the signal allowed to saturate over cloud.
A mirror can be used
to tilt the field of views away from areas where sunglint is expected, and narrow, optimally placed
can be is dramatically illustrated in Fig. 3 which shows processed data from the Coastal selected.
The
result
of
this
Zone Color Scanner on NIMBUS 7,
spectral bands
improvement
for the same area of ocean at
nearly the same time (3.5 hours earlier). Shades
of
grey
in
the
image
represent
phytoplankton
chlorophyll 5 and phaeophytin pigment concentrations (a standard measure of phytoplankton concentration) with the darkest shades corresponding 10 mg.m-3.
to
.05
mg.m-3
and
the
lightest
to
over
Comparing this with Fig. 1, the water colour image
is able to show more structure in the water, though Fig. 1 could possibly be further enhanced to bring out more structure in the colder water. very
similar,
anticorrelation concentration.
The features that appear on the two figures are illustrating between
the
commonly
temperature
and
observed
high
phytoplankton
The Gulf Stream is again darker in Fig. 3, but
5
F i g . 3. C Z C S p r o c e s s e d ( l e v e l 2 ) p i g m e n t image s h o w i n g t h e same a r e a a s F i g . 1 a t 1554 GMT o n May 7 , 1 9 7 9 ( 3 . 5 h o u r s e a r l i e r ) 1000 m d e p t h c o n t o u r ( d o t t e d ) a n d N e w E n g l a n d s e a m o u n t c h a i n ( t r i a n g l e s ) have been superposed. Grey t o n e s t e p wedge, below g i v e s pigment value.
here
because
compared
of
to more
its
low
pigment
t h a n 0.3
concentration
mg.m-3in
t h e more
( < 0.1
mg.m-3),
productive w a t e r s
further north. T h e complex mesoscale
eddy
field
Gulf Stream i s h e r e w e l l i l l u s t r a t e d . be q u a n t i z e d i n t e r m s of
(Gower
et
al.
1981) and
t h o s e s u g g e s t e d by t h e
in
the w a t e r
north of
the
Such s p a t i a l p a t t e r n s c a n
t h e i r t w o dimensional s p a t i a l spectrum variations
in
increase i n high
this
spectrum,
such as
frequency s t r u c t u r e on
t h e r i g h t s i d e o f t h e image, would be e x p e c t e d t o c o r r e l a t e w i t h t h e changing dynamics o f d i f f e r e n t ocean a r e a s .
case,
t h e c h a i n o f New England seamounts
crosses t h e
image w h e r e
this
change
in
In the present
( p l o t t e d as t r i a n g l e s ) structure
is observed.
T h e s e a m o u n t s e x t e n d u p from t h e b o t t o m a t 5000 m e t e r s t o d e p t h s
6
o f b e t w e e n 1000 a n d 2000 m e t e r s , a n d t h e r e f o r e i n t e r c e p t t h e G u l f Stream,
which
Richardson
flows
in
the
(1981) h a s r e p o r t e d
top
2500
meters
the e f f e c t o f
t h e s u r f a c e f l o w a s t r a c e d b y buoy t r a c k s .
of
ocean.
the
these seamounts on
H e o b s e r v e d meanders
a n d s m a l l ( 2 0 km s c a l e ) e d d i e s , n e a r a n d e x t e n d i n g e a s t w a r d s f r o m i n d i v i d u a l seamounts.
Fig.
3 i l l u s t r a t e s t h i s e f f e c t more f u l l y ,
w i t h s e v e r a l i n s t a n c e s of s m a l l e r e d d i e s n e a r t h e s e a m o u n t s , a n d a g e n e r a l l y more d i s o r d e r e d f l o w t o t h e e a s t .
The 1000 m e t e r
depth
contour
i s s u p e r p o s e d o n t h e image t o
show t h e p o s i t i o n o f t h e e d g e o f t h e c o n t i n e n t a l s h e l f .
The area
of h i g h p r o d u c t i v i t y c a u s e d b y t i d a l m i x i n g o v e r G e o r g e s Bank c a n be s e e n a s a l i g h t e r t o n e d a r e a e a s t o f Cape Cod.
Fine structure
i n t h i s a r e a f o l l o w s t h e form o f t h e s h a l l o w ( a b o u t 10 m ) s h o a l s on t h e b a n k . F i g . 4 shows a n . a r e a o f f t h e w e s t c o a s t o f S o u t h A f r i c a . Town i s a t t h e l o w e r r i g h t c o r n e r o f t h e image.
Cape
The l i g h t a r e a s
i n d i c a t e h i g h p r o d u c t i v i t y due t o coastal upwelling w h e r e pigment concentrations
can
reach
30
mg.nr3.
CZCS
images
of
these
Fig.4. CZCS p i g m e n t image s h o w i n g e f f e c t s o f u p w e l l i n g ( r i g h t ) and p o s s i b l y t h e Benguela c u r r e n t (bottom r i g h t ) , off S o u t h A f r i c a on November 3 , 1979. 1000 m d e p t h c o n t o u r ( d o t t e d ) superposed.
c o a s t a l a r e a s h a v e b e e n d i s c u s s e d b y Shannon e t a 1 ( 1 9 8 3 ) .
The
c h a n g e i n s p a t i a l s t r u c t u r e f u r t h e r o f f s h o r e , a t t h e lower r i g h t of
the
image,
again
a
suggests
dynamic
input,
here
from
the
Benguela c u r r e n t . Fig.
5 c o v e r s a s m a l l s t r i p o f t h e n o r t h e a s t P a c i f i c Ocean
along the s o u t h e r n edge o f the A l e u t i a n I s l a n d c h a i n ,
and s h o w s
t h e s u r f a c e s t r u c t u r e a s s o c i a t e d w i t h the Alaskan Stream on J u l y 1 0 , 1 9 7 9 i n terms o f p i g m e n t l e v e l v a r i a t i o n s i n t h e r a n g e 0 . 4 t o 1 m9.m-3.
stream
This
l e a v e s t h e Gulf
is
of Alaska.
major
the
current
by
which
water
f l o w s westward as a narrow
It
jet
a l o n g t h e c o n t i n e n t a l s l o p e , whose l a n d w a r d e d g e i s i n d i c a t e d i n Fig. the
5 by t h e 1 0 0 0 m c o n t o u r
image
confirms the
(dotted).
narrow
The s t r u c t u r e v i s i b l e i n
( 6 0 km)
width
deduced by
Royer
a n d shows t h e s t a r t o f a
(1981) f r o m c u r r e n t m e t e r observations,
r e c i r c u l a t i n g e d d y n e a r t h e bottom c e n t r e o f t h e image,
south of
Dutch Harbor on U n a l a s k a
i n which
Gulf
of
Alaska
to
observed Thomson
water
occur
(1972).
at
Island.
mixes
a
Such r e c i r c u l a t i o n ,
into
range
The position
the
of of
Pacific
longitudes
this
eddy
Ocean,
as
has
been
discussed
by
is i n the s t a r t o f
t h i s range and n e a r t h e p o s i t i o n f o r the s t a r t o f
recirculation
r e p o r t e d by W r i g h t ( 1 9 8 1 ) f o r March 1980.
F i g . 5. C Z C S p i g m e n t image showing t h e A l a s k a n stream o n J u l y 10, 1979 w i t h t h e 1000 m d e p t h c o n t o u r ( d o t t e d ) s u p e r p o s e d . Fig.
6
shows high concentrations of
phytoplankton
along the
e d g e o f t h e c o n t i n e n t a l s h e l f o f f t h e A r g e n t i n a c o a s t , b e t w e e n 40 and 45' S o u t h o n December 1 0 , 1 9 7 8 .
P r o d u c t i v i t y here i s r e l a t e d
t o mixing by s t r o n g t i d a l c u r r e n t s over the shelf.
A r e a s of high
0
Fig. 6. CZCS pigment image showing areas of high phytoplankton concentration on the edge of the continental shelf off Argentina on December 10, 1978. 1000 m depth contour (dotted) superposed. phytoplankton concentration have been elongated by current shear parallel to the coast, and further strips of pigmented water are visible
further
offshore.
Similar
reports by Darwin and others of
strips must
have
led
to
"great bands" of discoloured
water. Location of depth contours and seamounts on figures presented above makes use of latitude and longitude marks provided round Inaccuracies of about 30 k m in
the edges of processed images.
3
5
and
partially corrected by reference to visible coast features.
No
positions
of
these
marks
were
noted
in
Figs.
and
such correction is possible in Fig. 6 and the depth contour may therefore be mis-located by a similar distance. In all these figures the data is processed so that grey shades of the image will correspond to definite levels of phytoplankton chlorophyll by
a
and phaeophytin pigment concentration as indicated
the grey wedge shown under Fig.
observed
as
being
above
a
given
3.
near
All
land and cloud,
infrared
brightness
9
threshold are masked to black so as to suppress grey tones for which this correspondence will certainly not apply. The form of the colour change being detected in these images is shown in Fig. 7 (NASA, 1 9 8 2 ) . Low concentrations of pigment will absorb blue light at wavelengths shorter than 500 nm, leading to a change from a blue to a bluelgreen colour for the water. At higher pigment concentrations backscatter from the associated cellular material in the water increases the radiance observed at longer wavelengths as indicated, leading to a yellow or brown colouration.
^E
10
S
I
I
I
I
I
I
I
I
I
I
I
I
I
I
i d
1
ti
\
3 2)
0. 1
(D
S .A
2
Qz m
0.01
0
4
5
0.001
LD
4 0
t 0.0001
-c c (
400 450 500 550 600 650 700 Wavelength (nm)
Fig. 7. Sea-water leaving radiance spectra chlorophyll 5 pigment concentrations. (NASA, 1982)
for
several
The algorithms used in the processing are accurate only in so called case 1 water where phytoplankton and their covarying detrital material play the dominant role in determining the This is true in optical properties (Morel and Gordon, 1980). open ocean and many coastal areas. In other areas (case 2 water) suspended material from a shallow bottom, or dissolved or suspended material from land will be important. In clear shallow water light reflected from the bottom will also form part of the optical signal.
10 8 shows a p a r t
Fig.
of
the Gulf
of
Mexico a n d Grand Bahama
Bank a r e a w h e r e m o s t o f t h e g r e y s h a d e s a r e d u e t o a d d e d r a d i a n c e where
pigment
error,
but
encountered
are
levels it
by
typically
near
instrument's
the
I n m o s t cases t h i s w i l l n o t be a s e v e r e s o u r c e
detection l i m i t . of
clear waters
from the ocean b o t t o m t h r o u g h t h e v e r y
reflected
indicates
an
ocean
the
colour
variety
of
optical
scanner.
In
problems case
this
an
algorithm t h a t i n t e r p r e t e d observed o p t i c a l radiances i n t e r m s of
water
depth
a n d bottom
reflectance
(Lyzenga 19811,
might
well
produce u s e f u l r e s u l t s .
Fig.
8.
CZCS p i g m e n t
tge
image showing shallow w a t e r a r e a s i n
G u l f o f Mexico a n d o n Grand B a h a m a Bank on D e c e m b e r 2 , 1 9 7 8 .
PROCESSING OF CZCS WATER COLOUR IMAGES Water c o l o u r d a t a i s c o l l e c t e d b y t h e CZCS i n 4 b a n d s 2 0 nm w i d e c e n t r e d a t 443 6 7 0 nm ( r e d ) .
(blue),
(blue/green),
520
550
(green)
and
A f u r t h e r t w o b a n d s a t 750 nm a n d 11 u m a r e u s e d
f o r m a s k i n g c l o u d or l a n d a n d f o r p r o v i d i n g s i m u l t a n e o u s t h e r m a l images
respectively.
l o w e r q u a l i t y t h a n t h e AVHRR, data
with
a
sufficiently
band
thermal
The
i n t e r m i t t e n t l y and ceased working
i n 1981.
which c a n ,
small
time
operated
i n principle,
difference
t h e r m a l c h a n n e l o n t h e CZCS of l i m i t e d u s e .
only
Its output w a s
to
of
provide
make
the
11 Pigment concentrations and attenuation coefficients are computed using algorithms based on observed correlations of these quantitites with upwelling radiances from the ocean in the blue and green spectral regions (Clark, 1981). To deduce these radiances from CZCS data, the outputs of the first three bands need to be corrected for atmospheric and surface effects. The fourth band at 670 nm is used in making this correction as described below. The processing of CZCS images as carried out by NASA provides two levels of output (Hovis et a1 1980, Hovis 1981). Level 1 gives a set of "quicklook" images of the data in each band recorded by the satellite, and level 2 gives images of: computed sub-surface radiances, corrected
for
atmospheric and
surface
effects: the aerosol signal at 670 nm: the phytoplankton pigment concentration: the diffuse attenuation coefficient and the thermal radiance where this band was operating. Grey levels on the level 2 images relate to quantitative values of all these variables.
Figs. 3-6 and 8 above are examples of level 2 pigment
images. A number of papers have been published describing improvements that have been made in arriving at the present process, the most recent being Gordon et a1 (19831. The first step is €0 convert the measured signals into radiance units. This step has been complicated by a degradation of the reflection of the Sensor tilt mirror while in orbit. This is not monitored by the on-board sensor calibrations, but can be accurately followed by its effects on the resulting data, and time dependent calibrations have now been derived. Modifying the calibration for each band in this way will also compensate for errors in the assumed solar
.
spectrum The major
computation in the processing is to remove the
signal due to Rayleigh scattering of sunlight in the atmosphere over the slightly reflecting ocean, with allowance for ozone absorption in the upper atmosphere. Gordon et a1 (1983) have found that a single scattering approximation works well, but the computation must be carried out for the rather complex geometry of the sensor scan, about an axis tilted to avoid sunglint, over a curved earth. Since the signal depends on the total of gases in the atmosphere, it can be predicted fairly accurately, giving a well defined problem easily handled by computer software. The signal varies smoothly across the scene and can be interpolated after relatively few computations.
12
The Rayleigh signal and upper atmosphere ozone concentration have a slight seasonal and latitudinal dependence that is allowed for in five possible steps. Variations in atmospheric pressure, and in the surface water reflection with wind and waves, including foam cover, cannot be compensated without more data. Some
correction
is
provided
in
the
next
stage
of
aerosol
correction. Aerosol scattering in the atmosphere adds a signal which is much more variable in intensity, but which has a smooth spectrum which can be reasonably well approximated by a power law. At 6 7 0 nm the water radiance becomes very small, and the remaining signal after Rayleigh correction can be used as a measure of the varying aerosol signal in the scene at this wavelength. Extrapolation to the wavelengths of other bands, however, requires a knowledge of the exponent of the power law spectrum. In retrospect, at least two bands, at 6 7 0 nm and at a longer wavelength, would have been useful for measuring this exponent at each pixel. The 7 5 0 nm band included in the CZCS is of low sensitivity and is not suitable for this purpose. However Gordon (1981) showed that this exponent was often constant over large areas and Gordon and Clark (1981) proposed the currently used method of determining it from one "clear water" point in the scene, and applying the resulting aerosol spectrum to the whole image. The method makes use of the fact that the upwelling radiance from case 1 water containing phytoplankton at a pigment concentration of less than 0 . 2 5 mg.m-3 and no other significant scattering material, will be close to fixed values at 5 2 0 and 5 5 0 nm and will be very low at 6 7 0 nm (see Fig. 7 ) . The mean aerosol spectrum power law deduced from these three wavelengths
can then be extrapolated to 443 nm. The resulting "aerosol" correction can contain contributions from improperly corrected Rayleigh radiance, surface foam and residual calibration errors, and
will
tend
to
reduce these
effects where conditions are the same as at the "clear water" point. The correction will be less perfect in other areas of the scene especially if the aerosol properties change. Errors will also exist wherever any suspended material raises the water leaving radiance at 6 7 0 nm, since the signal in this band is used to map the varying aerosol contribution whatever its spectrum. An iterative process in which deduced pigment concentrations are
used to estimate the 6 7 0 nm radiance due to higher concentrations
13
of phytoplankton was proposed by Smith and Wilson (19811, but this has not been implemented in the standard NASA process. The final stage of the atmospheric correction consists of computing the subsurface radiances that will give observed, corrected satellite radiances. This must allow for the facts that surface refraction reduces the signal from beneath the water by about half, and that Rayleigh scattering and ozone absorption attenuate the signal passing out through the atmosphere. These subsurface radiances are then used as inputs to the pigment and attenuation coefficient algorithms. Since these algorithms are based on observations in case 1 waters where optical properhies are determined by phytoplankton concentration only, the two outputs are in fact highly correlated. The algorithms are in the form of mean power law relations with ratios of subsurface radiances in bands 1 and 3, and 2 and 3 as given by
Gordon
et
a1
(1983) and
SASC
(1983).
These
two
documents give details of most of the above processes, with the exception of the time dependent calibration, which is still being refined, and the method used to automatically select clear water areas. The above processing system seems to work well in that images are produced in which atmospheric aerosol patterns are largely suppressed. A limited evaluation given by Gordon et a1 (1983) shows that pigment concentration estimates can be accurate to 530%. However this is for scenes showing large clear areas containing good “clear water” reference areas, and refers only to Shannon et a1 the pigment concentration range 0 to 1.5 mg.m-3. (1983) studying CZCS images of the relatively cloud free Southern Benguela current region (Fig. 4) find differences between ship and satellite chlorophyll 5 pigment estimates over the range 0.1 to 20 mg.m-3, of about a factor of 2 . In many areas the observations will need to be made in smaller clear areas among cloud. Here the existence of good clear water reference areas becomes particularly critical.
Gordon et a1 (1983) show that the
effect of only 0.27 ~ng.m-~ of pigment in the “clear water’’ area can lead to a factor of 2 error in deduced‘pigment for other areas. A drawback of the present processing system is that the position of the assumed clear water pixel is not recorded on the final data, so that users cannot easily assess possible errors. It must be emphasized that the present problem in making aerosol corrections is largely due to the present design of the
14
CZCS.
Morel and Gordon
(1980) proposed
an
improved set of
spectral bands, since refined in the MAREX report (NASA, 1982), which would greatly reduce this problem. The examples shown above demonstrate the value of the data. The MAREX report (NASA, 1982) suggests how an improved, follow-on sensor could be used in a large scale program of primary productivity mapping with applications in fisheries, climate studies and physical oceanography.
TECHNICAL IMPROVEMENTS POSSIBLE FOR SATELLITE WATER COLOUR MEASUREMENTS Improvements which can increase accuracy and coverage of satellite water colour data have been mentioned above and by Morel and Gordon (1980), NASA (1982) and SCOR (1983). Table 1 summarizes these proposals, several of which are being implemented on the next Ocean Color Imager due to be launched by the U . S . by about 1986 on one of the NOAA weather satellites. A further major problem found with the CZCS was in the complexity of the required data processing, and the resulting long delays before data became available. The problems now seem to have been overcome and the data backlog, in some cases extending back five years, is now being reduced. Technical developments in the field of integrating optics with solid state electronics have resulted in sensor arrays that can be used for remote sensing, either in a pushbroom mode (where a one-dimensional line of sensors looks at contiguous points along a line of view which is moved at right angles to the line by motion of the satellite) or in an imaging spectrometer mode (where a two-dimensional array of sensors operates as many pushbroom scanners, each at a different wavelength). Such sensor arrays offer high sensitivity and the possibility of observing in more, or more precisely chosen, spectral bands. A typical sensor array might have 300 by 300 elements, which would allow pushbroom imaging of a 15" field of view with an angular resolution comparable to the CZCS, and a spectral resolution of 1.5 nm in the wavelength range 400 to 850 nm. Several arrays would be required to cover the wider CZCS field of view. If the outputs from all elements were read and digitized at the rate required for satellite imaging (about 10 times per second) then the volume of data would be enormous (about 50 times
15
TABLE 1 Suggested improvements in satellite ocean colour imagers (OCI) ~
Technical requirement
Current action
Improve aerosol correction
add infrared bands
include in next OCI
Improve pigment characterization
add visible bands
include in next OCI
Improvement
I
Map smaller pigment changes
I
Improve area coverage
I
increase sensitivity
include in next OCI
add onboard processing to reduce data volume
include in next OCI
add colour sensors to geosynchronous satellites
proposed but not yet implemented
add band near 400 nm
proposed but not imp1ement ed
Map natural fluorescence
increase sensitivity and add special bands
flexible airborne sensor being constructed
that
CZCS).
-
Distinguish yellow substance from phytoplankton pigments
r
from
digitally
the
present
combined
into
The
outputs
predetermined
can
spectral
however bands
be
thus
reducing the data band width to that required by mechanical scanners, and giving much greater flexibility and precision in selection of the bands. The selection can be changed under software control, allowing a variety of specialized band combinations to be formed for mapping different target signatures. This type of sensor is particularly suitable for attempting the mapping of naturally stimulated phytoplankton pigment fluorescence as discussed in the next section.
16
MAPPING OF NATURALLY STIMULATED PHYTOPLANKTON, CHLOROPHYLL A FLUORESCENCE IN SEA WATER The broad band colour changes that are mapped by the CZCS are caused by a combination of absorption and backscattering of incident light by phytoplankton. The resulting colour changes are illustrated in Fig. 7 and are often adequately characterized by green to blue ratios deduced from measurements in CZCS bands. Another familiar feature of phytoplankton chlorophyll a pigments is their fluorescence, which for the most commonly occurring process leads to emission at 685 nm. A slight increase in the radiance at this wavelength, due to natural stimulation of this fluorescence by sunlight, can be seen for all four spectra plotted in Fig. 7, where the amount of this increase, above a smooth baseline, is roughly proportional to the chlorophyll concentrations listed. Use of this signal for airborne remote sensing surveys was first suggested by Neville and Gower (1977) and Gower (19801, and for satellite observations by Gower and Borstad (1981). The fluorescence signal has been found to be proportional to the chlorophyll concentration, though the value of the proportionality constant has been found to vary in the case 2 waters where most tests have been made (Fig. 9). Observations of naturally stimulated fluorescence have been used successfully in airborne surveys of the British Columbia coast (Borstad et al, 1980) and in the eastern Canadian Arctic (Borstad and Gower, 1983). Gower and Lin (1983) report a characteristic vector analysis of reflectance spectra for coastal waters for which fluorescence appears to provide superior estimates of pigment concentrations compared to the estimates derived from green to blue ratios. This analysis has been extended to examine variations in the fluorescence emission for different phytoplankton (Lin, et al, this volume). An 8 band ocean colour scanner with a band centred at 685 nm, 23 nm wide, was also flown on the Space Shuttle in 1981 (Kim et al, 1982). Other similar bands at 655 and 787 can be used to interpolate a baseline from which the radiance difference at 685 nm may be related to chlorophyll 5 fluorescence. Unfortunately, apart from the low sensitivity of the sensor and the non-optimal widths and positions of the bands, there were problems with weather and timing of this shuttle flight. The best scene of the limited resulting data set is shown in Fig. 10
17
20.0
Fi *
5
/
(3
3
15.0
.2
/
t
,/ .3
a -I -I
I
a 0
a
10.0
0 -I
...-
I
0
w
0
5.0
a
3 u)
0.0
FLUORESCENCE LINE HEIGHT
Fig. 9. Relations between naturally stimulated fluorescence (expressed as apparent reflectance increase at 685 nm x lo5) and phytoplankton pigment concentrations observed in surveys on the British Columbia coast in 1979 ( 1 and 2), 1981 (3 and 4) and 1976 (5). with uncorrected radiance at 685 nm
(top) and the calculated
radiance difference at 685 nm from the linear The scene shows parts of the Yellow Sea and off the mouth of the Yangtze River so that colour changes will be related to suspended
baseline (bottom). the East China Sea most of the water sediment. Some of
the brightening in the lower image, for example near the coast of Korea (top right), may be due to pigment fluorescence. The sensitivity is such that fluorescence due to a few mg.m-3 of pigment should be detectable. The lower scene is much less affected by the aerosol change near Cheja Island (centre) and by the strong limb brightening both visible on the top image. This data has not yet been processed using the techniques described above. Apart from its use as an estimator of chlorophyll 5 concentration, the
fluorescence
signal will
provide
another
tracer of water flow, or mixing patterns. Fig. 11 shows a variation in the observed fluorescence signal between spectra (A
18
Fig. 10. Images from the OCS experiment on the OSTA-1 Space Shuttle flight on November 13, 1981, showing the mouth of the Yangtze River (left) and southern Korea (top right). Uncorrected 685 nm band (top), partly processed fluorescence image (bottom). and
B)
taken
a
few
minutes
apart
in
Kiel
harbour
(Gower,
unpublished). Curve C is the difference plotted with 20 times more sensitivity. The proportional change at 550 nm is much smaller than that in the fluorescence signal. For airborne and satellite remote sensing the fluorescence signal has the advantages of a narrow band width, which distinguishes it from the variable, broad band signals due to aerosols or water surface effects, and a position at the red end of the optical spectrum where the Rayleigh scattered radiance is low.
Absorption of light by the atmosphere occurs at wavelengths
close to that of the fluorescence signal, particularly on the longer wavelength side where water vapour absorbs with varying strength from 690 to 745 nm and oxygen from 687 to 694 and from 760 to 770 (Fig. 12).
19
400
500
600
700
800
WAVELENGTH (nm)
Fig. 11. Water radiance spectra (A and B ) observed at two points in Kiel Harbour on April 26, 1982 from the deck of a ship. The difference ( C ) is plotted at 20 times the vertical scale. The right hand peak in curve C, interpreted as caused by a change in chlorophyll 5 fluorescence, can be fitted by a Gaussian centred at 682 nm with a half height width of 24 nm (residual shown dotted). Spectrometer resolution is 12 nm. Observing bands will need to be fitted between these features with the relatively high precision of a few nanometers. Measurement of the fluorescence signal will be by analysing the radiance spectrum shape in the range 660 to 690 nm supplemented by measurements in the window at 745 to 760 nm, or in the almost transparent window at 708 to 714 (Fig. 12) to remove the smoother shape of the background radiance. Although such observations could be made with a
specially
configured mechanical scanner, an array sensor such as described above provides greater sensitivity and flexibility. Such a sensor, the Fluorescence Line Imager (FLI), is now being built as part of the remote sensing program of the Canadian Department of Fisheries and Oceans. This is an airborne prototype imaging
20
Fig. 12. Atmospheric optical depths between 500 and 8 5 0 nm due to absorption by oxygen and water vapour. Note the expanded vertical scale which shows faint features especially at wavelengths shorter than 680 nm.
spectrometer whose properties are listed briefly in Table 2 . Figure 13 shows the sensor head with four of its five cameras, which will together cover a 7 0 " field of view. Fig. 14 shows the layout of one of the cameras in which light is dispersed by a transmission grating and focussed onto the CCD array on the left side. Some of the readout electronics is also visible. Computer control will allow spectral band specification and will perform the processing needed to form these bands by signal summation. A real-time output is available for display of a mathematical combination of different bands. Flight programs are being planned to test use of this sensor over a variety of targets. Although the instrument was designed specifically for water colour observations, its parameters make it ideal for other remote sensing studies, for example in the fields of agriculture, forestry, geology and atmospheric sciences and for simulating the spectral responses of other optical imagers. Scientists interested in joint observing programs should contact the author.
21
Fig. 13. The sensor head of the Fluorescence Line Imager (FLI), being built for the Canadian Department of Fisheries and Oceans, with four of the five CCD cameras in position.
Fig. 14. One of the F L I cameras with covers removed, showing the layout of the optics and some of the digitizing electronics.
22
TABLE 2 Properties of Fluorescence Line Imager (FLI) Size of arrays used Number of arrays Total field of view Total number of pixels Total number of spectral elements Spectral coverage Spectral resolution Number of bands Location and width of bands Digitization Signal to noise Scan rate
385 x 288 5 70" 1925 288 410 to 850 nm 2 nm 8 under software control to 1.5 nm 12 bits 2000:l for a 30 nm band 10 per second
CONCLUSIONS
Processed
CZCS
imagery demonstrates the potential of ocean
colour imaging from space for physical as well as biological oceanography. Improved sensors should lead to more results, covering wider areas with greater regularity.
precise Imaging
of natural fluorescence also appears possible and should lead to further improvements.
23
REFERENCES Borstad, G.A., Brown, R.M., and Gower, J.F.R., 1980. Airborne remote sensing of sea surface chlorophyll and temperature along the outer British Columbia coast. Proceedings 6th Canadian Symposium on Remote Sensing, Halifax, N.S., May, pp. 541-549. Borstad, G.A. and Gower, J.F.R., 1983. Ship and aircraft measurements of phytoplankton chlorophyll distribution in the eastern Canadian Arctic. Arctic, in press. Clark, D.K., 1981. Phytoplankton pigment algorithms for the NIMBUS-7 CZCS. In: J.F.R. Gower (Editor), Oceanography from Space. Plenum Press, Marine Science, 13: 227-238. New York. Darwin, C.R., 1845. The voyage of the Beagle, 2nd Ed., Everyman Library Paperback, Dent, London. Gordon, H.R., 1981. A preliminary assessment of the NIMBUS-7 CZCS atmospheric correction algorithm in a horizontally inhomogeneous atmosphere. In: J.F.R. Gower (Editor), Oceanography from Space, Marine Science 13: 257-265. Plenum Press, New York. Gordon, H.R. and Clark, D.K., 1981. Clear water radiances for atmospheric correction of coastal zone color scanner imagery. Applied Optics, 20: 4175-4180. Gordon, H.R., Clark, D.K., Brown, J.W., Brown, O.B., Evans, R.H. and Broenkow, 1983. Phytoplankton pigment concentrations in the Middle Atlantic Bight: Comparison of ship determinations and CZCS estimates. Applied Optics, 22: 20-37. Gower, J.F.R., 1980. Observations of in situ fluorescence of chlorophyll 5 in Saanich Inlet. Boundary Layer Meteorology, 18: 235-245. Gower, J.F.R., Denman, K.L. and Holyer, R.J., 1980. Phytoplankton patchiness indicates the fluctuation spectrum of mesoscale turbulence. Nature, 288: 157-159. Gower, J.F.R. and Borstad, G.A., 1981. Use of the in vivo fluorescence line at 685 nm for remote sensing surveys of surface chlorophyll a. In: J.F.R. Gower (Editor), Oceanography from Space, Marine Science, 13: 329-338. Plenum Press, New York. Gower, J.F.R. and Lin, S., 1983. The information content of different optical spectral ranges for remote chlorophyll estimation in coastal waters, International -Journal of Remote Sensing. In press. Hovis, W.A., Clark, D.K., Anderson, F., Austin, R.W., Wilson, W.H., Baker, E.J., Ball, D., Gordon, H.R., Mueller, J.L., El-Sayed, S.Z., Sturm, B., Wrigley, R.C., and Yentsch, C.S., 1980. NIMBUS 7 Coastal Zone Color Scanner: System description and initial imagery. Science, 210: 60-63. Hovis, W.A., 1981. The NIMBUS 7 Coastal Zone Color Scanner (CZCS) program. In: J.F.R. Gower (Editor). Oceanography from Space, Marine Science, 30: 213-225. Plenum Press, New York. Kim, H.H., Hart, W.D. and van der Piepen, H., 1982. Initial analysis of OSTA-1 Ocean Color Experiment Imagery. Science, 218: 1027-1031. and Gower, J.F.R., 1980. LaViolette, P.E., Peteherych, S . Boundary Layer Meteorology, 18: 159-175. Lyzenka, D.R., 1981. Remote sensing of bottom reflectance and water attenuation parameters in shallow water using aircraft and Landsat data, International Journal of Remote Sensing, 2: 71-82.
24
Morel, A.Y. and Gordon, H.R., 1980. Report of the Working Group on Ocean Color. Boundary Layer Meteorology, 18: 343-355. NASA, 1982. The Marine Resources Experiment Program (MAREX) Report of the Ocean Color Science Working Group. Goddard Flight Center, R. Kirk (Coordinator). Neville, R.A. and Gower, J.F.R., 1977. Passive remote sensing of phytoplankton via chloropyll 5 fluorescence. Journal of Geophysical Research, 82: 3487-3493. Richardson, P.L., 1981. Gulf Stream trajectories measured with free-drifting buoys. Journal of Physical Oceanography, 11: 999-1010. Royer, T.C., 1981. Baroclinic Transport in the Gulf of Alaska Part I. Seasonal Variations of the Alaska Current. Journal of Marine Research, 39: 239-250. SASC, 1983. NIMBUS 7 CZCS derived products scientific algorithm description. Report no. EAC-7-8085-0027. Systems and Sciences Corporation, Hyattsville, MD., USA. SCOR, 1983. Remote Measurement of the Oceans from Satellites. Scientific Committee on Oceanic Research,.Workinq Group 70 report, in preparation. Shannon, L.V., Mostert, S.A., Walters, N.M. and Anderson, F.P., 1983. Chlorophyll concentrations in the Southern Benguela current region as determined by satellite (Nimbus 7 Coastal Zone Color Scanner). Journal of Plankton Research, 5: 565-583. Smith, R.C. and Wilson, W.H., 1981. Ship and satellite bio-optical research in the California Bight. In: J.F.R. Gower (Editor), Oceanography from Space, Marine Science, 13: 281-294. Plenum Press, New York. Thomson, R.E., 1972. On the Alaskan Stream. Journal of Physical Oceanography, 2: 363-371. Wright, C., 1981. Observations in the Alaskan Stream during 1980. NOAA Technical Memorandum ERL. PMEL-23.
26
CONTRIBUTION OF REMOTE SENSING TO MODELLING Jacques C.J. NIHOUL GHER, University of LiBge, Belgium
1. Application of remote sensing to the identification of processes
and structures and to the formulation of mathematical models of the marine system. One of the most decisive contribution of remote sensing has been the supplying, for the first time, of synoptic views of large sea areas and the identification of mesoscale and macroscale horizontal structures which had been overlooked in field studies and ignored in mathematical models. Digital image analysis of Landsat data has revealed, for instance, the penetration in the Harima Sea (Japan) of a pair of large scale vortices formed by amalgation of two series of coherent vortices, produced in the free boundary layers in the wakes of the Naruto Straits'Capes. The vortex pair, apparently carried along by the tidal currents in a first stage, was found to continue penetrating into the Harima Sea, after tide reversal, under self-induced driving forces (Maruyasu et al., 1 9 8 1 ) . This mechanism which plays a cogent role in local mixing could not have been identified without synoptic remote sensing views of the Set0 Inland Sea. NOAA 6 images of the Western Mediterranean have shown complicated seasonal circulation patterns, - including eddies, planetary solitons, upwellings, fronts, water intrusions, coastal currents which could not have been apprehended by restricted experimental
-
investigations (e.g. Philippe and Harang, 1 9 8 2 , Preller and Hulburt, 1 9 8 2 ) (fig. 1). The meandering of large scale currents like the Gulf Stream and the subsequent shedding of synoptic eddies has never been properly perceived and understood until remote sensing images of the area were available (e.g. Behie and Cornillon, 1 9 8 1 ) .
26
F i g . 1. c h a r t of s p r i n g s u r f a c e t e m p e r a t u r e f r o n t s i n t h e A d r i a t i c Sea (23-29 A p r i l , 1982) communicated by Lannion C e n t e r . General legend f o r f i g u r e s 1 t o 4 . Mean p o s i t i o n of a t h e r m a l f r o n t (AT 2 1 ° C ) p e r s i s t i n g t h e whole week; w a r m water on t h e d a s h e d s i d e .
7'7 ///
O c c a s i o n a l t h e r m a l f r o n t (AT 2 1 " C ) w i t h t h e d a t e o f observa t i o n . Permanent t h e r m a l b o r d e r w i t h o u t marked f r o n t a l f e a t u r e s .
ri"rOd actcea soi fo noabl stehrevramt iaol n bi on rddi ec ra t ewdi.t h o u t marked Up
,--.Upwelling
$1TC :: W a r m - _ _
'------0
4
a
I2
I6
T i m e , t-IO3 (sec) F i g . 2 . Time v a r i a t i o n or t h e a n i s o t r o p y parameter, a , f o r three experiments.
153
2.0
It
t,,,,,,,,,,., 2
0
4
8 Time, t
I2
'
I6
- I03 (sec)
20
24
I
Fig. 3 . Time variation of the diffusion velocity, several experiments.
-
p=lmax
for
2tmax
L
V
P) ffl
\
E
v
&! c ffl
4J
d
a, -ci
u
4
w w
0 0
u
P)
m
i
3X M
4 10' 8 102 2 4 6 Scale of phenomenon, R (m) Fig. 4 . Variation of the coefficient of horizontal diffusion
, with the scale II of the phenomenon estimated from air 2At photographs of floats. Experiment n O 1 took place in the Caucasus coastal region, experiment n02 in the Crimea coastal region. K = l -
154 a t buoy s t a t i o n s . The v e r t i c a l s h e a r c a n b e a s h i g h a s 10-1 s e c - l ; t h e mean v a l u e i s sec -1 which e x c e e d s t h e mean v a l u e s u s u a l l y o b s e r v e d i n t h e open o c e a n . The e l o n g a t i o n o f t h e d y e p a t c h e s u s u a l l y o c c u r s i n t h e d i r e c t i o n of t h e c u r r e n t i n t h e a b s e n c e of wind o r wheii t h e wind blows i n t h e d i r e c t i o n of t h e c u r r e n t . I f t h e d i r e c t l v n s of t h e c u r r e n t and of t h e wind a r e d i f f e r e n t , t h e a x i s of t h e p a t c h c a n b e a l i g n e d w i t h y e t a n o t h e r d i r e c t i o n ( F i g . 6 ) . The e l o n g a t i o n of a p a t c h c a n be i n f l u e n c e d by t h e v e r t i c a l v e l o c i t y s h e a r when d e e p e r l a y e r s of t h e p a t c h " l a g b e h i n d " n e a r s u r f a c e l a y e r s . An example of t h i s e f f e c t i s shown i n F i g . 7 . F o r p a t c h s c a l e s o f 1 t o 1 0 km, t h e e l o n g a t i o n of t h e p a t c h e s may b e i n f l u e n c e d b y h o r i z o n t a l v e l o c i t y s h e a r s . The e l o n g a t i o n and t h e d i s t o r t i o n o f t h e p a t c h e s c a n b e , i n a number of c a s e s , r e l a t e d t o d r i f t and Ekman s p i r a l c u r r e n t s . One s u c h example i s i l l u s t r a t e d i n F i g . 8 . T h e s e p h o t o g r a p h s of t h e p a t c h w e r e o b t a i n e d i n c o n d i t i o n s of s l i g h t wind and c a l m s e a , when t h e Ekman d e p t h a p p e a r e d n o t t o e x c e e d 1 0 - 2 0 m and t h e d y e was p e n e t r a t i n g t o a d e p t h o f 1 5 m. F o r s t r o n q e r w i n d s and rougher sea, i.e.
104
f o r l a r g e r Ekman d e p t h s , s i m i l a r p i c t u r e s
105
S p a c i n g between i n s t r u m e n t s A R
I06 (cm)
au
5 . V a r i a t i o n of t h e h o r i z o n t a l v e l o c i t y s h e a r , r = BQ w i t h t h e i n s t r u m e n t s p a c i n g , A Q . Experiment n O 1 t o o k p l a c e i n t h e C r i m e a c o a s t a l r e g i o n , experiment n 0 2 i n t h e Caucasus c o a s t a l region.
155
Fig. 6 . Schematic representation of the drift of diffusing patches of dye in the Crimea coastal region. The arrows indicate the wind and current directions.
of patch distortion were not observed. Air photographs often show a "streak" structure of the dye field. These streaks can be related to Langmuir circulations. According to our estimates the distance between these circulations varied from 5 - 1 0 to 2 0 - 3 0 m. This distance is smaller than the one observed in the open ocean by Assaf et al. ( 1 9 7 1 ) . The difference seems to be related to the fact that a sharp and fairly shallow ( 2 0 - 3 0 m) pycnocline was present in the region of our observations and that the wind velocity was not very large. Air photography of dye patches can also reveal interesting phenomena in frontal zones. Such a frontal zone was observed in a coastal region between transparent waters near shore and a "tongue" of more turbid waters seaward. Patches of dye were first released at a distance of 5 0 0 m on both sides of the front. The patches floated rapidly towards the front, they became parallel to the front in its vicinity, and they disappeared rapidly at the frontal line. The velocity at which the patches approaEhed the front reached 5 - 1 0 cm/sec. Based on the rates of disappearance of the
156
F i g . 7 . E l o n g a t i o n of a d y e p a t c h c a u s e d b y v e r t i c a l v e l o c i t y shear.
F i g . 8 . E l o n g a t i o n and d i s t o r s i o n of a d y e p a t c h u n d e r t h e a c t i o n of d r i f t c u r r e n t s ( t h e " h e a d " of t h e p a t c h i s i t s southern t i p )
.
157
patches, one can conclude that the vertical velocities in the frontal zone had the same order of magnitude. In a second experiment, we released 2 patches seaward of the front at distances of 1.3 and 3 miles. At these distances, the velocities at which the patches approached the front were small, and during 4-5 hours of observation the patches were not absorbed by the front. Obviously, aerial photography of dye patches in frontal zones can provide valuable data on the structure and the kinematics of these interesting and important features. The wide application of remote methods using dye as a tracer can provide detailed information on the interactions of currents, wave motions and turbulence in various hydrometeorological and orographical situations. The application of only contact methods of measurements would be insufficient for such an investigation in a number of cases.
REFERENCES Assaf,G,,Gerard,R. and Gordon,H., 1971. Some mechanisms of oceanic mixing revealed in aerial photographs. J.Geophys.Research, 76, 6550-6572. Djuric, D. and Leribaux, H.R., 1974. On che determination of turbulent diffusivity in shallow waters by serial photography of floating markers. Limnology and Oceanography, 19, 138-144. Fucuda, V . , Ito,N. and Sakahishi, S., 1964. Dispersion phenomena in coastal areas. Proc. of the 2-nd Inter. Conf. held in Tokyo, 3, E.A.Pearson (Editor). Pergamon Press. Ichiye, T., 1965. Diffusion experiments in coastal waters using dye techniques. Proc.Symp. Diffusion in Oceans and Fresh Waters at Lamont Geol. Obs., Palisades, N.Y. 54-67. Ichiye, T. and Plutchak, N.E., 1966. Photodensitometric measurements of dye concentration in the ocean. Limnology and Oceanography, 11, 364-370. Ichiye, T., 1967. Upper ocean boundary layer flow determined by dye diffusion. Physics of Fluids Supplement, 270-277. Ito, N., 1964, On the small-scale horizontal diffusion near the coast. J. Ocean. SOC. of Japan, 19, 182-186. Ito, N. and Fucuda, M., Tanicawa, V., 1966. Small-scale horizontal diffusion near the coast. Disposal Radioactiv. Wastes in the Seas, Oceans and Surface Waters, Vienna. James, W. and Burgess, F.J., 1970. Ocean outfall dispersion. Photogram. Engineering, 36, 1241-1250. Nanniti, T., 1964. Some observed results of oceanic turbulence. Science on Oceanography, 211-215. Pritchard, D.W., Okubo, A . and Carter, H., 1966. Observation and theory of eddy movement and discussion of an introduced tracer material in the surface layers of the sea. Disposal Radioact. Wastes in the Seas, Oceans and Surface Waters, Vienna, 397-424. Schott, F., Ehlers, M., Hubrich, L. and Quadfasel, D., 1978. Small scale diffusion experiments in the Baltic surface-mixed layer under different weather conditions. Dt. hydrogr. Z., 31, 195215.
This Page Intentionally Left Blank
159
SURFACE-WAVE EXPRESSION OF BATHYMETRY OVER A SAND RIDGE C. GORDON, D. GREENWALT and J. WITTING Naval Research Laboratory, Washington, DC 2 0 3 7 5
ABSTRACT Satellite SAR and airborne SLAR produce strong surface imagery of bottom features in shallow seas. Lagrangian and Eulerian current measurements have been made in the vicinity of a sand ridge that generates a visual and remotely-sensible wave manifestation known as a rip. The phenomenon is interpreted in terms of wave-current interaction in which the propagation of shorter waves is blocked by an adverse current. INTRODUCTION During the past decade wave patterns on the ocean surface have been observed using coherent imaging radar (Brown et al., 1 9 7 3 , 1 9 7 6 ; Moskowitz, 1 9 7 3 ; Larson and Wright, 1 9 7 4 ) . Progress in this field has been recently reviewed in detail by Alpers, et al. ( 1 9 8 1 ) This radar imagery is of great interest to oceanographers because the changes ih surface roughness responsible for the effect are related to physical oceanographic and meteorological parameters. For example, variations in coherent radar backscatter have been attributed to local wind, surface fronts, ocean swell, internal waves , surface siicks, currents, island shadowing and subsurface topography. The influence of the last of these sources affecting radar imagery is the subject of the work described here. This report is an initial interpretation of the surface-wave manifestation of a subsurface topographic feature (Phelps Bank, Nantucket Shoals) based on photographic, bathymetric and current measurements obtained during a field exercise aboard the USNS HAYES, July, 1 9 8 2 .
BACKGROUND The first investigators to note the appearance of subsurface topographic features in the sea-surface imagery of coherent radar (Side-looking Airborne Radar, SLAR) were DeLoor and Brunsveld van Hulten ( 1 9 7 8 ) . They reported that the surface wave pattern had. the same crest direction and wavelenght as the sand waves on the bottom of the North Sea where their measurements were taken. They tentatively and qualitatively attributed the effect to "interference of the tidal current with the bottom topography, producing a very weak wave-like pattern which modulated the capillary waves in combination with velocity fluctuations at the sea surface". With regard to the present work they made one particularly relevant observation, that is, no surface manifestations of subsurface sand-wave patterns were detected with turning of the tide, but rather only when diurnal current was present. The implication is that the topographic effect on the surface-wave pattern is probably current related. The influence of near-surface currents 0.1 the wavelenghts, amplitudes, speeds and directions of surface waves has been observed and treated theoretically for a long time (Unna, 1 9 4 2 ; Johnson, 1 9 4 7 ; Taylor, 1 9 5 5 ; Ursell, 1 9 6 0 ; Longuet-Higgins and Stewart, 1 9 6 0 , 1 9 6 1 ) . The appearance of current effects in the surface wave field as imaged by coherent radar was first noted by Brown et al. ( 1 9 7 6 ) but no quantitative explanation was offered. Tidal currents in shallow seas and the influence of bottom topography on their radar images has also been discussed in a qualitative way by DeLoor (1981). The explanation proposed by DeLoor ( 1 9 8 1 ) is that "through the tidal current the dune pattern at the bottom modulates the capillary and short gravity waves at the surface which are in resonance with the radar wave". The speculation was based on a comparison of radar imagery with bathypetric charts of the same area, rather than a detailed analysis. The sea depth at the location was about 2 5 m and the "wave height" of the subsurface sand dunes was about 4 m. The tidal currents involved ranged between 0.5 and 1 msec-l. SLAR imagery of the sea surface in the Southern Bight of the North Sea off the coast of Holland has been reported by McLeish et al. ( 1 9 8 1 ) to reflect sea-floor bed forms such as sand ridges and sand waves. The sand ridges were the order of 500 m wide and 1 0 to 2 5 m below the surface at their crests (Buitenbank, Schouwenbank and Bollen van Goeree). They occur in an area where currents are
161
dominated by rotary tidal components in the range of 0.25 - 0.50 -1 msec McLeish et al. (1981) conclude that water depth is the controlling variable responsible for changes in the coherent radar imagery. Basing their interpretation on the "hydrodynamic effect" of ocean waves on radar as proposed by Valenzuela (1978), McLeish et al. (1981) assume that the Bragg waves (order of cm) on the surface that are "seen" by the radar are changed in shape by convergence and divergence of the current flow as it passes over the topographic feature. They suggest that the surface convergence where the sea is deeper tends to increase the density of wave energy whereas divergence over the top of the shallower features will have the opposite effect on waver energy density. It was concluded that the surface manifestation requires two conditions : 1) water flow across a sufficient relative depth change and 2) wind speed sufficient to generate Bragg waves. These investiqators also observed that the appearance and disappearance of surface patterns from bottom features depended on the direction of the tidal currents. The features that disappeared in the radar imagery were those roughly parallel to the current direction. AS in the earlier work, the bathymetry effects on surface waves are associated with current flow. Some of the most spectacular indications of surface wave patterns reflecting bottom topography have been observed with the SAR (Synthetic Aperture Radar) aboard the SEASAT satellite, particularly in the Nantucket shoals area. As yet there are no detailed analyses or interpretations of this data. As pointed out by Phillips (1981) in his discussion of the SAR imagery and the interaction of short gravity waves with surface currents, there has been little systematic study of the spatial variability of surface currents induced by steady or tidal flow across irregular shallowbottom topography. Figure 1 is an illustration of SAR imagery in the Nantucket-Cape Cod area. The location of the subsurface sand ridge called Phelps Bank that is the object of this study is shown in the figure. As Kasischke et al. (1980) have pointed out, approximately 8 0 % of the surface features in this imagery were over or near distinct bottom features. The abrupt change in the character of the surface wave field in the vicinity of Phelps Bank is clearly seen from the air. Figure 2 is an aerial photograph that shows this sharp demarkation. The fact that surface currents interact with gravity waves to change their properties has been the subject of numerous studies.
.
162
0
w
u
a, C C
4J -4
-4
N
.
ffix 4 -c
.CO
cno m m m
163
F i g . 2 . An a e r i a l p h o t o g r a p h t a k e n n e a r P h e l p s Bank f;om a n a l t i t u d e o f a b o u t 4 6 0 m . The camera f o c a l l e n g t h was 15 c m . A s h a r p c h a n g e i n s u r f a c e waves i s c l e a r l y e v i d e n t . Unna ( 1 9 4 2 ) examined t h e c a s e i n which o n - s h o r e waves e n c o u n t e r e b b i n g t i d a l c u r r e n t s from a n e s t u a r y a t a p p r o x i r m t e l y 180Obetween t h e d i r e c t i o n s o f c u r r e n t f l o w and t h e wave p r o p a g a t i o n . H e found t h a t when waves t r a v e l i n g a t s p e e d Vo meet a c u r r e n t moving a t speed C t h e waves w i l l b r e a k when C = -1/4 Vo r e g a r d l e s s of t h e i n i t i a l s t e e p n e s s o f t h e wave. Thus i n a t i d a l r a c e o r on a b a r t h e r e c a n be s t a n d i n g l i n e s o f b r e a k i n g waves and t o t h e l e e w a r d of t h e p o s i t i o n where C = -1/4 Vo t h e sea w i l l b e r e l a t i v e l y c a l m . For example, h e found t h a t waves w i t h s l a c k - w a t e r w a v e l e n g t h s o f 3 0 m a r e e s s e n t i a l l y b l o c k e d by a c o u n t e r c u r r e n t o f 1 . 8 msec-l.
Johnson ( 1 9 4 7 ) l a t e r d e m o n s t r a t e d t h a t wave b l o c k i n g , b r e a k i n g and r e f r a c t i o n may o c c u r e v e n when t h e waves d o n o t a d v a n c e d i r e c t l y
164
into the current. His analysis implied that when waves encounter a current of 1.0 msec-l, all the waves with speeds less than 10 msec-' (7 sec period) that enter the current at an angle greater than 58' will break and will therefore be unable to cross the current. Johnson (1947) demonstrated the wave-current interaction with a remote sensing experiment using aerial photographs of waves passing through ebb and flood tidal currents at the entrance of Humboldt Bay, California. Taylor (1955) examined wave-current interaction theoretically for the practical purpose of utilizing the wave blocking phenomenon in designing a "current-flow'' breakwater for harbor protection. He found that the effect was not strongly dependent on the vertical distribution of the current flow. That is, the wave stopping efficiency of a uniform stream is equivalent to that of a stream with a uniform velocity gradient provided the total flow of inertia is the same. An interesting example of Taylor's (1955) results is that if a 4.6 msec-' current is present over a sufficiently long distance, it would only have to be 0.5 m deep to stop waves with 30 m wavelengths. More recent work by LonguetHiggins and Stewart (1960, 1961) and Ursell (1960) has treated the problem of changes in the form of short gravity waves by surface\ currents in rather rigorous mathematical detail. FIELD MEASUREMENTS On July 14, 1982, 1400 U.T. the USNS HAYES was conducting measurements of wave spectra in the vicinity of Phelps Bank, Nantucket Shoals (40'50'N-60°20'W). Two wave-recording buoys were deployed, one free and one tethered to the ship by about 100 m of cable. The ship was adrift with the exception of occasional slow maneuvers to maintain slack cable to the tethered buoy. The ship was carried in a south-westerly direction by the tidal current at a relatively rapid speed (0.8-1.5 msec-l) into an area labeled on the charts as Asia Rip (NOAA, 1979). It should be noted that the designation "rip" derives from ripple and implies an area of water made rough by opposing tides or currents. Its presence on a navigational chart also indicates that it is a visible and relatively permanent surface feature. The East to West course of the USNS HAYES over Phelps Bank during the drift into Asia Rip is shown as Track A in Figure 3. As anticipated from the navigational charts, the rip was indeed clearly visible with a rather sharp line of demarcation separating an area of considerable wave activity from
165
the relatively calm ambient sea. The photograph in Figure 4 provides a qualitative indication of the change in sea state approaching the rip. The change was roughly equivalent to moving from a sea at about Beaufort scale 2 with small wavelets and a glassy appearance to one at Beaufort scale 3 with large wavelets and some breaking crests. The wind at a height of 10 meters was approximately 5 msec-' at the time. Since neither the meteorological or hydrographic conditions changed significantly as the ship passed into Asia Rip, it was tentatively assumed that the sharp contrast in surface waves was a dynamic effect, probably related to wave interaction with current flow. Because the location of the rip was near the southern end of a prominant, 9 km-long bedform (Phelps Bank, 40°50'N-69020'W) and inasmuch as the ship course (Track A) showed no sharp directional changes indicating horizontal current shearing it was decided to pursue the hypothesis that the variations in the near-surface current responsible for the change in wave pattern are related to the bottom topography. During the at-sea exercise aboard the USNS HAYES, a chart recorder maintained a co?tinuous graphical plot of the depth as measured by a sonic fathometer. Time marks were placed on the bathymetry record by watch standers at intervals varying from a few minutes to a few hours, which makes it possible to calculate the depth at a given time based on the chart recorder speed. Furthemore, navigational information from the LORAN-C system was averaged over each minute and logged by computer, providing a time series record of both ship speed and position. By correlating the records for the appropriate times during the cruise it was possible to derive current and bathymetry information at Phelps Bank in general and to relate it to the crossing of Asia Rip in particular. The navigational and bathymetric data pertaining to the "drift" into Asia Rip are given in Table 1 and shown graphically in Figure 5. The bank profile in the figure represents the recorded depths along Track A plotted in terms of west longitude only. The approximate position of the USNS HAYES at the time of the photograph in Figure 4 is illustrated with some artistic license. The location of the change in surface wave pattern relative to the subsurface topography is presented schematically. The least interpretable information in the figure is the current or ship drift. A s mentioned earlier, the ship at the time, was maintaining position relative to a tethered buoy. It was primarily in a drifting
regime, but there is a possibility that some occasional maneuvering took place to accommodate the deployed buoy. Since this particular analysis was not anticipated at the time, no record was kept of ship propeller rotations so there is some ambiguity in the interpretation of the ship speed data. The LORAN-C positions in the area are quite accurate (less than 5 100 m) but the question of the contribution of short-term ship maneuvers to the speed curve remains open. In a qualitative sense, the significant correlation to be gained from Figure 5 is that the change in surface wave pattern coincides spatially with the lee edge of the bank and what appears to be a drop of 30-40 cmsec-I in current speed (assuming that the ship was moving as a Lagrangian drifter at the time). Later in the cruise (July 2 1 , 1 9 8 2 ) there was an opportunity to repeat an East-to-West ship drift across the bank (Figure 3 , Track B ) and eliminate the ambiguity associated with possible ship maneuvers. This was done by stopping all engines. Table 2 lists navigational and bathymetric data at two minute intervals for this pass across Phelps Bank. The basic premise in this measurement is that the USNS HAYES (keel depth 5.8 m) acts as a Lagrangian drifter when not under power, that is, the progressive series of LORAN-C fixes recorded aboard ship represent motion of the ocean current only. For this premise to be valid, it must be assumed that no other forces are acting on the ship. The only other force of any consequence would be the wind acting on the super-structure of the ship as a sail. Table 3 provides information on the relationship of the ship course, ship heading and the wind while traversing Track B. It is seen from the table that both the ship heading and the wind velocity remained relatively constant during the drift across the bank, that is, the wind remained on the starboard quarter. This is intgrpreted to mean that the wind effect is constant and if it has any influence, it will be a steady southerly displacement of the ship track. Such an effect could be subtracted out if the ship movement were known as a function of wind speed alone. Therefore, based on this rather qualitative argument, it is considered that the ship drift is probably linearly related to the current. The information in Tables 2 and 3 is summarized and correlated graphically in Figures 6 and I. The bathymetric profile of the bank along the drift track is shown in both figures for common reference. Figure 6 shows a segment of Track B, plotting LORAN-C
167
40'52
69' IS'
69'22'
69.21 69'20' I I PHELPS BANK BAT HY METRY -CONTOURS IN METERS I
40051'
40'52'
40051'
40050'
40'49'
40'48'
40048'
+----J2 ASIA R
40'47'
I
69'22'
1
65 !I' 69' 20' WEST LONGITUDE
I
I
69' 19'
40'47'
Fig. 3 . The general topography of Phelps Bank based on bathymetric measurements obtained during six ship crossings (Track A-F). Asia Rip is located near the southern end of the bank. posit-ions at 4 or 8-minute intervals. The "wind rose" for the duration of the drift is also shown in the figure. It should be noted that the wind remains constant within about 2 25' in direction 2 1.5 msec-' in speed. Figure 7 is equivalent to Figure 5 however, there were no accompanying surface wave observations
168 Depths, p o s i t i o n s and s h i p s p e e d s along Track A, J u l y 14, 1982 Speed (MSEC-1) Depth (M) N. L a t i t u d e W. Longitude 36.0 40" 48.45' 69" 19.49' 1.62 35.5 40" 48.34' 69" 19.57' 1.73 34.0 40" 48.25' 69" 19.65' 1.74 1.65 33.0 40" 48.17' 69" 19.73' 1.50 31 .O 40" 48.11' 69" 19.82' 30.5 40" 48.04' 69" 19.90' 1.42 1.36 29.5 40" 47.97' 69" 19.97' 28.5 40" 47.91 ' 69" 20.04' 1.25 1.22 27.5 40" 47.85' 69" 20.12' 26.5 40" 47.80' 69" 20.21' 1.29 25.5 40" 47.75' 69" 20.31' 1.34 1.34 24.5 40" 47.70' 69" 20.40' 1.28 24.0 40" 47.65' 69" 20.49' 69" 20.58' 1.23 23.5 40" 47.61 ' 1.19 23.5 40" 47.57' 69" 20.67' 22.5 40" 47.52' 69" 20.76' 1.20 ---21 .o Shall owest Point 24.0 40" 47.50' 69" 20.82' 1.09 0.84 38.0 40" 47.48' 69" 20.89' 0.89 37.5 40" 47.49' 69" 20.99' 1.13 37.5 40" 47.49' 69" 21.09' 37.0 40" 47.48' 69" 21.18' 1.12 37.0 40" 47.47' 69" 21.26' 1 .oo 0.89 37.0 40' 47.46' 69" 21.33' ---40" 47.45' 69" 21.39' 0.81 ---40" 47.44' 69" 21.46' 0.28 40" 47.44' 69" 21.53' 0.25 0.74 40" 47.44' 69" 21.59' ---40" 47.43' 69" 21.66' 0.76
Table 1. T4me (U.T.) 1420 1422 1424 1426 1428 1430 1432 1434 1436 1438 1440 1442 1444 1446 1448 1450 1451.5 1452 1454 1456 1458 1500 1502 1504 1506 1508 1510 1512 1514
-------
because i t was dark and foggy d u r i n a t h e p a s s (Local EDT i s 4 h r s e a r l i e r t h a n U . T . ) . The o b s e r v a t i o n of primary i n t e r e s t is t h e c u r r e n t v a r i a t i o n r e l a t i v e t o t h e topography of P h e l p s Bank. I n common w i t h F i g u r e 5 , t h e r e i s a marked change i n c u r r e n t ( s h i p d r i f t ) speed c o r r e l a t e d w i t h t h e leeward edge of t h e bank. However, t h e r e a r e fewer l a r g e f l u c t u a t i o n s i n c u r r e n t speed t h a n was t h e c a s e d u r i n g t h e " d r i f t " i n t o Asia Rip ( F i g u r e 5 ) . T h i s may be a t t r i b u t a b l e t o o p e r a t i n g w i t h e n g i n e s s t o p p e d o r may i n d i c a t e t h a t t h e r e a r e fewer t u r b u l e n t e d d i e s w i t h dimensions e q u i v a l e n t t o several shiplengths i n t h i s area. A particularly noticeable dynamic f e a t u r e i s t h e r a t h e r a b r u p t change i n t h e East-West c u r r e n t speed (U) g r a d i e n t around Longitude 6 9 ' 2 1 . 1 5 ' .
dU/dX
undergoes a change of a p p r o x i m a t e l y a f a c t o r o f f i v e a t t h i s p o i n t . sec -1 w h i l e t h e Upstream of t h e b r e a k p o i n t dU/dX i s 0 . 1 4 X downstream v a l u e i s 0 . 6 6 X sec- 1 There i s some i n d i c a t i o n t h a t t h e c u r r e n t changes a r e a t t r i b u t a b l e t o t h e bank a s s u c h . For example, F i g u r e 7 a l s o i n c l u d e s a c u r r e n t t h a t would be p r e d i c t e d
.
i n t h e absence of any t o p o g r a p h i c f e a t u r e . T h i s c u r r e n t p r e d i c t i o n
169
Fig. 4 . A photograph of the sea surface in the vicinity of Asia Rip showing the sharp contrast in surface wave activity. The field of view is directed from Phelps Bank towards the lee side of the feature (west). is based on Lagrangian measurements that were made west of Phelps Bank during an independent experiment using drogues to follow the trajectory of tidal flow in the vicinity. Even though the predicted current may be less than precise, the implication is that current across the shallow part of the bank is faster than would be expected in the absence of the topography while the current in the lee of the sand ridge is slower than anticipated. In the course of the remote sensing field exercise there were no other occasions when Phelps Bank was crossed in a drifting mode. During the other East-West passes over the bank shown in Figure 3 (Track C,D,E), the ship was under power. If it is assumed that the USNS HAYES was moving through the sea at constant speed (RPM), two of these passes (C and D) do provide some supplementary information on current variation over the bank. During the course of Track C
SHIP LOCATION
I
I
f i
I
3.2
$ z Y
WEST LONGITUDE
Fig. 5. A schematic diagram showing the spatial relationship of Phelps Bank, the surface-wave changes and the USNS HAYES at the time the photograph in Fig. 4 was taken. The ship drift speed across the bank is also illustrated. It should be kept in mind that some of the variation in ship speed may be a result of deliberate maneuvers. (ship traveling West to East), the predicted current was flowing East to West ( 2 8 0 ' ) at about 0.5 msec-l, that is, the current opposed the ship motion. Data on the ship speed and bathymetry for this track are given in Table 4. The observation of interest is that there is an approximately 0.23 msec-' increase in ship speed as it passes over the lee edge of the bank, an indication of a slower surface current at that location. This is in qualitative agreement with the results of the measurements when the USNS HAYES was in a drifting mode. Analogous information for Track D (Fig. 3) is given in Table 5. Here the ship is crossing the bank under power from East to West. The predicted tidal current at the time flows from West to East ( 1 2 5 ' ) and as in the case of Track C, opposes the ship motion. As seen from Table 5, the ship appears to slow down by -1 approximately 0.34msec asit passes over the "shallows" of the bank. Since the current opposes the ship's progress, this indicates that the faster current is over the bank with slower currents at both the upstream and downstream edges of the bank. Although the interpretation of ship speeds under power as current indicators are less reliable than observations in the drifting mode, it should be noted that they are consistent with the drift data. The influence of the sand ridge on the local fJow pattern is also confirmed by a comparison of simultaneous Eulerian and Lagrangian current measurements made near the site. Figure 8 is a
171
Table 2. Time (U.T.) 0943 0945 0947 0949 0951 0953 0955 0957 0959 1001 1003 1005 1007 1009 1011 1013 1015 1017 1019 1021 1023 1025 1027 1029 1031 1033 1035 1037 1039 1041 1043 1045 1047 1049 1051 1053 1055 1057 1059 1101 1103 1105 1107 1109 1111 1113 1115 1117 1119 1121 1123 1125 1127 1129 1129
Depths, p o s i t i o n s and s h i p speeds along Track B, J u l y 21, 1982 N. L a t i t u d e W. Longitude SDeed (MSEC-1) Depth (M) ' 1.09 40" 49.96' 29.0 69" 19.74' 28.0 1.03 40" 49.93' 69" 19.82' 25.5 1.01 40" 49.91 ' 69" 19.90' 26.0 40" 49.89' .93 69" 19.97' 25.0 40" 49.88' 69" 20.04' .85 24.0 40" 49.87' .82 69" 20.13' 24.0 40" 49.84' * 91 69" 20.21' 23.5 40" 49.81' .92 69" 20.26' 23.5 40" 49.77' 69" 20.32' .88 23.0 40" 49.74' 60" 20.39' .87 23.0 40" 49.71 ' 60" 20.45' .82 23.0 60" 20.52' .86 40" 49.69' 23.5 60" 20.60' .92 40" 49.66' 23.5 60" 20.65' .83 40" 49.63' 23.5 40" 49.61' 69" 20.71' .77 24.0 40" 49.59' 69" 20.77' .65 23.5 40" 49.58' 69" 20.82' .67 24.5 40" 49.55' 69" 20.87' .76 23.5 69" 20.94' .71 40" 49.52' 22.5 40" 49.50' 69" 20.98' .65 22.0 69" 21.03' .65 40" 49.49' 23.5 40" 49.48' 69" 21 . l o ' .67 24.5 .70 40" 49.45' 69" 21.16' 23.0 .69 40" 49.42' 69" 21.20' 23.5 .62 40" 49.41 ' 69" 21.24' 22.5 40" 49.41 ' 69" 21.28' .50 18.4 -50 69" 21.33' 40" 49.40' 31.5 40" 49.37' .55 69" 21.37' 37.0 40" 49.36' .50 69" 21.41' 38.5 -36 69" 21.42' 40" 49.35' 39.5 -32 40" 49.34' 69" 21.46' 40.5 69" 21.49' 40" 49.33' .35 41.5 40" 49.32' .35 69" 21.52' 41 .O 40" 49.32' .22 69" 21.54' 40" 49.32' .21 41.5 69" 21.55' 40" 49.31 ' .26 41 .5 69" 21.58' 42.0 .26 40" 49.29' 69" 21.59' .21 41.5 40" 49.30' 69" 21.61' .10 42.0 69" 21.63' 40" 49.30' .26 42.0 40" 49.29' 69" 21.66' 42.0 .26 40" 49.29' 69" 21.69' .26 42.0 40" 49.27' 69" 21.70' .15 42.0 40" 49.28' 69" 21.71' .10 42.5 40" 49.28 69" 21.73' .15 42.0 40" 49.28' 69" 21.75' .21 42.0 40" 49.27' 69" 21.77' 42.0 .21 40" 49.27' 69" 21.78' .05 41 .O 40" 49.28' 69" 21.78' .05 40" 49.28' 40.5 69" 21.79' 00 40.5 69" 21.81' 40" 49.28' 40" 49.29' 40.0 .10 69" 21.84' 40.0 .10 40" 49.29' 69" 21.85' 40.0 40" 49.30' .05 69" 21.84' 40" 49.31 ' 40.0 .15 69" 21.84' 40.0 .15 40" $9.31' 69" 21.84'
.
172 T a b l e 3. Time (U.T.)
7.3
0945 0947 0949 0951 0953 0955 0957 0959 1001 1003 1005 1007 1009 1011 1013 1015 1017 1019 1021 1023 1025 1027 1029 1031 1033 1035 1037 1039 1041 1043 1045 1047 1049 1051 1053 1055 1057 1059 1101 1103 1105 1107 1109 1111 1113 1115 1117 1119 1121 1123 1125 1127 1129 Note:
Wind c o n d i t i o n s and s h i p h e a d i n g d u r i n g a pass o v e r P h e l p s Bank ( J u l y 21, 1982) Course (Deg) Ship Hdng Wind Speed Wind D i r
246 250 256 257 254 250 237 233 236 245 244 241 240 236 244 245 237 238 248 257 242 236 243 249 25j 248 237 248 258 236 243 251 267 256 242 231 251 272 251 234 246 286 264 249 257 281 335 265 266 267 31 2 248
1) 2) 3) 4)
Ship Wind Wind Wind
1 90" 220" 235" 230" 21 0" 200" 220" 240" 240" 220" 220" 235" 235" 220" 240" 250" 230" 220" 240" 260" 250" 240"
2.8
7.8
326
9.5
347.4
9.9
354.5
9.8
3.9
10.4
7.9
270" 270" 250" 240" 2500 270" 250" 240" 250" 260" 270" 260" 240"
10.3
13.5
engines stopped a t 0938 speed i s i n m e t e r s p e r second d i r e c t i o n and s h i p s course and h e a d i n g a r e i n degrees speeds and d i r e c t i o n s a r e 15 m i n u t e averages
113
DEPTH PROFILE
-
0
? 0
,
d
I
0
0
W
n 3
k Ia
J
-0, d
b d
I ILL 0
z
a
2C Q
1
6' 22'
F i g . 6.
69"21' WEST LONGITUDE
69' 20'
Ship d r i f t a c r o s s P h e l p s Bank (Track 6 , F i g . 3 ) . Winds d u r i n g t h e d r i f t and t h e b a t h y m e t r i c p r o f i l e a l o n g t h e d r i f t p a t h a r e a l s o shown.
174
I
1
SHIP DRIFT ACROSS PHELPS BANK
I
,." .
- JULY
21.1982
I I CURRENT PREDICTION BASED ON DROGUE E JULY 10-11 TRAJECTORY
-
2.2
I
\
.-.-I
40
0-T
O .
000
O
A
69'22 0'
69'21 5 '
69'21 0' 69'20 5' WEST LONGITUDE
69'20 0'
0 69O19 5'
Fig. 7 . The drift speed of the U S N S HAYES along the path shown in Fig. 6 (Track B. Fig. 3 ) . computer plot showing the trajectory of a current-following drogue about 8 km west of Phelps Bank compared to the progressive vector diagram derived from a recording current meter moored at the sand The drogue and the current meter ridge ( 4 0 ' 5 0 . 0 7 ' N - 69'19.81'W). were at approximately the same depth (5-6 m). As seen from the figure, there is an almost two-fold increase in the East-West component of the tidal current as it passes over the sand ridge perpendicular to its major axis. We interpret this enhancement of current flow over Phelps Bank as a topographic effect, that is, an acceleration due to the vertical reduction in "channel depth". Based on these field measurements which are admittedly somewhat limited in both quantity and degree of reliability, the following conclusions can be drawn. Relatively large changes in surface current speed ( 2 5 to 5 0 cm/sec) and current gradient ( 0 . 2 0 - 1.5 x l o m 3 sec-l) occur in the vicinity of this topographic feature, with faster currents over the shallow portion of the bedform. The most significant variations in current are spatially correlated with the limits of the bank. The change at the Western edge of Phelps Bank was particularly noticeable and on at least one occasion was clealy associated with a sharp contrast in the appearance of the surface wave pattern. It seems reasonable to attempt to relate the surface wave variations to the bathymetry through the mechanism of wave-current interaction. This will be explored in the discussion to follow.
175 Table 4.
T'ime (U.T.) 1715 1716 1717 1718 1719 1720 1720.5 1721 1722 1723 1724 1725 1726 1727 1728 1729 1730 1731 1732 1733 1734 1735
Depths, p o s i t i o n s and s h i p speeds a l o n g Track C, J u l y 14, 1982 Speed (MSEC-l) Depth (M) N. L a t i t u d e i4. Longitude 4.97 44.5 40" 49.89' 69" 22.48' 69" 22.31' 4.72 44.0 40" 49.91 ' 4.54 43.0 40" 49.93' 69" 22.15' 4.53 43.0 40" 49.94' 69" 21.94' 4.65 43.0 40" 49.96' 69" 21.73' 4.72 39.0 40" 49.97' 69" 21.53' ---19.5 4.76 23.5 40" 49.97' 69" 21.33' 4.65 24.0 40" 49.95' 69" 21.15' 4.58 23.0 40" 49.95' 60" 20.95' 4.51 24.0 40" 49.94' 60" 20.76' 4.48 22.0 40" 49.93' 60" 20.57' 4.52 26.5 40" 49.91' 60" 20.37' 60" 20.18' 4.54 24.0 40" 49.91 ' 4.54 26.5 40" 49.90' 69" 19.98' 4.54 28.0 40" 49.91 ' 69" 19.78' 4.52 30.5 40" 49.91' 60" 19.57' 4.52 31 .O 40" 49.92' 60" 19.39' 4.55 35.5 40" 49.93' 69" 19.19' 4.54 35.0 40" 49.94' 69" 19.00' 40" 49.94' 69" 18.81' 4.57 41 .O 4.59 42.5 40" 49.95' 69" 18.60'
Table 5. Time (U.T.) 1530 1531 1523 1533 1534 1535 1536 1537 1538 1539 1540 1541 1542 1543 1544 1545 1545.3 1546 1547 1547.3 1548 1548.2 1549 1550 1552 1553 1554 1555 1556
Depths, p o s i t i o n s and s h i p speeds along Track J u l y 19, 1982 Speed (MSEC-1) Depth-(M) N. L a t i t u d e W. 40" 50.18' 4.22 48.0 40" 50.20' 39.0 4.29 4.33 40" 50.22' 39.0 40" 50.23' 38.0 4.36 40" 50.25' 37.0 4.32 40" 50.25' 34.0 4.18 33.0 4.10 40" 50.27' 40" 50.29' 4.10 37.0 34.0 40" 50.31' 4.11 4.08 32.0 40" 50.32' 29.0 40' 50.35' 4.08 27.0 40" 50.36' 4.07 25.0 40" 50.38' 4.03 25.0 40" 50.39' 3.98 26.5 40" 50.41 ' 3;98 3.93 40" 50.43' 22.0 19.5 40" 50.44' 25.0 3.98 4.06 40" 50.47' 21 .o ---_ 18.0 21 .o 40" 50.46' 4.12 18.0 37.5 4.15 40" 50.45' 4.20 40" 50.44' 43.0 40" 50.43' 45.0 4.32 4.39 40" 50.42' 45.0 40" 50.41 ' 43.0 4.38 4.38 40" 50.40' 44.0 40" 50.39' 42.0 4.36
---_
---_
D,
Longitude 69" 18.10' 69" 18.28' 69" 18.47' 60" 18.65' 60" 18.82' 60' 19.00' 60" 19.17' 60" 19.35' 69" 19.53' 69" 19.69' 60" 19.87' 69" 20.04' 69" 20.20' 60" 20.37' 69" 20.54' 69" 20.72' 69" 20.89' 69" 21.07' 69" 21.24' 69" 69" 60" 69" 69" 69" 60"
21.43' 21.62' 22.00' 22.19' 22.38' 22.56' 22.75'
176
DISCUSSION For purposes of discussion, we will consider the measurements described in terms of simplified fluid mechanics. Figure 9 is a two-dimensional picture of the problem. The figure does some injustice by deletion to the complicated oceanographic situation but includes its essential features. A strong current in relatively shallow water flows over a bathymetric feature and becomes less strong in the deeper water. The water surface in the lee of the feature is covered with waves, some breaking. The obvious waves are shorter than the water depth ( % 2 0 - 4 0 m), so we take the relevant water waves to be deep water waves. Let us first consider, in the simplest possible way, how the currents may be modifying a pre-existing wave field to the extent observed, namely an apparent blocking of the shorter waves. The approach is similar to that of Unna ( 1 9 4 2 ) and is purely kinematic. The velocity C1 of deep-water waves moving over still water is proportional to the square root of their wavelenght.
If the waves encounter an opposing current, u, the wave velocity relative to the moving current Cr will be
where h 2 is the wave length associated with velocity Cr. The new velocity over the bottom is C2 = Cr - u. From the velocity equations, it can be seen that the ratio
However, the period o f the waves relative to the bottom does not change as they propagate into the current, so
177
Two aspects are worth noting, that is
cr -
I2 =
u
-q--
(5)
or the new wavelength is shorter than that in deep water and,that the ratio
c
Fig. 8. Comparison of a Lagrangian drogue trajectory 8 km west of Phelps Bank with a progressive vector diagram from a moored current meter on the bank for the same period of time. from the Equation (3). This yields a quadratic equation in
c 2 (
949
953
957
961
965
969
GRID POINT
Figure 11. An expanded view of the 27th (top) and 28th profile from the bottom of Fig. 3 . The horizontal and vertical scales are the same, so that ratios of wave height to depth and slopes are viewed without distortion. The 28th profile has a wave height and slope exceeding that of a spilling breaking wave.
203 4.
DISCUSSION T h i s is e s s e n t i a l l y a p r o g r e s s r e p o r t o f t h e a p p l i c a t i o n of t h e
u n i f i e d waves model t o w a v e - c u r r e n t wave b l o c k i n g i n p a r t i c u l a r .
i n t e r a c t i o n s i n g e n e r a l , and t o
The s e t t i n g i s a s t e a d y n o n u n i f o r m
flow i n t o which a wave p a c k e t p r o p a g a t e s . and h i g h e r .
The waves become s h o r t e r
Some of t h e i n c r e a s e of a m p l i t u d e is d u e t o t h e c h a n g e
of c h a n n e l b r e a d t h ( l e s s t h a n h a l f ,
i f G r e e n ' s law,
1838, a p p l i e s ) .
The r e s t of t h e c h a n g e o f a m p l i t u d e r e s u l t s f r o m w a v e - c u r r e n t
i n t e r a c t ion. The model g i v e s c h a n g e s of w a v e l e n g t h and p o s i t i o n of b l o c k i n g t h a t a g r e e w e l l w i t h what l i n e a r w a t e r wave t h e o r y p r e d i c t s .
There
is no e v i d e n c e o f s i g n i f i c a n t r e f l e c t e d waves, n o r o f s i g n i f i c a n t e v a n e s c e n t waves a h e a d of t h e b l o c k i n g p o s i t i o n .
The b a s i c p i c t u r e
t h a t t h e u n i f i e d waves model d r a w s is t h a t of b r e a k i n g waves a t t h e place t h a t t h e g r o u p s p e e d of t h e waves is matched by t h e c u r r e n t , and l i t t l e e l s e . REFERENCES DeLoor, G.P. and B r u n s v e l d van H u l t e n , H.W., 1978. Microwave measurements o v e r t h e North Sea. Boundary-Layer M e t e o r o l o g y , 13: 119-131. DeLoor, G.P., 1981. The o b s e r v a t i o n of t i d a l p a t t e r n s , c u r r e n t s and b a t h y m e t r y w i t h SLAR i m a g e r y of t h e s e a . I E E E J o u r . of 124-129. O c e a n i c Eng., OE-6, N o . 4: Gordon, C., G r e e n e w a l t , D., and W i t t i n g , J . , 1983. A s i a r i p : NRL Memorandum R e p o r t S u r f a c e wave e x p r e s s i o n of b a t h y m e t r y . 5027, 40pp. Surface-wave Gordon, C., G r e e n e w a l t , D. , and W i t t i n g , J. e x p r e s s i o n of b a t h y m e t r y o v e r a s a n d r i d g e . T h i s volume. Green, G., 1838. On t h e m o t i o n o f waves i n a v a r i a b l e c a n a l o f small d e p t h and w i d t h . Camb. P h i l . T r a n s . , 6: 457-462. Longuet-Higgins, M.S. and S t e w a r t , R.W., 1960. Changes i n t h e form of s h o r t g r a v i t y waves on l o n g waves and t i d a l c u r r e n t s . J. F l u i d Mech., 8: 565-583. Longuet-Higgins, M.S. and S t e w a r t , R.W., 1961. The c h a n g e s i n i n a m p l i t u d e of s h o r t g r a v i t y waves on s t e a d y n o n - u n i f o r m currents. J. F l u i d Mech., 10: 529-549. V l i e g e n t h a r t , A.C. , 1971. On f i n i t e - d i f f e r e n c e m e t h o d s f o r t h e J. Eng. Math. , 5: 137-155. Korteweg-deVries e q u a t i o n . Witting, J . M . , 1982. A u n i f i e d model f o r t h e e v o l u t i o n of n o n l i n e a r w a t e r waves. NRL Memorandum R e p o r t 5001, 64 pp. S u b m i t t e d f o r p u b l i c a t i o n i n J. Comp. Phys. Witting, J . M . , 1983. A d d i t i o n a l c a p a b i l i t i e s of t h e u n i f i e d waves NRL Memorandum R e p o r t 5194, 52 pp. model. Witting, J.M. and McDonald, B . E . , 1982. A c o n s e r v a t i o n - o f - v e l o c i t y law f o r i n v i s c i d f l u i d s . NRL Memorandum R e p o r t 4977, 1 2 pp. S u b m i t t e d f o r p u b l i c a t i o n i n J. Comp. Phys. ACKNOWLEDGEMENT T h i s work is s u p p o r t e d by t h e C o a s t a l S c i e n c e s Group, Ocean S c i e n c e s D i v i s i o n , O f f i c e of Naval R e s e a r c h . b
This Page Intentionally Left Blank
205
REMOTE SENSING OF O I L SLICK BEHAVIOUR P. P. G . DYKE Mathematics and Computer S t u d i e s D e p t . , Sunderland. SR1 3SD. U.K.
Sunderland Polytechnic,
ABSTRACT
This s h o r t c o n t r i b u t i o n h i g h l i g h t s t h e r o l e remote s e n s i n g h a s played i n o u r u n d e r s t a n d i n g of t h e b e h a v i o u r of sea s u r f a c e o i l . I n p a r t i c u l a r , it p o i n t s o u t t h e i m p o r t a n c e o f Langmuir C i r c u l a t i o n s an e f f e c t t h a t r e c e i v e d d e r i s o r y a t t e n t i o n b e f o r e t h e advent o f o p t i c a l and i n f r a - r e d a e r i a l - p h o t o g r a p h y . F i n a l l y , some a t t e m p t i s made t o f o r e c a s t t h e f u t u r e r o l e of r e m o t e s e n s i n g i n o i l s l i c k monitoring. INTRODUCTION
The p r i n c i p a l a i m of t h i s p a p e r i s t o assess t h e i m p a c t t h a t remote s e n s i n g h a s had upon o u r u n d e r s t a n d i n g o f t h e b e h a v i o u r of
oil s l i c k s i n open sea. Remote s e n s i n g h a s b e e n a r o u n d s i n c e t h e Second World War, and as a " r e v o l u t i o n " seems t o have been l a r g e l y o v e r l o o k e d by t h e general p u b l i c .
E s s e n t i a l l y , remote s e n s i n g i s as o l d a s t h e
camera, it i s t h e t e r m " r e m o t e s e n s i n g " t h a t i s new, and t h i s phrase h a s been c o i n e d i n r e s p o n s e t o t h e b e w i l d e r i n g a r r a y of new instruments t h a t have appeared i n t h e l a s t decade o r so.
Every
remote s e n s i n g d e v i c e i s a t r a n s m i t t e r and r e c e i v e r o f s i g n a l s , the most u s e f u l d e v i c e s f o r o i l s l i c k e x a m i n a t i o n u s e t h e o p t i c a l
to radio-wave
s e c t i o n of t h e e l e c t r o m a g n e t i c spectrum.
The atmos-
p h e r e i s t o o opaque t o s h o r t e r w a v e l e n g t h s , a l t h o u g h t h e u s e o f ultra-violet
s e n s o r s h a s been r e p o r t e d , t o u t i l i s e o i l s l i c k
'
luminescence. The main t e c h n i q u e s u s e d t o s t u d y sea g o i n g p a t c h e s o f s u r f a c e o r n e a r s u k f a c e o i l are ( i ) o p t i c a l p h o t o g r a p h y and ( i i ) i n f r a red l i n e s c a n t o g e t h e r w i t h employment o f s a t e l l i t e s .
Acoustic
techniques have been u s e d , b u t a r e mainly r e s e r v e d f o r beneath-the-
sea n a v i g a t i o n .
The u s e o f i n f r a - r e d d e v i c e s i s p a r t i c u l a r l y
u s e f u l i n t h e d e t e c t i o n of i l l e g a l dumping of w a s t e o i l which u s u a l l y t a k e s p l a c e a t n i g h t o r i n low v i s i b i l i t y c o n d i t i o n s .
206 F i r s t o f a l l , l e t u s examine t h e r o l e o f a e r i a l p h o t o g r a p h y . The a d v a n t a g e s o f t h i s t e c h n i q u e a r e i n i t s s i m p l i c i t y and i n t h e e a s e of i n t e r p r e t a t i o n of t h e r e s u l t s .
The r e s u l t s o f c o m p e t e n t
a e r i a l p h o t o g r a p h y c a n be e a s i l y u n d e r s t o o d by t h e layman, a n i m p o r t a n t f a c t o r i f p r e s s u r e f o r new l e g i s l a t i o n i s b e i n g a p p l i e d . A e r i a l p h o t o g r a p h s g i v e a good o v e r a l l i m p r e s s i o n of what i s r i n g , b u t o f t e n do n o t show d e t a i l .
OCCUP
Only i n t h e most f a v o u r a b l e
c i r c u m s t a n c e s d o e s it r e v e a l a n y t h i n g a b o u t what i s h a p p e n i n g u n d e r t h e surface of t h e sea. E a r l y i n v e s t i g a t o r s found t h a t s h i p b o r n e s e n s o r s w e r e i n a d e q u a t e f o r o i l s l i c k monitoring.
The o b l i q u e a n g l e o f o b s e r v a t i o n and t h e
c l o s e n e s s o f t h e s h i p ' s h o r i z o n made a l a r g e s u r f a c e o i l s l i c k d i f f i c u l t even t o d e t e c t , l e t a l o n e monitor.
I t w a s t h e Torrey
Canyon d i s a s t e r o f 1 9 6 7 t h a t f i r s t showed how u s e f u l IRLS c o u l d b e . I n 1 9 7 7 , t h e E k o f i s k blowout w a s m o n i t o r e d u s i n g 8-140m I R L S and a 50kW(H) p o l a r i s e d Q-band SLAR ( S i d e Looking A i r b o r n e R a d a r ) o v e r f l y i n g a s p i l l o f 5 t o n n e s of E k o f i s k c r u d e ( P a r k e r (1979)).
Parker
&
&
Cormack
Cormack ( 1 9 7 9 ) g i v e a n a c c o u n t o f c o n t r o l l e d
s p i l l s and d e t a i l how t h i c k n e s s e s o f s l i c k s c a n be measured u s i n g t h e IRLS t e c h n i q u e .
Their paper i s mainly concerned with t h e
c o m p a r i s o n between IRLS and SLAR c o n c l u d e t h a t t h e f o r m e r g i v e s b e t t e r r e s o l u t i o n and more i n f o r m a t i o n on s l i c k t h i c k n e s s t h a n t h e latter. The two f a c t o r s t h a t g o v e r n t h e u s e f u l n e s s o f a remote s e n s i n g device f o r o i l s l i c k examinations are ( i ) t h e p e n e t r a t i o n capabili t y o f t h e c h o s e n r a d i a t i o n , and ( i i ) t h e v a r i a t i o n of t h e incoming s i g n a l , b e i t r e f l e c t e d o r b a c k - s c a t t e r e d ,
w i t h parameters
of i n t e r e s t s u c h a s t e m p e r a t u r e , d i s s o l v e d s u b s t a n c e s , suspended
matter e t c . , i n t h e r e g i o n o f s e a b e i n g s t u d i e d . I n v i e w of t h e s e f a c t o r s , t h e infra-red l i n e scan technique i s used. IRLS s u c c e s s f u l l y d e t e c t s t h e d i f f e r e n c e s i n t h i c k n e s s of s u r f a c e o i l s l i c k s b e c a u s e t h e t h i c k e r p a r t of t h e s l i c k i s w a r m e r t h a n i t s s u r r o u n d i n g s due t o t h e a d s o r p t i o n of h e a t r a d i a t i o n from t h e s u n .
This
d i f f e r e n c e i n temperature a l t e r s t h e c h a r a c t e r of t h e backscattered r a d i a t i o n ( t h a t is t h e o v e r a l l s i g n a l t o noice ratio of t h e detecto r ) and shows up as a b r i g h t area on t h e IRLS p h o t o g r a p h (see Figure 1 ) .
207
.T
scale 1 :93,333 I RL
t
N
Fig. 1 . An a e r i a l p h o t o g r a p h u s i n g an i n f r a - r e d t e c h n i q u e of o i l on t h e s e a s u r f a c e a t 2 8 t h A p r i l , 1 9 7 7 . T h i s h a s been t r a c e d from two p l a t e s i n Audunson ( 1 9 7 8 ) s o t h a t t h e b l a c k s t r e a k s a r e o i l ( t h e y w e r e w h i t e on b l a c k i n t h e o r i g i n a l ) . The d o t s 'T' a r e v e s s e l s m o n i t o r i n g t h e s l i c k from t h e s u r f a c e and 'P' i s t h e Bravo platform i t s e l f . Note t h e b r e a k u p i n t o s t r e a k s .
208
A s mentioned a t t h e o u t s e t , t h e purpose of t h i s p a p e r i s t o
h e l p t o u n d e r s t a n d t h e b e h a v i o u r of t h e o i l s l i c k s , n o t i n a d e t a i l e d a p p r a i s a l o f p a r t i c u l a r remote s e n s i n g d e v i c e s .
So,
a l t h o u g h it i s r e c o g n i s e d t h a t i n t e r p r e t a t i o n and d a t a a n a l y s i s p r o c e d u r e s a r e e x t r e m e l y i m p o r t a n t , t h e s e w i l l r e c e i v e no a t t e n t i o n i n what f o l l o w s .
I t i s f e l t s a f e t o do t h i s a s t h e m a j o r i t y of t h e
f a c t s p r e s e n t e d h a v e c o m e t h o u g h o p t i c a l and i n f r a - r e d p h o t o g r a p h y , a b o u t which t h e r e a r e f e w a r g u m e n t s nowadays.
O i l Slick Constituents An i m p o r t a n t q u e s t i o n t o a d d r e s s i s what c o n s t i t u e n t s o f o i l s l i c k s a r e h a r m f u l , and t o what d e g r e e d o e s t h e s e a ' s dynamics
s e p a r a t e o r comingle t h e s e c o n s t i t u e n t s ?
O f course, detailed
a p p r a i s a l o f t h e v a r i o u s c r u d e o i l s i s w i t h i n t h e r e a l m of t h e m a r i n g c h e m i s t , however, p h y s i c a l o c e a n o g r a p h e r s must be aware of some of t h e v a s t d i f f e r e n c e s i n d e n s i t y , v i s c o s i t y e t c . , o f s a y , bitumen and l i g h t c r u d e . I t i s p e r h a p s u s e f u l h e r e t o d i s t i n g u i s h between t a n k e r a c c i d -
e n t s and o i l p r o d u c t i o n p l a t f o r m b l o w o u t s .
I n general, it i s
tanker accidents t h a t provide t h e g r e a t e s t r i s k t o t h e marine. ecosystem. with.
Oil l i k e Kuwait c r u d e i s heavy and d i f f i c u l t t o d e a l
Blowouts o n o f f s h o r e p l a t f o r m s u s u a l l y g i v e l e s s o i l , and
t h e o i l i t s e l f i s much l i g h t e r w i t h a t e n d e n c y t o d i s p e r s e and e v a p o r a t e ( e . g . The Bravo (1978)).
( E k o f i s k ) blowout of 1 9 7 7 , Audunson
I t i s much more i m p o r t a n t , t h e r e f o r e ,
t o u n d e r s t a n d how
t h e h e a v i e r o i l s o f t h e Middle E a s t behave i n t h e s e a . When o i l i s d i s c h a r g e d i n t o t h e s e a s e v e r a l t h i n g s happen simultaneously.
I t s e p a r a t e s by d e n s i t y , l i g h t e r c r u d e f l o a t i n g
and h e a v i e r s i n k i n g , sometimes r e s i d i n g a t an i n t e r m e d i a t e d e n s i t y i n t e r f a c e s u c h a s t h e t h e r m o c l i n e ( t h e r e was e v i d e n c e of t h i s i n t h e p h o t o g r a p h i c d a t a o f t h e I X T O C I blowout i n t h e Gulf o f Mexico where o i l w a s b e i n g t r a n s p o r t e d t o w a r d s Texas a p p r o x i m a t e l y 5Om below t h e s e a s u r f a c e , p r o b a b l y a t a p y c n o c l i n e
-
Attwood ( 1 9 8 0 ) ) .
Some o f t h e h e a v i e s t o i l s go i n t o t a r b a l l s which s e p a r a t e o u t from a s l i c k .
T h e p r e c i s e m a n n e r i n which t h i s i s a c h i e v e d i s n o t
w e l l u n d e r s t o o d b u t i t i s promoted by t h e u s e o f d e t e r g e n t s . A f t e r a w h i l e , t h e o i l and water may form a " c h o c o l a t e mousse", an e m u l s i o n c o n t a i n i n g o v e r 5 0 % w a t e r and r e m a i n i n g a t t h e s e a s u r f a c e
a s a f i l m o v e r Imm t h i c k .
Smith ( 1 9 6 8 ) r e p o r t e d s u c h e m u l s i o n s
moving from t h e T o r r e y Canyon.
The o i l and w a t e r a l s o i n t e r a c t
209 c h e m i c a l l y i n a manner d e p e n d e n t on f a c t o r s s u c h a s t h e t e m p e r a t u r e and s a l i n i t y o f t h e s e a , and t h e c o m p o s i t i o n o f t h e c r u d e .
The
dynamic i n t e r a c t i o n of oil and w a t e r i s b e t t e r b u t c e r t a i n l y n o t c o m p l e t e l y , u n d e r s t o o d and i s t h e s u b j e c t of t h e n e x t s e c t i o n . O i l S l i c k Movement A s o f f s h o r e oil was d i s c o v e r e d ,
l a r g e r and l a r g e r o i l t a n k e r s
w e r e b e i n g b u i l t , and s t r i c t e r l e g i s l a t i o n was b e i n g d r a f t e d .
As
t h e r i s k of c a t a s t r o p h i c o i l s p i l l s i n c r e a s e d , so t h e i n s u r a n c e companies needed more c a l c u l a t i o n s and d a t a upon which t o b a s e their levies. Van K l e e f
This lead t o publications such a s Dietzel, Glass
(1976).
&
I n t h e body of t h i s r e p o r t had t o b e an answer
t o a v e r y d i f f i c u l t q u e s t i o n , namely:-
i f a g i v e n o i l w e l l had a
blowout o f f s h o r e a n d a g i v e n amount o f a s p e c i f i e d c r u d e o i l escaped
into
t h e sea a t a g i v e n l o c a t i o n t h e n how much of it would
r e a c h l a n d i n a g i v e n d i r e c t i o n a g i v e n d i s t a n c e away.
Of course,
a g r e a t d e a l of s t o c h a s t i c m o d e l l i n g had t o be d o n e , b u t t h e u n d e r l y i n g d y n a m i c a l a s s d m p t i o n w a s t h a t t h e sea s u r f a c e b o r n e c o n t a m i n a n t moved a t 3 . 4 % of t h e wind s p e e d i n a d i r e c t i o n 1 5 " t o the r i g h t of i t ( i n t h e northern hemisphere).
C l e a r l y , however,
t h e e m p h a s i s i n s u c h m o d e l s i s n o t on t h e dynamic a s p e c t s . The a g e n c i e s t h a t c a u s e w a t e r b o r n e o i l t o move i n c l u d e d i f f u s i v e s p r e a d i n g , wind and wave a c t i o n , s e a c u r r e n t s ( i n c l u d i n g t i d e s , c o n v e c t i o n c u r r e n t s and wind d r i v e n c u r r e n t s ) and Langmuir Circulations.
A g e n c i e s t h a t c a u s e t h e o i l t o b r e a k up i n c l u d e t h e
formation of emulsions, chemical r e a c t i o n s , v a r i a t i o n s i n o i l d e n s i t y and t h e r e f o r e ,
i n particle sinking rates, biological
a c t i o n , e v a p o r a t i o n and p r e c i p i t a t i o n , and a t m o s p h e r i c o x i d a t i o n . T h e r e i s no d o u b t t h a t r e m o t e s e n s i n g h a s g i v e n a c l e a r e r i n s i g h t i n t o some o f t h e s e p r o c e s s e s . I t i s n o t p o s s i b l e t o s e p a r a t e t h e s e a g e n c i e s from o n e a n o t h e r
s i n c e t h e y a l l c o n t r i b u t e t o oil s l i c k b r e a k u p .
I t s h o u l d be
p o i n t e d o u t t h a t even w i t h a l l t h e s e a g e n c i e s i n o p e r a t i o n , o i l s l i c k d i s p e r s i o n seldom o c c u r s n a t u r a l l y .
Most l a r g e o i l s l i c k s
n e e d m a n ' s h e l p i n c l e a n - u p p r o c e d u r e s , t h e most p e r f e r a b l e b e i n g c o l l e c t i o n by booms o r p o l y s t y r e n e , o r b u r n i n g . s i o n i s used a s a l a s t r e s o r t .
Chemical d i s p e r -
N o t e , however, t h a t t h e c h o c o l a t e
mousse e m u l s i o n m e n t i o n e d e a r l i e r w i l l n o t b u r n , t h e r e i s t o o much w a t e r i n i t .
210 The n e x t s e c t i o n i s d e v o t e d t o a mechanism m e n t i o n e d a b o v e t h a t h a s n o t p r e v i o u s l y r e c e i v e d t h e a t t e n t i o n i t d e s e r v e s as a mechani s m f o r moving s u r f a c e c o n t a m i n a n t s ,
namely Langmuir C i r c u l a t i o n s .
Langmuir C i r c u l a t i o n s Langmuir C i r c u l a t i o n s a r e h e l i c a l r o l l v o r t i c e s i n t h e s u r f a c e l a y e r s o f t h e o c e a n whose e x i s t e n c e h a s b e e n d r a m a t i c a l l y e s t a b l i s h e d i n r e c e n t y e a r s by t h e b e h a v i o u r of sea s u r f a c e o i l a n d i t s s u b s e q u e n t d e t e c t i o n by r e m o t e s e n s i n g .
The two p h o t o g r a p h s ,
Plate 1. An a e r i a l p h o t o g r a p h o f a s u r f a c e p a t c h o f o i l . Notethe b r e a k i n g up i n t o s t r e a k s , and t h e windrows ( a b o u t 1 0 0 m a p a r t ) . ( P h o t o g r a p h c o u r t e s y of Warren S p r i n g s L a b o r a t o r y a n d p u b l i s h e d with t h e i r permission).
211
Plate 2. An a e r i a l p h o t o g r a p h o f a s m a l l s l i c k b e i n g e r o d e d by waves. N o t e t h e s t r e a k y a p p e a r a n c e , p o s s i b l y c a u s e d by Langmuir Circulations. ( P h o t o g r a p h c o u r t e s y o f Warren S p r i n g s L a b o r a t o r y and p u b l i s h e d w i t h t h e i r p e r m i s s i o n ) .
P l a t e s 1 a n d 2 , show t h e o i l o n t h e s e a s u r f a c e t e n d i n g t o s p l i t i n t o s t r e a k s i n a manner d i c t a t e d by t h e h y d r o d y n a m i c s of Langmuir Circulation.
I t i s p e r h a p s w i s e t o c o n s i d e r t h i s phenomenon more
c a r e f u l l y and s t u d y t h e c o n s e q u e n c e s . Langmuir C i r c u l a t i o n s (named f r o m t h e s e m i n a l p a p e r Langmuir ( 1 9 3 8 ) ) have l o n g been r e c o g n i s e d a s r e s p o n s i b l e f o r t h e f a m i l i a r l i n e s o f foam a b o u t 5 m e t r e s a p a r t t h a t o c c u r a t r i g h t a n g l e s t o t h e wave c r e s t l i n e s i n i n l a n d w a t e r s
(e.9. l a k e s and r e s e r v o i r s ) .
Remote s e n s i n g h a s shown s i m i l a r "windrows" a t s e a .
This t i m e ,
212 however, t h e s p a c i n g i s o f t h e o r d e r 100m which makes d e t e c t i o n
a t t h e sea s u r f a c e i t s e l f
( f o r example from a b o a t ) d i f f i c u l t .
Mathematical t h e o r i e s o f Langmuir C i r c u l a t i o n s have a p p e a r e d i n t h e l a s t d e c a d e o r so, and from an e x a m i n a t i o n of them it seems t h a t t h e r e i s no obvious e x p l a n a t i o n f o r t h e s e c i r c u l a t i o n s . S e v e r a l d i f f e r e n t t h e o r i e s have been p r o p o s e d , and t h e i n t e r e s t e d r e a d e r i s d i r e c t e d towards t h e r e c e n t e x c e l l e n t r e v i e w of Leibovich (1983).
I f a coherent s p a t i a l s t r u c t u r e e x i s t s i n the surface
waves, t h e n a s p a t i a l l y p e r i o d i c S t o k e s d r i f t w i l l be i n d u c e d . T h i s p e r i o d i c d r i f t w i l l produce a t o r q u e due t o h o r i z o n t a l variations i n t h e vortex force t h a t w i l l d i r e c t l y drive the r o l l motions.
However, an i n s t a b i l i t y mechanism i n t h e u n d i r e c t i o n a l
c u r r e n t w i l l a l s o produce Langmuir C i r c u l a t i o n s even w i t h o u t any c o h e r e n t s u r f a c e waves.
I n t h i s l a t t e r case, t h e v o r t e x f o r c e i s
b a l a n c e d by a v e r t i c a l p r e s s u r e g r a d i e n t .
D i r e c t wind-driven
Langmuir C i r c u l a t i o n s can a l s o t a k e p l a c e ( C r a i g and L e i b o v i c h (1976)). Some of t h e b i o l o g i c a l and e c o l o g i c a l consequences of Langmuir C i r c u l a t i o n s a r e h i g h l i g h t e d i n Dyke & Barstow ( 1 9 8 3 ) .
The
i m p o r t a n t f e a t u r e o f t h e c i r c u l a t i o n s i s t h e j u x t a p o s i t i o n of e a c h v o r t e x , p r o d u c i n g l i n e s o f downwelling and u p w e l l i n g zones
(see F i g u r e 2 ) .
The magnitude o f t h e u p w e l l i n g and downwelling
i n t h e d i v e r g e n c e and convergence zones ( r e s p e c t i v e l y ) w i l l govern p r e c i s e l y what w i l l remain n e a r t h e s u r f a c e and what w i l l s i n k beneath it.
B i o l o g i s t s have known f o r some t i m e t h a t convergence
zones c a u s e enhanced p r o d u c t i o n by c o n c e n t r a t i n g t h e p l a n k t o n i n t o l i n e s , b u t t h e same convergence zones c a n a l s o h e l p t o c o n c e n t r a t e s u r f a c e o i l i n t o t h i c k n e s s e s t h a t a r e p o t e n t i a l l y harmful t o t h e environment.
Dyke & Barstow ( 1 9 8 3 ) r e p o r t d e t a i l s of many o b s e r -
v a t i o n s t h a t s u p p o r t t h e c o n j e c t u r e t h a t t h e convergence zones a c t a s a s o u r c e of n e g a t i v e d i f f u s i o n .
T h i s i s p a r t i c u l a r l y dangerous
as h i t h e r t o n o n - t o x i c l e v e l s of p o l l u t a n t c o u l d be c o n c e n t r a t e d up, t o become t o x i c .
All of t h e r e s e a r c h i n t o Lanqmuir C i r c u l a t i o n s mentioned above The s t r u c t u r e of t h e c i r c u l a t i o n s i s s u c h t h a t o n l y by d e t e c t i o n from a d i s t a n c e c a n t h e o v e r a l l p a t t e r n s emerge. T h i s i s shown g r a p h i c -
owes a great d e a l t o t h e t e c h n i q u e s o f remote s e n s i n g .
a l l y by t h e p l a t e s h e r e and t h o s e i n Dyke
&
Barstow ( 1 9 8 3 ) .
213
Fig. 2 .
Future
An i d e a l i s e d r e p r e s e n t a t i o n of Langmuir C i r c u l a t i o n .
U s e s of Remote S e n s i n q
Remote s e n s i n g i s i d e a l l y s u i t e d t o m o n i t o r i n g t h e b e h a v i o u r o f o i l s l i c k s , simply because o i l s l i c k s p r o v i d e a wholly u n p l e a s a n t environment.
The f u r t h e r one i s away, t h e b e t t e r !
Perhaps t h e
g r e a t e s t advance i n t h e n e x t few y e a r s w i l l be i n t h e u s e o f s a t e l l i t e borne i n f r a - r e d d e v i c e s f o r o i l s l i c k m o n i t o r i n g .
Alreadv t h e
motion and g e n e r a l b e h a v i o u r o f t h e l a r g e r s l i c k s , f o r example t h e very u n p l e a s a n t s p i l l i n t h e Middle E a s t , a consequence of s h a l l o w water p r o d u c t i o n p l a t f o r m b e i n g damaged i n t h e I r a n - I r a q c o n f l i c t , can be t r a c k e d by s a t e l l i t e .
With improvements i n m i c r o p r o c e s s o r
c o n t r o l l e d o p t i c s , i t can be e x p e c t e d t h a t t h i s t r a c k i n g a b i l i t y w i l l soon be e x t e n d e d t o c o v e r s l i c k s o n l y a few s q u a r e metres i n s u r f a c e area.
What i s c e r t a i n i s t h a t a g e o s t a t i o n a r y s a t e l l i t e
w i t h i n f r a - r e d s e n s o r s t r a i n e d on o f f s h o r e l o a d i n g and u n l o a d i n g t e r m i n a l s would be a s i g n i f i c a n t d e t e r r a n t f o r t h o s e who p e r s i s t in i l l e g a l l y flushing out t h e i r tanks a t sea.
A s s a t e l l i t e s become more common, it may be p o s s i b l e t o under-
t a k e s p a t i a l m o d e l l i n g o f o i l s l i c k s i n much t h e same way a s i s b e i n g done i n meteorology.
T h i s t e c h n i q u e r e q u i r e s many s a m p l e s ,
c
v i r t u a l l y a "film" t o deduce dynamics o l o g y due t o GARP
o f t h e p a r t i c u l a r s l i c k ' s movements from which T h i s t e c h n i q u e i s f u r t h e s t advanced i n meteort h e G l o b a l Atmospheric Research Programme) b u t
o c e a n o g r a p h e r s a r e a l r e a d y examining t h e p o s s i b i l i t y of improving o u r knowledge o f o c e a n i c f r o n t s and s i m i l a r m e s o s c a l e phenomena. t h a t t h e r e a r e s t i l l t e c h n i c a l probl e m s t o b e overcome b e f o r e t h e e x i s t i n g i n f r a - r e d t e c h n i q u e s c a n be I t must be s t r e s s e d , however,
used w i t h c o n f i d e n c e t o deduce t r u l y t h r e e d i m e n s i o n a l b e h a v i o u r a t
sea. I n c l o s i n g , mention o u g h t t o be made of t h e work of Shearman (1980).
H e d e m o n s t r a t e s t h e u s e o f i o n o s p h e r i c b a c k s c a t t e r where-
by r a d i o waves a r e r e f r a c t e d back t o t h e d e s i r e d area of sea by t h e ionosphere.
T h i s t e c h n i q u e i s s u i t a b l e f o r s u r f a c e wave d e t e c
t i o n and, s i n c e o i l damps o u t waves, c a n t h e r e f o r e be a p p l i e d h e r e . However, t h e t e c h n i q u e remains c o n t r a v e r s i a l and t h e a c c u r a c y o f r e s u l t s i s s t i l l open t o d o u b t . Acknowledgement The a u t h o r would l i k e t o t h a n k D r . Stephen Barstow who c a r r i e d o u t much of t h e p r e l i m i n a r y work w h i l s t working a s a R e s e a r c h A s s i s t a n t w i t h him a t Heriot-Watt shore Engineering.
U n i v e r s i t y ' s Department o f Off-
215 REFERENCES Attwood, D. K . ( e d ) , 1980. P r e l i m i n a r y R e s u l t s from t h e September 1 9 7 9 RESEARCHER/PIERCE IXTOC I C r u i s e . U . S . Dept. Commerce NOAA/RD/MP3, B o u l d e r , C o l o r a d o , U.S.A. Audunson, T . , 1978. The Bravo Blowout. I K U R e p o r t No. 9 0 , Trondheim, Norway. C r a i k , A. D. D. and L e i b o v i c h , S . , 1976. A R a t i o n a l Model f o r Langmuir C i r c u l a t i o n s , J . F l u i d . M e c h . , 401-426. D i e t z e l , G. F. L . , G l a s s , A. W . and Van K l e e f , P . J . , 1976. A computer s i m u l a t o r f o r t h e p r e d i c t i o n o f s l i c k "Sliktrak. movement by n a t u r a l means, c l e a n - u p and p o t e n t i a l damages a r i s i n g f r o m o i l s p i l l s o r i g i n a t i n g from o f f s h o r e o i l w e l l blowouts. I t s d e v e l o p m e n t and a p p l i c a t i o n t o t h e N o r t h S e a " . S h e l l I n t e r n a t i o n a l e P e t r o l e u m , M a a t s h a p p i j , B . V . , The Hague, The N e t h e r l a n d s , R e p o r t EP-47436, December, 1 9 7 6 . Dyke, P . P . G . and B a r s t o w , S . F . , 1983. The i m p o r t a n c e o f Langmuir C i r c u l a t i o n s t o t h e Ecology o f t h e mixed l a y e r p p 486-497 i n " N o r t h S e a Dynamics" e d s . Sundermann and L e n z , Springer-Verlag. S u r f a c e m o t i o n o f w a t e r i n d u c e d by w i n d , Langmuir, I . , 1938. S c i e n c e 87, 119-123. L e i b o v i c h , S . , 1983. The f o r m and dynamics o f Langmuir C i r c u l a tions. Ann.Rev.Fl.Mech I 5 391-427. P a r k e r , H . D . & Cormack, D . , 1 9 7 9 . Evaluation of i n f r a - r e d l i n e s c a n s (IRLS) a n d s i d e - l o o k i n g a i r b o r n e r a d a r (SLAR) o v e r c o n t r o l l e d o i l s p i l l s i n t h e North Sea. W a r r e n S p r i n g s L a b o r a t o r v R e p o r t LR315 (OP), S t e v e n a a e , U . K . 2513~. Shearman, E . D . L . , 1980. Remote s e n s i n g o f t h e s e a s u r f a c e by decametric radar. R a d i o and E l e c t r o n i c E n g i n e e r i n g 50, 611623. Smith, J . E . ( e d ) , 1968. " T o r r e y Canyon" p o l l u t i o n and m a r i n e l i f e C.U.P.
12,
This Page Intentionally Left Blank
217
AN INTERCOMPARISON OF GEOS-3 ALTIMETER AND GROUND TRUTH DATA OFF THE NORWEGIAN COAST
Lygre, Asle Oceanographic Department, Continental Shelf Institute, P.O. Box 1883, 7 0 0 1 Trondheim (Norway)
ABSTRACT An intercomparison has been carried out between wave heights and wind speeds estimated from the GEOS-3 altimeter data off the Norwegian ‘coast. The main result of this study has been that the absolute mean difference between GEOS-3 measurements of significant wave height (HS) and corresponding waverider measurements is 1.18 m. Meanwhile the wind measurements show a corresponding difference of 2.43 m/s. In addition the tendency was towards overestimation of HS by GEOS-3 compared to HS measured by the waveriders. We have tried to account for this overestimation but no obvious explanation has #been found. However, we might suggest the following as possibilities: spatial non-homogeneities in the wave field caused by, for example, the coastal shielding effect, instrument malfunction in the altimeter as the satellite ground track passes from land to sea, possibly due to the change in reflectivity. Surprisingly this systematic overestimation of HS by GEOS-3 has not been reported in earlier comparisons*, indicating that altimeters are possibly not suitable for measuring waves in near coastal areas. To assess if and why there are inconsistencies between satellite and waverider measurements off the Norwegian coast further investigations must be carried out. Unfortunately, the data collected by SEASAT in 1978 comprises only a few passes over Norwegian waters. It was not possible to give a definite answer to the question as to whether or not there exist positive wave gradients in a direction away from the coast. However, the satellite measurements did show situations with relatively large geographical variations in HS. In the Halten area, increasing wave height with distance away from the coast always corresponded to approaching frontal systems. The GEOS-3 passes giving data across frontal systems shows how powerful satellite measurements can be in monitoring the sea state, but before this can be achieved accurately, an algorithm for correcting the effects of clouds and heavy precipitation is required.
*
Except for comparisons of HS recorded early in the GEOS-3 mission (Fedor et al., 1 9 7 9 ) . The error was detected and we have assumed that this has been corrected for in the algorithm of concern.
218
INTRODUCTION The GEOS-3 satellite was launched on April 9th 1 9 7 5 from the Air Force Western Test Range. One of the purposes was to test the possibility of collecting geophysical data by means of a radar altimeter. Data was acquired from April 21st 1975 until December 8th 1978. The data used in this comparison was provided on tape by NOAA's satellite division in Washington DC. The tapes contained the corrected data set from the GEOS-3 mission and covered the North Atlantic area. In this paper we will concentrate on the GEOS-3 wave measurements and compare them to measurements of significant wave height provided by three waverider buoys located at Utsira, Brent and Halten off the Norwegian coast - see fig. 1. We will also compare wind measurements at Utsira to wind speed deduced by the scattering of the radar return signal. Wave profiles in a direction away from the coast are also considered to see if buoy measurements near the coast line can be used to reflect the sea state further off the Norwegian coast. Two passes of the GEOS-3 satellite over meteorological frontal systems are also presented.
Fig. 1. Map showing the waverider locations at Halten (A), Brent ( C ) and Utsira (B).
219
THE COMPARISON OF GEOS-3 AND IN-SITU DATA For those who are interested in a description of the altimeter and the GEOS-3 satellite we refer to the special GEOS-3 volume of Journal of Geophysical Research vol. 84, and Barrick (1972).
When comparing GEOS-data to in-situ data one should be aware of the following: 1 ) Wind and wave measurements obtained by GEOS-3 are in effect
moving averages along the ground track. In this report HS is averaged over approximately 140 km. 2) In-situ measurements are based on time series at a fixed point on the sea surface.
3) There are space and time lags between the ground measurements and the GEOS-3 measurements. Considering points 1 to 3 it is obvious that differences between GEOS and in-situ measurements may occur due to for example, effects of rapid changes in the surface weather situation both in time and space. To a certain extent it is possible to control this feature by an inspection of weather maps to assure reasonable stationarity in the weather situation. In the following comparison the difference in time between ground and satellite measurements seldom exceeds 2 hours and the geographical distance between the waverider and the point of nearest approach lies between 8 and 140 km. Most of the data considered here is from February 1 9 7 6 , but data was also chosen from other months in 1 9 7 5 and 1 9 7 6 . Figs. 2 to 4 show the ground tracks of the GEOS-satellite during the selected passes in the vicinity of the waveriders. Table 1 further gives the time and date of the passes. As is readily seen there is a geographical spread in the passes near the buoy positions. Sometimes it was difficult to pick out a
.
220
representative estimate of HS due to rapid variations in HS along the track close to the waverider. However, as a rule the nearest measured value was chosen, but there also arose situations where it was necessary to average the HS estimates along the :rack close to the waverider position and use this mean value when comparing to the waverider data. This averaging is in addition to the averaging performed by the computer algorithm and will certainly cause additional smearing of the estimate. In an attempt to minimize the effect of time lag between ground measurements and satellite data, the buoy values were interpolated linearly with time. Buoy measurements of HS were available every third hour. With regard to the comparison of wind speed at Utsira Lighthouse to wind estimates obtained by GEOS-3, the same procedure as outlined above was used. The wind data at Utsira Lighthouse was provided by the Meteorological Institute in O s l o . In Fig. 5, scatter plots of GEOS-3 and waverider data are shown in addition to a scatter plot of wind speed measured at Utsira and by GEOS-3. Two lines of regression are fitted to the data points, one of which is forced to run through the origin. For the wind speed data and the Brent data only one line of regression is shown in the scatter diagrams as the two lines coincide.
Fig. 2. Ground t r a c k s of GEOS-3 p a s s e s c l o s e t o t h e w a v e r i d e r a t U t s i r a . a n d t i m e of e a c h p a s s i s g i v e n i n T a b l e 1.
The d a t e
N N N
Fig. 3. Ground tracks of GEOS-3 passes close to the waverider at Brent. The date and time of each pass is given in Table 1.
HALTEy
65"
21 12 13
64O
63"
I?
jZo
52'-
Fig. 4. Ground tracks of GEOS-3 passes close to the waverider at Halten. The date and time of each pass is given in Table 1. N N W
224
U T S l RA
HALTEN DATE
TIME
1.
090276
8.13
2.
120276
7.30
BRENT
DATE
TIME
1.
210276
11.56
2.
020676
23:43
DATE
TIME
1.
020676
23.42
2.
260276
12.22
3.
030276
9.40
3.
150276
8.26
3.
141075
20.44
4.
130276
8.55
4.
170775
10.45
4.
170276
11.16
5.
130276
7.16
5.
160276
10.29
5.
130276
10.34
6.
080276
6.49
6.
070276
11.21
6.
040276
9.26
7.
060575
22.07
7.
030276
9.40
7.
080776
23.20
8.
140276
7.02
8.
170276
11 . I 5
8.
110276
6.07
9.
080276
8.28
9.
280775
6.30
9.
180776
00.28
10.
061 175
20.09
10.
220276
6.47
10.
070776
23.34
050776
22.25
11.
150276
6.49
11.
230475
21.66
11.
12.
220475
22.10
12.
180276
6.05
12.
060276
5.40
13.
190276
9.07
13.
020875
6.57
13.
200276
7.16
14.
110276
8.11
14.
241075
16.42
15.
170276
9.36
15.
080276
5.11
16.
220276
10.03
17.
180276
9.22
BUOY POSIT ION :
18.
220276
8.24
N59'
19.
230276
9.48
20.
170276
7.57
21.
100276
7.59
22.
160276
9.50
23.
230276
8.10
BUOY POSITION : N64'
11'
Ego 8'
BUOY POSITION: N6I0 4'
18'
E4'
E l 0 48'
48'
Table 1. The date and time of the GEOS-3 passes close to the waveriders at Utsira, Brent and Halten as shown on the maps in figs. 2 to 4. Each pass is numbered according to the numbers given on the trajectories in figs. 2 to 4.
225
a UTSIRA
HALTEN
t k:
CORRELATION COEFFICIENT 0 95
3
9
s m 'IHSGE (D
Q
hl
0
0 WINDSPEED GEOS
2
A
6
a
1'0
1'2 m
HS GEOS
UTSIRA
BRENT
HS GEOS
Fig. 5. In-situ measured wind speed and wave heights plotted against corresponding GEOS-3 measurements. For the Halten and Utsira wave data 2 lines of regression are specified one of which runs through the origin. The correlation coefficient is given on top of each scatter plot. HSGE: HS from GEOS-3. HSWR: HS from waverider. WSU: Wind speed from Utsira Lighthouse. WSGE: Wind speed from GEOS-3.
226
UTSIRA UTSIRA
TOTAL BRENT
C.L. L.S. 'ARSONS FEDORel
ABSOLUTE MEAN DIFFERENCE
0.94 rn
1.18 m
0.94 m
1.14 m
0.88 m
L X I HSGE - HSWR ) N MEAN DIFFERENCE
&Z
(HSGE - HSWR )
STANDARD DEVIATION OF
0.61 rn
MEAN DIFFERENCE
CO R R E LATlON
0.83 COEFFICIENT
MEAN HS
2.27 NAVE R IDE RS/BUOYS
'ERCENTAGE OF SEOS DATA WITHIN
28.6 %
r0.5 m OF THE NR - DATA.
I
-I.. TF 0.75 m
0.71 m
0.69 m
0.79 m
0.24 m
0.19 m
0.53 rn
1.02 rn
0.91
0.67
high
low
iea state
sea state
2.80
38.5 %
5.6%
33.3%
22.2 %
33 %
JUMBER OF
14 'ASSES (N)
45
19
27
I
Table 2. Summarizes t h e comparison parameters in this study and the comparisons carried out by Fedor et al. (1979) and Parsons (1979) HSGE: HS from GEOS-3 HSWA: HS from waverider
227
DISCUSSION In the report by Fedor et a1 (19791, an intercomparison of several algorithms is carried out, and GEOS-3 estimates of HS using the different algorithms are in addition compared to buoy measurements. For the algorithm used here, developed by Hayne (1977), the absolute mean difference was calculated to be 0.88 m - based on 27 GEOS-3 passes. Parsons, (19791, also presents an intercomparison of buoy and GEOS-3 measurements using the same algorithm as in our case - see table 2. We find an overall absolute mean difference for Halten, Utsira and Brent to be 1.17 m based on a total of 45 passes. This is higher than the value reported by Fedor et all and the difference may partly be due to larger space lag between the satellite ground track and the buoy positions in our case. Fedor et a1 give a correlation coefficient C of 0.667 between buoy and altimeter data while our correlation coefficient based on all three stations is estimated to be 0.91. Thus, we find a higher correlation between buoy and GEOS-3 measurements than Fedor et al. However, our results suggest that GEOS tends to overestimate HS compared to the waverider - see table 2. In Fedor et a1 (1979) it is also found that 23% of the satellite measurements are within 2 0.5m of the buoy measurements for the algorithm in question. In our case we found 28.6% for the Utsira data, 38.5% for the Brent data and 5.6% for the Halten data. Of the 45 GEOS passes near the waveriders, 22.2% of the measurements were within 0.5m of the waverider values. In fig. 6 on the next page we have plotted (HSGE-HSWR) as a function of HSWR. No clear relationship between sea state and (HSGE-HSWR) can be detected from the scatter plot. But the ratio (HSGE-HSWR)/HSWR seems to decrease as HSWR increases showing that the relative error of the altimeter decreases during higher sea states. This was also expected as the radar altimeter was primarily designed to operate during high sea state conditions.
228
m
I
Fig. 6. The difference in significant wave height from GEOS-3 and the waveriders (HSGE-HSWR) plotted as a function of significant wave height from the waveriders ( H S W R ) .
A natural question to ask now is why the results presented here seem to be different from previous intercomparisons. Though the number of passes is quite limited, the probability that almost all the satellite measurements are greater than the buoy measurements is very small provided both satellite and buoy measurements are sampled from the same probability distribution. The probability that the N first samples are 2 greater than the N last is given by (N!) / (2N!). If N = 45 as in our case it is evident that this probability is practically zero. Considering that only 3 of the altimeter measurements were greater than the waverider measurement we conclude that: 1) The GEOS-3 measurements and the waverider measurements are not sampled from the same distribution or 2) There is an error in the satellite data.
229
Strictly speaking the waverider should also be questioned, but we have used data from three buoys and waveriders are regarded as more reliable than the altimeter. Excluding the values from Halten in the above intercomparison changes the situation somewhat. We then find the absolute mean difference to be 0.88 m, the same as found by Fedor et all and the correlation coefficient to be 0.81. For + these two stations 33.4% of the data lies within - 0.5m of the buoy data, - the same as found by Fedor et al. (1979) for this specific algorithm. Thus, the absolute mean difference and tendency towards overestimation ceases while only using values from Brent and Utsira. But the bias in the altimeter data is still present. One main difference between Brent/Utsira and Halten is the possible effect of the coastal shadow - see Fig. 4 . As southerly and south westerly winds are common in this area and noting that most of the GEOS-3 measurements near Halten are from an area which is further north and further away from the coast than the buoy measurements, then under suitable wind direction conditions this increase in fetch might account for some of the larger discrepancies between GEOS and buoy data at Halten compared with those at Utsira and Brent. It might also be the case that the wave field in general is more inhomogeneous in the Halten area than in the Utsira and Brent areas. In order to check the possible effect of the coastal shadow in a loose sense, wind directions deduced from weather maps were used to split the Halten data set into two groups: one in which the coastal shielding effect was assumed to be of significance. The absolute mean difference was calculated for both data sets and it turned out that it was 0.2m higher for the data assumed to be affected by the shielding effect. However, this small increase was not regarded as significant for drawing any firm conclusions. But the increase supports the fact that the coastal sheltering effect will give rise to higher waves in the more exposed offshore waters during the appropriate wind conditions and thus probably will give rise to higher differences between satellite data and waverider data if the spatial spread of satellite data is biased in an offshore direction as in the present study. This is consistent with a comparison between the waverider at Halten and the ODAS 490
230
Norwave data buoy situated further off the coast (65O2'N, 7O33'E). Another way of investigating inhomogeneities in the wave field is to inspect the wave profiles measured by GEOS-3 along the ground track. This was done both for the Halten, Utsira and Brent areas. It was observed that quite large spatial variations in HS occurred. Differences of around 2 m within 100 or 200 km were not unusual, but it is not possible to say that increasing wave height in a direction away from the coast is a dominating feature. However, the 7 cases at Halten when the wave height increased with distance away from the coast always corresponded to an approaching frontal system or one which had just passed. This correspondence was not that clearly demonstrated in the Brent and Utsira areas were respectively 5 out of 7, and 2 out of 6 events corresponded to frontal systems. Regarding the present investigation in relation to those of Parsons (1979), Fedor et al. (1979) and other intercomparisons of in-situ wave/wind measurements and GEOS-3 data, it is worthwhile noting that the latter were carried out using data measured in more open ocean areas whilst our measurements are taken from just off the coast. From table 2 we see that the Brent data gives the least absolute difference, and Brent is also the waverider position furthest off the coast. The satellite also travelled in an east-west direction and as the HS estimates are moving averages, there is a slight possibility that reflections of radar signals from land masses will affect the near coast estimates. More important is probably the change in reflectivity as the ground track passes from land to sea. This change may affect the altimeter so that a bias is introduced in the near coast GEOS-3 data. As a conclusion we have found some evidence for the presence of spatial non-homogeneities in the wave field - at least in the Halten area - leading to the apparent result that GEOS-3 measurements and the buoy measurements are not sampled from the same probability distribution. In addition some of the bias in the GEOS-3 data may possibly be caused by the effects introduced when the ground track passes from land to sea.
231
GEOS-3 PASSES OVER METEOROLOGICAL FRONTS Figs. 7 and 8 show two examples of GEOS-3 passes over meteorological fronts. The surface weather maps presented are based on the 0600 GMT analysis. The satellite passed about two hours later. Thus, the frontal systems would have been positioned further towards north-east than shown on the weather maps. Wind speed observations taken from the weather maps are shown as single points on the plots showing the wind profiles along the surface track. On February 22nd there may be seen from the weather map to be a series of fronts approaching the Norwegian coast from the south west. The satellite passes over one warm front and proceeds towards another situated to the south of Iceland. Both the wind speed and significant wave height increase as the satellite approaches the first front, straight after which there is a significant drop in both these parameters. The wave height and wind speed then start to grow again as the satellite moves over the second front to the southwest of Iceland. The next day, February 23rd, the weather situation has worsened and the pass over .the frontal system suggests strong activity in the frontal region which gives rise to very strong gradients both in the wind and wave field in a direction away from the coast. However, the measured values of significant wave height and, especially, wind speed on February 23rd, must be considered to be rather high if we take into account that they represent average values over a rather long distance and the observed wind speeds on the weather map. It is very likely that the heavy precipitation and clouds in connection with frontal systems will affect the return radar signal, and this should be taken into consideration. However, no specific algorithm for correcting such errors is to our knowledge yet available, and this is surely a future demand. These two examples illustrate the ability of satellites to scan the sea surface and provide measured wind and wave profiles which are hardly available by other methods. The present results are promising enough to suppose that satellites may become a powerful tool in the acquisition of real time wind
232
0
o
zoo
BOO
POO
eoo
1000
1200 KM
:lo,,; ,
,
,
*
-
,
,
,
N
0
200
400
eoo
eoo
iooo
1200 KM
Fig.7. Wind and wave profiles from GEOS-3 on February 22nd 1976 at 0824 GMT. The arrow indicates the satellite propagation direction. Significant wave height (HS) is given in m and wind speed in m/s. The dots represents nearby wind speed observations taken from the surface weather map based on the 0600 GMT analysis.
233
N-
O
.
0
200
400
SO0
BOO
1000
1200
I
1400 Kt4
Fig. 8. Wind and wave profiles from GEOS-3 on February 23rd 1976 at 0810 GMT. The arrow indicates the satellite propagation direction. Significant wave height (HS) is given in m and wind speed in m/s. The dot represents a nearby wind speed observation taken from the surface weather map based on the 0600 GMT analysis.
234
and wave data in the future in particular as measuring techniques and computer algorithms are improved. ACKNOWLEDGEMENTS The preparation of this paper has been sponsored by the Royal Norwegian Council for Scientific and Industrial Research (NTNF) through the project "Analysis of Wave Data", 1810.7890 and the joint Norwegian measurement program "Oceanographic Data Acquisition Project" (ODAP). ODAP is financed by Norsk Hydro, Saga Petroleum, Statoil, British Petroleum, Phillips Petroleum and with additional funding from NTNF. REFERENCES Barrick, D.E., Determination of Mean Surface Position and Sea State from the Radar Return of a Short-pulse Satellite Altimeter, Sea Surface Topography From Space, Volume 1, NOAA Technical Report ERL 228-AOML, February 1972. Brown, Gary S., Estimation of Surface Wind Speeds Using Satellite-Borne Radar Measurements at Normal Incidence, J. Geophys. Res. 84 No B8, 3974-3978, 1979. Fedor, L.S, Godbey, T.W., Gower, J.F.R., Guptill, R., Hayne, G.S., Rufenach, C.L. and Walsh, E.J. Satellite Altimeter Measurements of Sea State - an Algorithm Comparison. J. Geophys. Res., 8 4 , No B8, 3991-4001, 1979. Hayne, G.S., Initial Development of a Method of Significant Wave height Estimation for GEOS-3, NASA CR-141425, August 1977. Krogstad, H.E., Gundersen, P., Computer Listings of Wind and Wave data from GEOS-3 North Sea/Norwegian Sea. IKU report P201/2b/81, Continental Shelf Institute, Trondheim. Krogstad, H.E., Gundersen, P., Wind and Wave Profiles from GEOS 3 North Sea/Norwegian Sea. IKU report P201/2c/81. Continental Shelf Institute, Trondheim. Parsons C.L., GEOS-3 Wave Height Measurements: An Assessment During High Sea State Conditions in the North Atlantic, J. Geophys. Res. 84, No B8, 4011-420, 1979.
235
SATELLITE IMAGERY OF BOUNDARY CURRENTS T. Carstens, T.A. McClimans and J.H. Nilsen Norwegian Hydrodynamic Laboratories, Trondheim, Norway
ABSTRACT Fronts along coastlines are often associated with boundary currents, and satellite imagery has increased our ability to study such currents. Their causes are discussed, and it is concluded that persistent boundary currents are due to the deflection of runoff by the earth's rotation. The much studied Norwegian Coastal Current is used to illustrate the dynamics of boundary currents. The basic front disturbancies as seen from satellites are reproduced in a small scale physical model. Although very useful, satellite imagery is frequently limited by clouds, and does not give current speeds. Moored current meters yield current speeds exceeding present model predictions, indicating strong meteorological forcing. REAL AND PSEUDO BOUNDARY CURRENTS Fronts are not always associated with longshore currents. Vertical flows are known to produce fronts with significant temperature jumps, with or without corresponding density jumps. Whether or not there is a density difference across the front, it is unstable and may show configurations not unlike the fronts of the boundary currents described below. Accordingly, thermal images can be misinterpreted so that a pseudo boundary current with negligible longshore flow by an untrained eye is assumed to be a real boundary current. Normally the front of a pseudo current has more random perturbations than the typical eddies and whirls of a real boundary current. The so-called thermal bar between water masses of equal density but different temperature is easily deformed, and the balance of an arrested front between water masses of different densities is sensitive to stochastic variations. If the cause of the front is winddriven upwelling, the stochastic component is readily ascribed to the wind field. If the driving force is a heat flux, spatial variations are generatedfor many reasons, among which topography is an important one. In any case the absence of significant shear prevents the formation of the more predictable disturbances of the real boundary currents.
In very sheltered areas the thermal images may be mapping the temperature field of an undisturbed surface film. This field reveals spatial variance on a small scale and may limit the usefulness of future reductions in pixel size. For the study of coastal currents this source of noise is unimportant, but in small fjords and lakes we may eventually face a problem here. Boundary currents are readily generated by many different mechanisms and are therefore frequently observed in water bodies of any size. On the geophysical scale we find such currents occurring regularly in lakes, in fjords, in nearshore coastal waters as well as offshore along continental shelves. In fact, boundary currents seem to be the rule rather than the exception. The exceptions are those lakes in which the temperature gradients vanish for a few days each year and those shallow seas in which the density gradients vanish seasonally due to cooling and ice formation. In the discussion below we have excluded the large scale oceanic circulations along continental shelves. Satellite pictures have provided an overwhelming amount of evidence to prove the occurrence of boundary currents.
CAUSES OF BOUNDARY CURRENTS Runoff When a river discharges into a lake or on an open coast, it will normally remain an identifiable entity for a considerable distance from the source, which is the river mouth. As soon as the solid boundary (or, in the case of alluvials, not so solid boundary) is replaced by a fluid interface, mixing begins. In cases with weak tides the higher turbulence of the river, compared to the recipient, drives a one-way entrainment process by which the surrounding water is pulled into the now free-floating river. Gradually the composition of the river water changes so that it resembles more and more the receiving water. In many cases this process begins already far inland when a salt wedge lifts the river away from its bed. In cases with strong tides there is a two-way horizontal exchange by turbulent diffusion. Although the salinity at the mouth may approach that of the sea, there will normally be enough of a density deficit to make detection of the river possible even far away from the mouth.
231
Jet deflection For a river to form a boundary current in a lake or along a coast, it will have to change its course. Changes of direction occur for a number of reasons, among which cross flow is perhaps the most obvious one. However, we shall point out the other reasons as well and demonstrate that in the final analysis the earth's rotation provides a satisfactory explanation to our observations. The momentum balance for the laterally constrained river flow is usually written in the axial direction only. The lateral forces are small in comparison and their consequences negligible with a few exceptions. One such exception is a bend in an alluvial river, where centrifugal forces acquire importance. In any case the balance is easily achieved by a lateral surface slope compensating for the possible imbalance of the other lateral forces: Coriolis force, centrifugal force, wind stress and lateral friction. When the river leaves its mouth, the lateral slope can no longer be maintained. The river is thrown into a lateral imbalance which causes it to deflect and to spread side-ways. River trajectories have been computed for many different cases. The simplest case is that of a discharge for a stagnant sea as shown in Fig. 1.
(WHIRL) --T
'
"
''''
' "2
L '"
*',-
X
Fig. 1. Coriolis deflection of a river plume to form a boundary current. (McClimans, 1983) After a travel of about
r
=
u/f
the river has turned 90' and become parallel to the coast.
Here
u is the speed of the jet and f is the so-called.Coriolis para-
238
meter. Depending on the width of the river, it will continue to bend until it makes contact with the coast. For large river widths B >> r the outflow will be longshore within a distance x = B. For B 1. However, in a distorted r model ((B/H)r < 1 ) the importance of interfacial friction is reduced nearly to the conditions in nature.
Advection of oil spills We went to the Department of Environment with our discoveries and obtained a small contract to find what whirls might do to oil drift. The idea was to establish a forecasting routine that would improve existing routinsand which could be activated in case of a spill. An example of an oil spill being blown across a coastal current whirl is shown in Fig. 9 (McClimans & Nilsen, 1983) and in Table 1. An average whirl could speed up the landfall of a spill by 1 day from 3 to 2 and reduce the drift to 15 km. On the other hand it could also delay the land-fall by 2 days and deliver the oil 90 km away. The authorities apparently have not yet found these and other possibilities for improved forecasts sufficiently interesting. Pollution, however, was only one of several issues concerned with the NCC.
250
100
r
-
(OIL SPILLS)
IKU MODEL
OFFSHORE DISTANCE (Kml 50
-
COAST
Fig. 9. Position of an oil slick originally (day 0 ) at positions a-i illustrated as a front after 1 and 2 days. The current field is that of an average whirl originally (day 0 ) centered at L = 350 km and moving 15 km/day northward. Particle paths € o r three oil patches (at d, e and i) are drawn with dashed lines. A convergence zone for the oil is shaded. (Mc Climans & Nilsen, 1982)
TABLE 1 release
a b C
AT
days 3 3.5 5.2
I
d e f ¶
h i me an
d r i f t model 1 d r i f t model 2
4.8 2.1 2.5 2.8 2.9 3 3.3
I
3 3
AL
km 37 49 78 90 15 22 22 32 34 42
-
34 50
Design and operation of offshore structures In March 1981 the NCC itself came to our rescue by hitting a moored current meter with unprecedented high velocities (Fig. 10). The current meter rig was part of a monitoring programme at the
251
150 100
cm/s 50
0 Q)
CD
a
s
0
0
'C
8 7 6 5 4
3
2
3
4 5 6 7 MARCH 1981
8
F i g . 10. Current velocity and temperature measurements at 2 km depth ca 20 km offshore (Courtesy A/S Norske Shell)
252
Troll Field, believed to be the largest offshore gas field in the North Sea. The time history observed is neatly explained. If the speed increase is directed towards NE it should be followed by a period of strong constant N current before it veers to NW and decelerates. This indicates that the moored current meter is near the outer edge of a whirl. At the same time, a drop in temperature is registered as the colder coastal water passes the moored current meter. Routine statistics, f.inst. exceedance or duration diagrams form the basis for decision-making. Risk levels are assumed and this leads to probability of design events and eventually to the selection of the design event itself. In the case of the offshore bursts of coastal water, it is felt that satellite images can help fill in the statistics. The laboratory simulations revealed a relationship between the maximum current speed and the phase speed of a northward propagating whirl. Use of consecutive thermal images has been shown to give a reasonable phase speed compared to the laboratory results (Johannessen & Mork, 1979). Thus it is possible to use satellite thermal images for several quantitative analyses. It should be remarked that the laboratory simulations did not include wind or other variability. Our predictions were that the disturbances would extend 100 km offshore and cause velocities of 1 m/s or about twice what was expected in the absence of wind. After a rather short observation period of 2 years much larger currents have been observed. OTHER COASTAL CURRENTS Our example with the Baltic outflow turning right and becoming the Norwegian Coastal Current is but one of a widely distributed type of runoff-generated boundary currents. Fig. 11 shows a map of the winter currents in the East China Sea. The runoff to Pohai Bay starts the boundary current which is joined by other rivers and runs all along the Chinese coast. Fig. 12 shows a satellite picture of the Sea of Japan. As described by Sugimura et al. in the present volume, the Tsushima Current can be seen to flow northward along the eastcoastof Japan as a boundary current.
At the same time pseudo boundary currents
are visible along the Sibirian coast.
253
Fig. 11. Winter currents in the East China Sea, with boundary current flowing southward. After Guan and Mao (1982).
BOUNDARY CURRENTS IN FJORDS Rivers discharging to large fjords also reveal boundary currents held shoreward by the influence of the earth’s rotation. Fig. 13 shows a black and white copy of a colour enhanced LANDSAT picture revealing the pattern of outflow of glacial flour in the surface waters of Gaupnefjord in the inner reaches of the Sognefjord. The resolution of this picture is 80 m. As the resolution improves, the application of remote sensing increases.
254
Fig. 12. Gray tone enhanced thermal image of the Sea of Japan, with boundary current flowing northward along the east coast of Japan. (Courtesy Remote Sensing Technology Center of Japan).
CONCLUSIONS The use of remote sensing can provide quantitative measures for boundary current processes in lakes, rivers, fjords and the oceans.
Both termal images and optical windows provide useful
data for clear-weather situations.
As the resolution improves
(smaller pixels) more classes of boundary processes will be subjected to remote sensing and analysis.
ACKNOWLEDGEMENTS This work has been supported by the fund of License Fees.
255
Fig. 13. Black and white copy of colour enhanced thermal image of a narrow fjord.
REFERENCES Braaten, B.R. and Szetre, R., 1 9 7 3 . Farming salmon in Norwegian coastal waters. Fisken i havet, ser. B (9) (In Norwegian) Eggvin, J., 1 9 4 0 . The movements of a cold water front. Rep. Norw. Fishery Mar. Invest. 7:1-151. Fan, L.N., 1967. Turbulent buoyant jets into stratified or flowing ambient fluids. Cal. Inst. of Tech., Report No KH-R-15.
256
Guan Bingxian and Mao Hanli, 1 9 8 2 . A note on circulation of the East China Sea. C.J. of Oceanology and Lirnnology, V o l . 1, No 1. Helland-Hansen, B. and Nansen, F., 1 9 0 9 . The Norwegian Sea. Report on Norwegian Fishery and Marine Investigations, V o l . 2. Johannessen, O . M . and Mork, M., 1 9 7 9 . Remote sensing experiment in the Norwegian coastal waters. Spring 1 9 7 9 . Samarbeidsprosjektet den Norske Kyststr0m. Report 3 / 7 9 . Geophysical Institute, University of Bergen. McClimans, T.A., 1 9 8 3 . Laboratory simulation of river plumes and coastal currents. ASME symposium on modeling of environmental flow (in press). McClimans, T.A. and Nilsen, J.H., 1 9 8 2 . Whirls in the Norwegian Coastal Current and their importance for evaluating oil pollution. Symposium on physical processes related to oil movements in the marine environment. Tvaarminne, Finland. November, 1 9 8 2 . Vinger, A . and McClimans, T.A., 1 9 8 0 . Laboratory studies of baroclinic coastal currents along a straight, vertical coastline. River and Harbour Laboratory. Report STFGO A80081.
261
TURBULENCE DISTRIBUTION OFF USHANT ISLAND MEASURED BY THE OSUREM HF RADAR
P . PIAU and C. BLANCHET' ' I n s t i t u t FranCais du P e t r o l e , 1 B 4 , avenue de Bois Preau, 92506 Rueil-Malmaison Cedex, France.
ABSTRACl
The HF radar OSUREM was i n s t a l l e d f o r t h e spring o f 1982 on t h e c o a s t s of Brittany (France) i n order t o measure wind and waves o f f Ushant Island. This type of radar provides electromagnetic s p e c t r a . The p a r t of t h e s e spectra containing t h e most energy i s the so c a l l e d f i r s t - o r d e r spectrum composed of two l i n e s which a r e generated by a Bragg r e f l e c t i o n process. The width o f these l i n e s i s representative o f t h e energy d i s t r i b u t i o n of turbulent c u r r e n t s in t h e resolution c e l l , t y p i c a l l y ten kilometers wide. In the experiment described here, t h e w i d t h of t h e First-order l i n e was s u p r i s i n g l y g r e a t , due t o the strong t i d e c u r r e n t s encountered in t h i s a r e a . IJe can e x t r a c t t h e current d i s t r i b u t i o n in t h e area from t h e shape of t h e l i n e s , depending on t h e time and strength of t h e tide. INTRODUCTION The use of high frequency (HF) electromagnetic waves f o r t h e remote sensing of t h e sea surface has been increasing i n recent years. HF radars measure the Doppler spectra of t h e r e t u r n power coming back from t h e s e a . If we use a narrow-beam HF radar, t h e main f e a t u r e in t h e Doppler spectrum i s t h e so-called f i r s t - o r d e r spectrum. I t i s composed of two l i n e s t h e mean frequency of which i s r e l a t e d t o t h e sea c u r r e n t . In a l o t of geographic a r e a s , i t i s very easy t o e x t r a c t t h i s c u r r e n t from t h e f i r s t - o r d e r Doppler spectrum. Off Ushant Island, we will show t h a t t h i s f i r s t - o r d e r spectrum i s more complicated, due t o t h e turbulent c u r r e n t d i s t r i bution in the resolution c e l l o f t h e radar. We will not only e x t r a c t t h e c u r r e n t speed with good accuracy b u t a l s o obtain a measurement of t h e variance of t h e c u r r e n t speed in t h e d i r e c t i o n of t h e radar beam. DESCRIPTION OF THE OSUREM HF RADAR
1.
1.1
What i s a narrow-beam ground-wave radar ?
OSUREM i s a ground-wave radar. This means t h a t t h e electromagnetic waves
follow t h e sea surface i n going t o the measurement area and i n coming back t o the radar. Another way of propagation i s r e f l e c t i o n by t h e ionosphere which i s used by sky-wave radars.
268
Sky-wave r a d a r s may measure a t d i s t a n c e s of t h e o r d e r o f some thousand k i l o m e t e r s . But f o r t h e moment they cannot o b t a i n s p e c t r a good enough t o p r o v i de accurate wave and c u r r e n t measurements. Ground-wave r a d a r provides v e r y good Doppler s p e c t r a w i t h good t i m e coverage. I t does n o t depend d i r e c t l y on t h e v a r i a t i o n s o f t h e c h a r a c t e r i s t i c s o f t h e
ionosphere. OSUREM i s a l s o a narrow-beam radar.The r e s o l u t i o n c e l l i s a small one
( t y p i c a l l y 15 km x 15 km) and i s s i t u a t e d f a r away from t h e r a d a r ( f r o m 20 t o 150 km). The azimuthal r e s o l u t i o n i s o b t a i n e d by t h e narrow-beam o f t h e recept i o n antenna. T h i s antenna i s 180 meters l o n g and composed o f 16 l i t t l e antennas spaced 12 meters a p a r t . 1.2
P r i n c i p l e o f t h e measurement The OSUREM r a d a r provides Doppler s p e c t r a l i k e t h e one i n F i g u r e 3. This
spectrum uses a dB v e r t i c a l scale. I t follows t h a t t h e t h r e e main peaks i n t h e spectrum c o n t a i n a l o t more energy than a l l o f t h e remaining power. The c e n t r a l peak a t t h e Doppler zero i s an e l e c t r o n i c a r t e f a c t . The two peaks w i t h quasi
t
( f . 2 3 Hz) compose t h e f i r s t - o r d e r spectrum. B They a r e c a l l e d t h e Bragg l i n e s . The remaining power i n t h e spectrum i s c a l l e d
o p p o s i t e Doppler frequency
f
t h e second-order spectrum. 1.2.1
Ihe-f~rlt~order-reertrum
I t s name comes from an analogy w i t h t h e Bragg d i f f r a c t i o n i n c r y s t a l s . The electromagnetic i n c i d e n t wave ( F i g . 1) w i t h wavenumber
Ki
i s backscattered by
t h e water waves whose wavenumber i s :
Those water waves have t h e same d i r e c t i o n as t h e r a d a r beam and a wavelength h a l f o f t h e electromagnetic wavelength. The Bragg waves a r e g r a v i t y water waves, and t h e i r v e l o c i t y i s :
v= kB
-
The Bragg Doppler frequency i s t h e Bragg water wave frequency
Expressed w i t h r a d a r c h a r a c t e r i s t i c s , t h i s frequency becomes:
259
x w=x 1/2
Electromagnetic surface waves
I :-:EI
-
Waves
f k ffj
Backscattered electromagnetic wave (Doppler)
1
IREAL-TIME PROCESSSING
( iomn)
1
- H 113, Fm, long waves direction - Wind direction - Currents F i g . 1. P r i n c i p l e o f HF r a d a r w i t h Fr = r a d a r frequency, 3 t o 30 MHz ; c = speed of t h e l i g h t . t o t h e value o f t h e d i r e c t i o n a l
The power o f a Bragg l i n e i s p r o p o r t i o n a l sea spectrum S ( Z ) a t t h e p o i n t frequency.
E
E
tB,w i t h
E
b e i n g t h e s i g n o f t h e Doppler
i s p o s i t i v e f o r t h e waves t r a v e l i n g t o w a r d t h e r a d a r . Then t h e
f i r s t - o r d e r Doppler spectrum p r o v i d e s t h e v e l o c i t y and power of two k i n d s of waves w i t h wavenumbers t
tBand -
zB. So i t i s
used t o c a l c u l a t e t h e w i n d d i r e c -
t i o n and t h e c u r r e n t speed. The w i n d d i r e c t i o n i s i n f e r r e d f r o m t h e d i r e c t i o n of Bragg waves which a r e h i g h - f r e q u e n c y waves s t r o n g l y dependent on t h e w i n d . T h i s d i r e c t i o n i s o b t a i n e d f r o m t h e r a t i o S ( t B ) / S ( - t B ) which i s maximum i n upwind c o n d i t i o n s (20 t o 30 dB) and equal t o 0 dB i n c r o s s w i n d c o n d i t i o n s . Thismethod was used w i t h groundwave r a d a r 111 and w i t h skywave r a d a r C21. There i s an a m b i g u i t y because two symmetrical d i r e c t i o n s i n r e l a t i o n t o t h e r a d a r beam g i v e t h e same w i n d d i r e c t i o n . The accuracy o f t h e method i s improved and t h e a m b i g u i t y suppressed by t h e use o f t h r e e r a d a r beams C31. The measurement o f t h e c u r r e n t c h a r a c t e r i s t i c s i s deduced f r o m t h e p o s i t i o n i n frequency o f t h e Bragg l i n e s , which i s n o t always developed 1 a t e r .
fB. T h i s p o i n t w i l l be
260
1.2.2
Th_e_second~o_rd_er-leectrum
This spectrum i s t h e r e s u l t o f second-order i n t e r a c t i o n s between F o u r i e r components o f t h e h e i g h t o f t h e sea s u r f a c e . These i n t e r a c t i o n s appear i n t h e equations f o r hydrodynamics and electromagnetism. I t f o l l o w s t h a t two waves a r e i n v o l v e d i n one elementary i n t e r a c t i o n . For t h e hydrodynamic p a r t , t h e r a d a r measures t h e second o r d e r waterwave
spectrum w i t h wavenumber
tB. This
spec-
trum c o n t a i n s power f o r every Doppler frequency. So i t d i f f e r s from t h e f i r s t o r d e r water wave spectrum which i s a D i r a c f u n c t i o n f o r a g i v e n wavenumber
cB.
The electromagnetic p a r t a l s o g i v e s a continuous spectrum, which i s added t o t h e hydrodynamic one. The shape o f t h e second-order spectrum, s t a r t i n g from a Bragg l i n e , i s d i r e c t l y r e l a t e d t o t h e shape o f t h e low-frequency p a r t o f t h e waterwave spectrum.
For one peak i n t h e waterwave spectrum, t h e r e a r e f o u r peaks i n t h e r a d a r spectrum, on each s i d e o f each Bragg l i n e .
A l o t o f work has been done t o e x t r a c t t h e waterwave d i r e c t i o n a l spectrum from t h e r a d a r Doppler spectrum. Narrow-beam r a d a r i s the o n l y one which can measure a t a g r e a t d i s t a n c e from t h e r a d a r . But f o r t h i s type o f radar, t h e maximum power i n t h e Doppler spectrum i s n o t always provided by t h e dominant d i r e c t i o n o f t h e low frequency waves. This i s t h e case f o r waves t r a v e l i n g p e r p e n d i c u l a r t o t h e r a d a r beam. T h i s problem has been s o l v e d f o r t h e OSUREM r a d a r . This r a d a r provides:
-
t h e d i r e c t i o n o f t h e h i g h frequency waterwave t h e h e i g h t o f t h e h i g h frequency waterwave t h e d i r e c t i o n o f t h e low-frequency waterwave w i t h an accuracy o f 10" t h e spectrum o f t h e low-frequency waterwave. The accuracy i s t h e frequency o f t h e maximum which i s t h e same as w i t h a buoy
-
t h e t o t a l h e i g h t o f t h e waterwave w i t h an accuracy o f 20% i n a l l conditions.
Extensive experiments were performed w i t h H 1/3 from 2 t o 7 meters. F i g u r e 2 shows a wave spectrum o b t a i n e d by t h e OSUREM r a d a r and a Datawell buoy i n a complicated sea, w i t h t h e wind b e i n g p e r p e n d i c u l a r t o t h e low-frequency waterwaves.
2.
TURBULENCE MEASUREMENT : PRINCIPLE The c u r r e n t c h a r a c t e r i s t i c s a r e deduced from t h e f i r s t - o r d e r Bragg l i n e s .
U s u a l l y t h e c u r r e n t speed i s r e l a t e d t o t h e p o s i t i o n o f t h e Bragg l i n e s . T h i s paper shows t h a t t h e shape o f t h e Bragg l i n e s g i v e s t h e s p a t i a l d i s t r i b u t i o n o f the turbulent current.
261
H 113 = 5.43 m
.05
.I0
X
OSUREM
o
BUOY
Frequency (Hz)
F i g . 2. Comparison o f l o n g wave s p e c t r a o b t a i n e d w i t h OSUREM r a d a r and a Datawell buoy.
2.1
Mean c u r r e n t measurement I f t h e mean c u r r e n t i n t h e r e s o l u t i o n c e l l o f t h e r a d a r has a non z e r o compo-
nent i n t h e d i r e c t i o n o f t h e r a d a r beam, t h e e n t i r e e l e c t r o m a g n e t i c spectrum i s sh i f t e d by :
A f c r = - 2 - 'cr -
Fr C
A fcr : Doppler s h i f t due t o t h e mean c u r r e n t .
Vcr c/Fr
: speed o f t h e mean c u r r e n t . : r a d a r wavelength.
T h i s t y p e o f s i m p l e measurement was t h e f i r s t a p p l i c a t i o n o f HR r a d a r , i m p l e mented i n t h e U.S.A.
[5] and i n France [6].
S i n c e t h e Bragg l i n e s have a non z e r o w i d t h , i t i s necessary t o d e f i n e t h e i r p o s i t i o n w i t h t h e g r e a t e s t accuracy. The c e n t r o i d s of t h e Bragg l i n e s bounded by t h e v a l u e s 3 dB under t h e maximum, may b e used C71. I n t h i s case, t h e s t a n d a r d d e v i a t i o n o f t h e f r e q u e n c y e s t i m a t i o n i s g i v e n by:
T : t o t a l d u r a t i o n o f t h e measurement
N = -A f I/ T
A f : w i d t h o f t h e peak
262
The accuracy i s independent o f t h e way o f a v e r a g i n g i n c o h e r e n t s p e c t r a and depends o n l y on t h e t o t a l d u r a t i o n T. The a c c u r a c y v a r i e s w i t h t h e w i d t h o f t h e peak. U s u a l l y , t h e Bragg l i n e s a r e v e r y sharp. A l l t h e energy i s concentraded i n a few samples,Figure 3 shows one o f t h i s k i n d of s p e c t r a o b t a i n e d d u r i n g t h e Marsen experiment i n t h e N o r t h Sea w i t h OSUREM r a d a r . S p e c t r a l i k e t h i s one were a l s o o b t a i n e d o f f t h e S h e t l a n d I s l a n d s , i n t h e Bay o f B i s c a y o r i n t h e M e d i t e r r a n e a n Sea. B u t t h e spectrum shown i n F i g u r e 4 was measured o f f Ushant I s l a n d . The energy d i s t r i b u t i o n i n t h e Bragg l i n e i s w i d e and c a n n o t be assumed t o be Gaussian. So we choose f o r t h i s experiment t h e l i m i t s o f t h e Bragg l i n e s a t 10 dB under t h e maximum. T h i s l i m i t i s chosen i n r e l a t i o n t o t h e v a l u e of t h e f i r s t s i d e l o b e o f t h e r e c e p t i o n antenna, which i s - 1 5 dB i n r e l a t i o n t o t h e main beam. Ne w i l l see l a t e r t h a t we may e x t r a c t an a c c u r a t e t i d e stream f r o m t h e Doppler s p e c t r a w i t h t h i s method.
3/10/1979
I
15h
Radar frequency : 5.285 MHz
Af
it--
0 0
7
-1.
-.5
0 Frequency (HA
.5
F i g . 3. P r i n c i p l e o f t h e mean c u r r e n t measurement w i t h
8
narrow beam HF r a d a r .
263
5/5/1982
-I
? -1.
19h45 I 3h after high water at Brast )
Radar frequency = i4Wr direction = 302 dlstance 26 km
-.5
0 Frequency (Hzj
Radar measurements : current = .OS m/s 6 = .I8 m/s
.5
F i g . 4. T y p i c a l Doppler spectrum o b t a i n e d w i t h OSUREM r a d a r o f f Ushant I s l a n d . The Bragg l i n e s a r e v e r y wide. There a r e two measurements of t h e c u r r e n t , w i t h t h e p o s i t i v e and t h e n e g a t i ve Bragg l i n e s . We use o n l y s p e c t r a w i t h a d i f f e r e n c e between t h e two measurements l o w e r t h a n t h e t h e o r e t i c a l s t a n d a r d d e v i a t i o n d e f i n e d e a r l i e r . T h i s i s t h e case i n g e n e r a l . I t i s an i m p o r t a n t r e s u l t o b t a i n e d w i t h HF r a d a r s : t h e Bragg l i n e s a r e always n e a r l y a t t h e p o s i t i o n deduced from t h e d i s p e r s i o n equat i o n f o r t h e waterwaves, even w i t h v e r y rough seas. The c u r r e n t i s t a k e n as t h e mean of t h e two measurements. The d e p t h a t which t h e c u r r e n t i s measured i s a f r a c t i o n o f t h e r a d a r wavel e n g t h . A t 10
2.2
MHz, t h i s d e p t h i s a few m e t e r s .
Turbulence measurement
As we have seen, t h e Bragg l i n e s a r e f a r f r o m a D i r a c f u n c t i o n f o r t h e Ushant experiment. Some work was done on u s i n g t h e w i d t h o f t h e Bragg l i n e s t o e x t r a c t w i n d speed. I t seems t h a t t h i s w i d t h i s e s s e n t i a l l y due t o t h e advect i o n o f t h e Bragg waves by inhomogeneous c u r r e n t s i n t h e r e s o l u t i o n c e l l o f t h e r a d a r . These c u r r e n t s may b e o r b i t a l motions o f l o n g waves o r t u r b u l e n t s u r f a c e c u r r e n t s . F o r t h e Ushant experiment, o r b i t a l m o t i o n s a r e one o r d e r o f magnitude l e s s t h a n t h e Doppler w i d t h o f t h e Bragg l i n e s . So we e x p e c t t h i s w i d t h t o be r e l a t e d t o t h e t u r b u l e n t surface c u r r e n t .
264
The power d e n s i t y a t a g i v e n frequency i n a Bragg l i n e i s r e p r e s e n t a t i v e o f t h e sea a r e a o c c u p i e d b y t h e c u r r e n t w i t h t h e c o r r e s p o n d i n g speed. So what i s measured i s t h e sea area where t h e c u r r e n t has t h i s g i v e n speed. More e x a c t l y
i t i s t h e component o f t h e t u r b u l e n t c u r r e n t a l o n g t h e r a d a r beam w h i c h i s used. The power r e c e i v e d f r o m a sea a r e a ( F i g . 5) i s p r o p o r t i o n a l t o t h e Bragg w a t e r wave energy i n t h e area and t o t h e a r e a . So t h e t o t a l power i n t h e spectrum f o r a g i v e n Doppler s h i f t i s p r o p o r t i o n a l t o t h e t o t a l a r e a where t h e c u r r e n t speed corresponds t o t h i s Doppler s h i f t . B u t t h i s i s t r u e o n l y i f t h e r e i s no c o r r e l a t i o n between wave power and c u r r e n t speed. T h i s p o i n t w i l l be d i s c u s s e d i n t h e l a s t s e c t i o n o f t h e paper.
.I
Radar beam /
i-
cell resolution
F i g . 5. P r i n c i p l e of s p a t i a l t u r b u l e n c e measurement. From t h e s t a n d a r d d e v i a t i o n o f t h e Bragg l i n e s c o n s i d e r e d as a d i s t r i b u t i o n , we deduce a c h a r a c t e r i s t i c of t h e t u r b u l e n t c u r r e n t , i . e . t h e s t a n d a r d d e v i a t i o n o f t h e s p a t i a l d i s t r i b u t i o n of i t s component i n t h e r a d a r beam d i r e c t i o n . As i s w e l l known i n t u r b u l e n c e t h e o r y , t h i s s p a t i a l measurement i s v e r y d i f f e r e n t f r o m c o n v e n t i o n a l p o i n t measurement e v o l v i n g w i t h t i m e . An a t t e m p t was made t o s t u d y t h e e f f e c t of t h e measurement d u r a t i o n on t h e r e s u l t s . There was no s i g n i f i c a n t d i f f e r e n c e between 3 and 20 m i n u t e s . 3. 3.1
THE USHANT EXPERIMENT The r a d a r s i t e The OSUREM r a d a r was i n s t a l l e d f o r t h r e e months on t h e B r i t t a n y c o a s t oppo-
s i t e Ushant I s l a n d ( F i g . 6 ) . The r a d a r was l o o k i n g i n f i v e d i r e c t i o n s . The 302" d i r e c t i o n met a p o i n t c a l l e d p o i n t 429. A t t h a t p o i n t , t h e t i d e stream had been measured by t h e French Hydrographic O f f i c e (SHOM). 3.2
The t i d e o f f Ushant I s l a n d The K e l v i n wave coming f r o m t h e s o u t h e n t e r s t h e channel as a p r o g r e s s i v e
wave ( F i g . 7 ) . The t i d e stream may b e r e p r e s e n t e d as a h o r i z o n t a l e l l i p s e . The c u r r e n t t u r n s c l o c k w i s e w i t h a s e m i d i u r n a l p e r i o d ( F i g . 8 ) . The c l o s e r we g e t t o t h e coast, t h e f l a t t e r t h e e l l i p s e becomes. F a r from t h e c o a s t , a t about
266
100 km, t h e e l l i p s e i s almost round
Radar dlrectlon
-7 20km
F i g . 6. The measurement area o f f Ushant I s l a n d
55km u
liner of constant range F i g . 7. The t i d e o f f t h e c o a s t o f B r i t t a n y .
266
m/
1.
POINT 429 .5
High water
/\Neop
Tlde/
-1.
S F i g . 8. T i d a l e l l i p s e s a t p o i n t 429 ( f r o m t h e t i d e t a b l e s o f SHOM). SHOM p r o v i d e s two e l l i p s e s measured f o r neap and s p r i n g t i d e s . These e l l i p s e s
a r e measured i n a s h o r t campaign, so t h e accuracy i s n o t v e r y good, and t h e d i r e c t i o n o f t h e t i d e stream i s assumed t o be independent o f t h e t i d e power. I n France, t i d a l power i s s c a l e d w i t h a c o e f f i c i e n t which i s p r o p o r t i o n a l t o t h e t i d e g e n e r a t i n g f o r c e . The mean neap t i d e has t h e c o e f f i c i e n t 45, and t h e mean s p r i n g t i d e t h e c o e f f i c i e n t 95, i n d e p e n d e n t l y o f t h e a b s o l u t e v a l u e o f t c u r r e n t speed o r o f t h e t i d e h e i g h t . O f f Ushant I s l a n d , t h e c u r r e n t speed i s g r e a t e r t h a n 1 m/s f o r t h e mean s p r i n g t i d e and i n an area 50 km wide. A l l t h area i s i n c l u d e d i n t h e c o n t i n e n t a l s h e l f around 100 meters deep. D u r i n g a s e m i d i u r n a l p e r i o d , a w a t e r p a r t i c l e f o l l o w s an e l l i p s e 10 km w i d e .
So i n t h e area t h e t i d e i s v e r y s t r o n g and i s t u r n i n g around an i s l a n d . The d e p t h i s g r e a t enough n o t t o d i s s i p a t e t u r b u l e n c e , b u t s h a l l o w enough t o g e n e r a t e t u r b u l e n c e w i t h i t s main topography. 3.3
M e t e o r o l o g i c a l and oceanographical f e a t u r e s The dominant wind comes f r o m t h e west, f r o m t h e A t l a n t i c Ocean. The s t r o n g -
261
e s t winds u s u a l l y t u r n f r o m southwest t o n o r t h w e s t as t h e d e p r e s s i o n moves northward. D u r i n g t h e Ushant experiment i n t h e s p r i n g o f 1982, t h e winds were almost g e n t l e i n a l l d i r e c t i o n s . The t h e r m o c l i n e i s e s t a b l i s h e d i n J u l y . A t t h a t t i m e t h e r e a r e a l o t o f i n t e r n a l waves coming f r o m t h e southhest, and oceanic f r o n t s appear i n t h e a r e a . B u t i n t h e s p r i n g , we may assume t h a t t h e t h e r m o c l i n e i s n o t e s t a b l i s h e d , even i f we have n o t measured i t .
4.
MEASUREMENT OF THE TIDAL STREAM As s a i d e a r l i e r , t h e mean c u r r e n t i s deduced f r o m t h e b a r y c e n t e r s o f t h e
Bragg l i n e s . The measurement i s accepted i f t h e two Bragg l i n e s g i v e t h e same c u r r e n t speed. The r e s u l t s a r e compared t o t h e t i d e t a b l e p u b l i s h e d by SHOM 181. There a r e s e v e r a l p o i n t s o f measurement o f t h e t i d a l stream. P o i n t 429 i s c l o s e t o a r a d a r c e l l ( F i g . 6 ) . The d e p t h o f t h e SHOM measurement i s 3 t o 5 m e t e r s . To compare t i d e streams i n d e p e n d e n t l y o f t h e t i d e ranges, we n o r m a l i z e d them i n t h e f o l l o w i n g way : we assumed t h a t t h e t i d e stream f o r a g i v e n c o e f f i c i e n t i s o b t a i n e d by i n t e r p o l a t i o n between t h e c o e f f i c i e n t s 45 and 95 ; we c a l c u l a t e t h e e q u i v a l e n t t i d e stream f o r c o e f f i c i e n t 95. The comparisons o f r a d a r c u r r e n t speed w i t h t h e p r o j e c t i o n o f t h e t i d e stream on t h e r a d a r beam a r e shown i n F i g u r e s 9, 1 0 and 11. Each p o i n t i s one r a d a r measurement. The dashed l i n e s a r e t h e t i d e stream v a r i a t i o n s deduced f r o m t h e t i d e t a b l e s . The crosses r e p r e s e n t t h e mean and t h e s t a n d a r d d e v i a t i o n w i t h i n one h o u r . The r e s u l t s a r e a c c u r a t e f o r l o w ranges o f t i d e . R e s u l t s a r e a l s o f a i r l y good f o r g r e a t c o e f f i c i e n t s b u t a l i t t l e d i f f e r e n t . S i n c e t h e aim o f t h e experiment was n o t t h e c u r r e n t measurement, t h e r e were a few measurements f o r t h o s e c o e f f i c i e n t s when t h e t u r b u l e n c e was v e r y s t r o n g . Me may c o n c l u d e t h a t t h e mean c u r r e n t measurement i s a good one. This r e s u l t i s i m p o r t a n t because i t means t h a t t h e Bragg l i n e s a r e p r o b a b l y what we t h i n k t h e y a r e , i . e . a r e p r e s e n t a t i o n of t h e c u r r e n t i n a c e r t a i n r e s o l u t i o n c e l l o f t h e r a d a r we know, c l o s e t o p o i n t 429. The r e s u l t s a r e v e r y bad i f we t r y t o compare r a d a r measurements i n t h i s r e s o l u t i o n c e l l t o o t h e r p o i n t s o f measurement o f SHOM, even i f t h e y a r e n o t f a r away. So t h e d e g r a d a t i o n o f t h e r e s u l t s on t u r b u l e n c e , which we w i l l see
n t h e next section,
i s n o t p r o b a b l e . These d e g r a d a t i o n s s h o u l d l e a d t o a change
n t h e mean c u r r e n t
speed o b t a i n e d by t h e r a d a r . 5.
TURBULENCE MEASUREMENTS : RESULTS As we saw b e f o r e , we c h a r a c t e r i z e t h e t u r b u l e n c e w i t h t h e s t a n d a r d d e v i a t i o n
CJ
o f t h e c u r r e n t d i s t r i b u t i o n g i v e n b y t h e shape o f t h e Bragg 1 i n e s . o i s c a l c u -
l a t e d o n l y w i t h t h e most p o w e r f u l Bragg l i n e . R e s u l t s f o r u a r e n o t n o r m a l i z e d by t h e t i d e range. So F i g u r e s 1 2 and 13 g i v e a b s o l u t e v a l u e s o f u v e r s u s t i m e
268 i n a t i d e period.
1:-
Current (cm/s)
90 60 -
Polnt 429
r
-
30
pf
0 -
-
-30
-
Range of tide (adlmenslonnal) = < 55 Radar direction = 302' distance = 26 km
-
-60
I
11
-90
0
2 4 6 8 10 12 hours Time after high water at Bred
F i g . 9. T i d a l stream component a l o n g t h e r a d a r beam. Dashed l i n e : SHOM measurements P o i n t s : OSUREM measurements w i t h mean and s t a n d a r d d e v i a t i o n shown by t h e crosses. Low ranges of t i d e .
269
Current (cm/s) a
-
60 90
.I
I
tI \
-
/
/-Point
429
30 -
-
0-
-60 -30
.
1
-90
Range of tide ladimensionnail = 55-60 Radar direction = 302' distance = 26 km
I
I
0
1
2
1
1
4
1
6
1
8
~
~
10
Time after high water at Brest
F i g . 10. L i k e F i g u r e 9, b u t f o r m i d d l e ranges of t i d e .
~
'
-
12 hours
'
1
270
Current (cm/s)
1
90
Range of tide lodlmanslonnal) = > 80 Radar dlrectlon = 302'
-
60
-
30
-
26 km
-
0 -
-30 -
-90-60
I 1
0
1
1
2
1
1
4
1
1
6
1
,
8
1
1
1
10
1
w
12 hours
Time after high water at B r e d
F i g . 11. L i k e F i g u r e 9 and 10, b u t f o r h i g h ranges o f t i d e
271
Range of tide (adirnensionnal) = 55 - 80 Radar direction = 302' distance = 26 krn
6 (cm/s)
50 40
I
a
0
0
a 'I
-
30 -
20 10
0
0
2
4
6
8
10
12 hours
Time after high water at Brest
F i g . 1 2 . Turbulence measurement f o r middle ranges of t i d e .
272
Rang. of tld. (adlmmslennal) --
-- > -.-.-< a
80
55 50
- 80
Radar draction = 302' dlstonce = 26 km
\
\
Time after high water at Brest F i g . 13. Turbulence measurement. Dependence on t h e range o f t i d e .
The t u r b u l e n c e i s seen t o be connected t o t h e c u r r e n t v e l o c i t y , i . e . t o i t s v a r i a t i o n i n t h e s e m i d i u r n a l t i d e p e r i o d , and t o t h e t i d e range. When somebody measures t h e c u r r e n t
versus t i m e a t a p o i n t i n t h e sea, he
c o n s i d e r s as normal v a r i a t i o n s o f t h i s c u r r e n t o f 10%. Here, t h i s v a l u e l e a d s t o 13 cm/s f o r t h e maximum c u r r e n t w i t h t h e maximum t i d e range ( a p p r o x i m a t e l y t h e c o e f f i c i e n t 1 1 0 ) . The r e s u l t s o b t a i n e d here a r e more t h a n two t i m e s h i g h e r . The d i s c u s s i o n o f why t h e t u r b u l e n c e i s so h i g h i n t h a t area i s n o t t h e purpose o f t h i s paper. We may suppose i t i s a c h a r a c t e r i s t i c o f t h a t area, b u t a l s o t h a t s p a t i a l measurement o f t h e t u r b u l e n c e i s n o t u s u a l . Everybody knows t h a t ocean i c d i f f u s i o n i s o f t e n h i g h e r t h a n d i f f u s i o n c a l c u l a t e d w i t h s i m p l e models. So turbulent dispersion i s usually calculated w i t h empirical coefficients. 6 . DISCUSSION
T h i s s e c t i o n argues about t h e r e s u l t s t o show t h a t no a r t e f a c t was found w h i c h would have been a b l e t o g i v e such w i d e Bragg l i n e s , e x c e p t f o r t u r b u l e n c e . Oceanographic remote s e n s i n g i s a new s c i e n c e . The sensors used i n t r o d u c e
273 new concepts, and i t i s n o t easy t o connect t h e i r measurements w i t h more convent i o n a l ones. I t i s always necessary t o s t u d y t h e i r r e s u l t s c a r e f u l l y i n o r d e r t o e l i m i n a t e e v e n t u a l a r t e f a c t s which may damage t h e r e s u l t s . I n t h i s s e c t i o n we w i l l r e v i e w t h e p o s s i b l e phenomena w h i c h may produce t h e broadening o f t h e Bragg l i n e s .
6.1.
The s i d e l o b e s o f t h e r e c e p t i o n antenna
The r e c e p t i o n antenna i s composed o f s i x t e e n s i m p l e o m n i d i r e c t i o n a l antennas. The f i r s t s i d e l o b e i s
-
16 dB below t h e main l o b e . There i s a n a t u r a l p r o t e c -
t i o n a g a i n s t l o b e s a t more t h a n 90" f r o m t h e r a d a r beam by more t h a n 10 km o f land. An a t t e m p t was made t o s t u d y t h e importance of s i d e l o b e s . We chose a p e r i o d when t h e wind was up t o t h e r a d a r . I n t h i s case t h e e f f e c t o f s i d e l o b e s i s below t h e l i m i t o f
-
I0 dB which i s used. There i s no d i f f e r e n c e between these
r e s u l t s and t h o s e o b t a i n e d i n o t h e r w i n d d i r e c t i o n s . Another i m p o r t a n t i t e m i s t h a t t h e mean c u r r e n t i s e f f e c t i v e l y compared t o t h e t i d e stream, and t h a t even i n upwind c o n d i t i o n s , t h e mean c u r r e n t i s t h e same f o r t h e n e g a t i v e and p o s i t i v e Bragg l i n e s . The shape o f t h e Bragg l i n e s i s o f t e n f l a t l i k e i n F i g u r e 4. S i d e l o b e s would have produced o t h e r shapes, w i t h o u t any f l a t c e n t r a l p a r t . F i n a l l y , t h e r e i s no d i f f e r e n c e between t h e two f r e q u e n c i e s 7 and 14
MHz.
Perhaps t h e l o b e s a r e t o t a l l y d i f f e r e n t . So we may say t h a t t h e r e s u l t s a r e n o t due t o t h e r a d a r c h a r a c t e r i s t i c s and
t h a t a l l t h e s i g n a l s came from t h e s e l e c t e d r e s o l u t i o n c e l l .
6.2.
The e f f e c t o f t h e wind
We t r i e d t o f i n d a c o r r e l a t i o n between t h e w i d t h o f t h e Bragg l i n e s and t h e wind speed i n t h e area. The w i n d was o f t e n v e r y l o w . O b v i o u s l y no c o r r e l a t i o n was found i n t h i s case. B u t even w i t h s t r o n g winds, we o b t a i n e d t h e same r e s u l t . We suppose t h a t t h e w i n d i s r e s p o n s i b l e f o r some d i s p e r s i o n i n t h e r e s u l t s f o r t h e mean c u r r e n t . B u t s i m p l e c o r r e c t i o n s which were s u c c e s s f u l f o r t h e Marsen experiment 161, were n o t i n t h i s one. Stokes d r i f t i s u s u a l l y low, much l o w e r t h a n t h e r e s u l t s o b t a i n e d f o r u
1 9 1. W i t t e [ l o 1 found t h a t d i s p e r s i o n was a l o t h i g h e r t h a n t h a t e x p l a i n e d by t h e Stokes d r i f t . S i n c e t h e r e i s a s t r o n g w a v e - c u r r e n t i n t e r a c t i o n i n some p a r t s o f t h e a r e a c l o s e r t o t h e c o a s t [111, we s t u d i e d i t s e f f e c t on t h e Doppler spectrum. I n t h e case o f c r o s s w i n d c o n d i t i o n s , t h e Bragg l i n e s would be symmetrical about a frequency n e a r z e r o . B u t t h e shapes observed a r e t h e e f f e c t o f a 2 fB t r a n s l a t i o n o f one Bragg l i n e on t h e o t h e r , i n s t e a d o f a symmetry.
274
Other f e a t u r e s The u value d o e s n ' t depend on the time s c a l e . Measurements were made f o r 3 and 20 minuts. The r e s u l t s a r e already the same. So we a r e not measuring basic a l l y time-varying process. Since the r e s u l t s a r e independent of the radar frequency, they d o n ' t depend on the current v a r i a t i o n with depth i n t h e f i r s t meters. This i s not s u r p r i s i n g f o r a turbulence on t h e s c a l e of kilometers. 6.3
Finally, t h e r e a r e a few v a r i a t i o n s of 0 with the distance from the c o a s t . of t h e c e l l . So t h e r e i s no B u t the f a r t h e r we go the g r e a t e r i s t h e width d e f i n i t e conclusion on t h a t point. CONCLUSION The HF narrow-beam radar may measure a turbulence c h a r a c t e r i s t i c , i n addit i o n t o wind d i r e c t i o n , long-wave directional s p e c t r a , t o t a l wave height and current speed. This turbulence value, which i s the s p a t i a l standard deviation of t h e current speed, i s not a c l a s s i c one. I t s values a r e surprinsingly high o f f Ushant Island. I t i s well known t h a t t h e wind e f f e c t on current i n t h a t area i s very g r e a t ... and d i f f i c u l t t o c a l c u l a t e . We may assume t h a t some variat i o n s of current with time have been supposed t o proceed from t h e wind e f f e c t , and a r e in f a c t due t o turbulence on t h e space s c a l e of 5 t o 10 km.
REFERENCES [l] Broche, P . ,
121 131 [4]
151 161 171 [8] [9]
1979. Sea s t a t e directional spectra observed by HF Doppler radar. Agard Conf. Proc., 263, 31.1-31.12. Parent, J . and Delloue, J . , 1982. Determination de l a d i r e c t i o n d u vent a l a surface de l a mer au moyen d ' u n radar a r e t r o d i f f u s i o n ionospherique. Ann. Geophys., t . 38, f a s c . 6, pp.863-873. Gay, H., Blanchet, C . , Nicolas, J . and Piau, P . , 1982. Determination of wind and s h o r t wave d i r e c t i o n a t g r e a t distances with OSUREM r a d a r . In Wave and Wind D i r e c t i o n a l i t y . Ed. Technip, P a r i s . Forget, P . , Broche, P . , De Maistre, J.C. and Fontanel, A . , 1981. Sea s t a t e frequency f e a t u r e s observed by ground wave HF Doppler r a d a r . Radio Science, Vol. 16, No 5, pp.917-925. Lipa, B . and Barrick D . , 1982. Codar measurements of ocean surface parameters a t ARSLOE. Preliminary r e s u l t s . Oceans 82. Janopaul, M.M. e t a1 ., 1982. Comparison of measurements of sea currents by HF radar and by conventional means. I n t . 3 . Remote Sensing, vol. 3, NO 4, p p . 409-422. Barrick, D. and Snider, J . B . , 1977. The s t a t i s t i c s of HF sea-echo Doppler s p e c t r a . I . E . E . E . Trans. on Antennas and Propagation, vol. AP-25, No 1. Service Hydrographique e t Oceanographique de l a Marine, 1968. Tome No. 550. Courants de maree dans l a Manche e t s u r l e s c6tes franGaises de 1 'Atlantique. Broche, P . , de Maistre, J . C . and Forget, P . , 1983. Mesure par radar decametrique coherent des courant5 s u p e r f i c i e l s engendres par l e vent. Oceanologica Acta, Vol. 6, n o 1.
275
[lo] Witte, H . e t al.,
1982. Small s c a l e d i s p e r s i o n measurements o f d r i f t e r buoys i n t h e N o r t h Sea. F i r s t i n t . cong. on m e t e o r o l o g y and a i r / s e a i n t e r a c t i o n o f t h e c o a s t a l zone. The Hague, May 10-14. I l l ] Cavani@, . A . , E z r a t y , R. and G o u i l l o n , J . P . , 1982. T i d a l c u r r e n t modulat i o n s o f wave d i r e c t i o n a l s p e c t r a parameters measured w i t h a p i t c h and r o l l buoy west o f Ushant i n w i n t e r , F i r s t i n t e r n a t i o n a l conference on m e t e o r o l o g y and a i r / s e a i n t e r a c t i o n o f t h e c o a s t a l zone. The Hague. May 10-14.
This Page Intentionally Left Blank
277
A Q U A S I GEOSTROPHIC MODEL OF THE CIRCULATION OF THE MEDITERRANEAN S E A
Laurent LOTH (*) and Michel C R E P O N (**)
(*) I N R I A
-
Domaine de Voluceau - Rocquencourt - B.P. 1 0 5 - 78150 - L E CHESNAY -
France
(**) Laboratoire d'Oc6anographie Physique - Museum National d' Histoire Naturelle LA175
-
CNRS
-
-
43 Rue Cuvier - 75005 P A R I S - France
Abstract A quasi geostrophic model o f t h e Mediterranean sea i s solved b y using a f i n i t e element
method.
The barotropic
and baroclinic
mode are computed independently.
The Alboran Sea gyre i s observed i n both models but it i s less intense than i n nature. When penetrating t h e Mediterranean sea t h e Alboran sea current overshoots t o t h e North, then becomes trapped by t h e Algerian shore.
1.INTR 0 D U CTIO N The Mediterranean Sea is a concentration basin. Evaporation creates a mass d e f i c i t i n t h e whole basin which i s compensated by an inflow o f Atlantic water passing through the s t r a i t o f Gibraltar and through t h e s t r a i t o f Sardinia. The incoming A t l a n t i c water which i s l i g h t i s t r a n f o r m e d i n t o dense water by a complicated convective process (Gascard
-
1978). This dense water f o r m s a deep l a y e r which f l o w s out i n t o t h e A t l a n t i c
ocean.These fluxes strongly influence t h e general circulation of t h e sea. From a schemat i c "point o f view",
t h e Mediterranean sea can be considered as a , t w o l a y e r ocean.
In t h e subsequent we focus our i n t e r e s t on t h e barotropic and baroclinic circulation
o f t h e western basin f o r c e d b y t h e fluxes through t h e t w o straits.
2. T H E MODEL Since we are interested i n low frequency phenomena we deal with t h e quasi geostrophic version o f t h e shallow water equations. The governing equations are, i n a coordinate f r a m e with x positive east-ward and y positive northward.
where Y
i s t h e stream function (u=-%;,
v=E)
R i s t h e i n t e r n a l Rossby radius o f deformation. (1/R2 i s set equal t o zero t o obtain t h e barotropic mode) 5 t h e variation r a t e o f t h e Coriolis parameter P
( 5 = 2.10- 11s -1m -1)
t h e density
D t h e depth A t h e horizontal turbulent viscosity c o e f f i c i e n t E
t h e bottom f r i c t i o n parameter (
E
=
( A = 512.m2s-')
5.11)-~s-l)
We only study t h e motion generated by fluxes o f water f l o w i n g through t h e straits o f Gibraltar and Sardinia. I n t h i s study t h e f o r c i n g due t o t h e wind i s neglected. I n order t o satisfy t h e mass continuity (Pedlosky, 1979) it can be shown t h a t
Thus, t o solve (1) subject t o (2) we l e t (Holland, 1978) YJ = Y o + c(t) Y
where Y
1
i s a solution o f
a(2,
-+)
at
t h a t t h e t i m e independent f i e l d Y function Y
= 0 with
Yl
=
1 on boundaries. Note
needs t o be determined only once. The Stream
i s a solution o f (l), w i t h Y
= Y
0
= 0 on t h e south boundary ( A f r i c a n
- y o N i s equal t o t h e f l u x o f water f l o w i n g through t h e strait, with Coast) and Y N yo = Y on t h e northern boundary (European coast). Now, condition (2) determines c(t) a t each instant, i-e.
279
3. M E T H O D O F SOLUTION I n order t o approximate t h e coastline geometry as closely as possible, we have chosen a numerical f i n i t e element approach, using a triangular grid (Fig. 1). The model i s f o r c e d by imposing velocity profiles a t t h e t w o straits, t h e fluxes o f which are equal.
We s t a r t f r o m r e s t a t t = 0 and t h e t w o fluxes reach a constant
value i n one month. A t each strait, t h e boundary condition i s imposed a t t h e end o f a canal t h e length o f which i s f o u r grid size. This allows t h e f l u i d t o adjust i t s e l f before entering t h e sea and prevents unrealistic forcings i n t h e basin. The t i m e discretization i s a leap f r o g scheme with a Matsuno scheme every nine steps. The f i n i t e elements are interpolated by linear functions (Dumas e t a1
-
-
1982, Dumas
1982).
The bottom i s assumed t o be f l a t . The barotropic and baroclinic modes are solved separatly (1/R2 = 0 f o r t h e barotropic mode i n 1). This implies t h a t baroclinic unstabil i t y i s n o t taken i n t o account. The depth o f t h e upper layer i s 200 m and t h e reduced
g r a v i t y parameter g' i s 10-Zms-2 (g'=g AP / p ) i.e t h e i n t e r n a l radius o f deformation R i s equal t o 40 km. This value i s l a r g e r than t h e actual one, b u t allows us t o deal w i t h a minimum number o f triangles and t o respect t h e dynamical constraints between t h e i n t e r n a l radius o f deformation and t h e grid size which i s taken about h a l f o f t h i s value i.e. 20 km. A t t h e coast a f r e e slip condition i s used.
Fig. 1 : F i n i t e elements grid used f o r t h e Mediterranean sea.
280
Many runs were performed i n order t o check t h e sensitivity o f t h e model t o t h e width o f t h e s t r a i t o f Gibraltar, t o t h e velocity profile, t o t h e magnitude o f t h e incoming f l u x e and t o t h e eddy viscosity coefficient. The f i n a l runs were done with r e a l i s t i c parameters. The width o f t h e s t r a i t o f Gibraltar (Sardinia) was 20 km (160 km).
According t o Lacombe and Richez
t h e incoming (and out-going) f l u x was 0.32
Sverdrup f o r t h e barotropic
-
1982
-,
model and
1.6 Sverdrup f o r t h e baroclinic one. The viscosity c o e f f i c i e n t A was 512 m2s-'.
In
t h e s t r a i t o f Gibraltar t h e grid size i s 10 km. This makes it possible t o vary t h e velocit y p r o f i l e o f t h e forcing.
I n t h e following runs assym metric parabolic profiles are
used (Fig. 3). Equilibrium i s reached a f t e r 400 days i n t h e barotropic case, a f t e r 700 days i n t h e baroclinic case. The t i m e step i s 4 hours.
4. RESULTS I n both runs, t h e main c u r r e n t i s deviated t o t h e northern coast o f t h e Alboran sea (Fig. 2 and 4). A p a r t o f t h i s f l o w i s recycled southward and f o r m s one o r t w o anticyclonic eddies. I n t h e barotropic case one observes t h e f o r m a t i o n o f t w o weak anticyclonic eddies which are separated a t t h e l e v e l o f Cape Tres Forcas (Fig. 3). I n t h e baroclinic case, there is one strong anticyclonic eddy which extends through t h e whole sea (Fig. 5, 6). When penetrating t h e Mediterranean sea, t h e c u r r e n t overshoots t o North. This overshooting could be responsible f o r t h e f r o n t which extends between t h e Balearic Islands and Sardinia and which i s o f t e n observed on i n f r a - r e d s a t e l l i t e images (Fig. 7) (Deschamps e t a1
-
1984). Then t h e c u r r e n t bends southward
and f l o w s along t h e Algerian coast. The p a t t e r n o f t h e circulation i n t h e Alboran sea i s strongly dependent on t h e v o r t i c i t y o f t h e forcing. If t h e v o r t i c i t y i s positive, t h e Alboran gyre is enhanced, if t h e v o r t i c i t y i s negative, t h e main c u r r e n t i s n o t any more deviated t o t h e coast
o f Spain b u t f o l l o w s t h e coast o f Morocco and t h e gyre disappears. Following Holland (1978), several length scales o f i n t e r e s t are defined Wi = ( u / ~)'I2 = 70 km W
S
= E / B = 10 km
m = 2(+'13
=
60 km
1
where u i s a t y p i c a l velocity ( u = 0.1 ms- ) The length scales Wi,
W s and W m are respectively t h e width o f t h e western boun-
d a r y c u r r e n t when i n e r t i a l e f f e c t s dominate and when bottom f r i c t i o n dominates and when l a t e r a l f r i c t i o n dominates. These values show t h a t t h e circulation i n t h e Alboran sea i s strongly dependent on i n e r t i a l e f f e c t and l a t e r a l f r i c t i o n .
Thus, an analysis
o f t h e motion i n t e r m s o f v o r t i c i t y balance must include t h e f r i c t i o n term.
281
Fig.2. equal t o 0.32
I .\
Stream lines o f t h e barotropic Sverdrup.
model. The fluxes a t t h e straits are
The distance between t w o stream lines i s 0.032
Sverdrup.
m e t r i c parabolic
n the Strait
Fig.3.
Barotropic model. Enhanced p i c t u r e o f t h e stream lines i n t h e Alboran
sea. The distance between t w o stream lines i s 0 . 0 0 3 Sverdrup. The velocity p r o f i l e o f t h e f o r c i n g i n t h e Gibraltar s t r a i t i s shown i n t h e upper l e f t corner.
282
5 O
Fig.4.
O0
Stream lines o f t h e baroclinic
5 E
model. The fluxes a t t h e straits are
equal t o 1.6 Sverdrup. The distance between t w o stream lines i s 0.16 Sverdrup.
Fig.5. sea.
Baroclinic model. Enhanced p i c t u r e o f t h e stream lines i n t h e Alboran
The distance between t w o stream lines i s 0.05
of t h e f o r c i n g i s t h e same as i n Fig.3.
Sverdrup.
The velocity profile
283
ALGERIA
Fig.6. Baroclinic model. Velocity vectors in the Alboran sea.
Fig.7. Pattern of the thermal front between the Balearic Islands and Sardinia a t different months of the year 1978 observed from N O A A satellite (from Deschamps e t al. 1984) - (5 is May, 6 June, 7 July, 8 August).
284
5. CONCLUSION The f i n i t e element technics i s a valuable t o o l t o study t h e mediterranean circulation. The model supports r e a l i s t i c i n f l o w s o f A t l a n t i c water passing through t h e s t r a i t o f Gibraltar. Despite t h e over-simplification o f t h e model, many features o f t h e circulat i o n as t h e Alboran gyre (Lanoix - 1974) are reproduced. But it i s noted t h a t t h e circulat i o n i n t h e northern p a r t o f t h e Basin i s not obtained. I n particular, t h e strong cyclonic gyre (Crepon e t al. 1982) existing between France and Corsica i s n o t observed. The n e x t stage i s t o include t h e wind stress and t h e bottom topography by dealing with a t w o layer model which can t r i g g e r t h e Alboran sea gyre more intensely. The computations were done on t h e Cray 1 o f French Research.
A C K N 0 W L E D G E M E N TS This work was supported by f r e n c h c o n t r a t D r e t N081/1117 and by C N R S and C N E X O . The f i n i t e element model was k i n d l y provided by C. L e Provost. Discussion with P.
Delecluse and J.C.
Gascard,
C.
M i l l o t have been very helpful1 throughout
t h i s work.
REFER E N C ES Wald L. and Monget J.M. - 1982 - L o w frequency waves i n t h e Ligurian
Crepon M.,
sea during December 1977. J.G.R Oeschamps P.Y.,
Vol. 82 C 1 pp 595-600.
Frouin R. and Crepon M. - 1984
-
Sea surface temperature o f t h e
coastal zones o f France observed by t h e H C M M satellite - J.G.R ( i n Press) Dumas E. - 1982 - Modelisation des circulations oceaniques ge'nerales par des mgthodes aux 6 E m e n t s finis. These Dumas E.,
-
University o f Grenoble - June 1982.
L e Provost C. and Poncet A.
-
1982 - Feasibility o f f i n i t e element methods
f o r oceanic general circulation modeling. I n Proc. o f 4th. Int. Conf. on f i n i t e elements i n Water Res. H A N O V E R - 1982. Gascard
J.C.
-
1978 -
Mediterranean
deep
water f o r m a t i o n ; baroclinic
and ocean eddies - Oceanologica Acta - Vol. 1, NO3
- pp.
315-330.
instability
285
Holland B. ocean; VOl.
-
1978
numerical
-
The r o l e of Experiment
mesoscale eddies i n t h e general circulation of t h e
Using a wind-driven
quasi geostrophic
model.
J.P.O.
8 NO3 pp. 363-392.
Lacombe ti. and Richez C.
-
1982 - The regime o f t h e s t r a i t o f Gibraltar i n hydrody-
namics o f semi-enclosed seas. I n hydrodynamics o f semi-enclosed seas by J.C.J
Nihoul
(Editor), Elsevier, Amsterdam, pp. 13-73. Lanoix F. - 1974 - Project Alboran
-
Etude hydrologique e t dynamique de l a mer d'Albo-
r a n - N A T O technical r e p o r t 66, 39 p. Pedlosky J. - 1979 - Geophysical Fluid Dynamics - Springer Verlag - 624 p.
This Page Intentionally Left Blank
287
SOME APPLICATIONS OF REMOTE SENSING TO STUDIES I N THE BAY O F BISCAY, CELTIC SEA AND ENGLISH CHANNEL R.D.
PINGREE
I n s t i t u t e of Oceanographic S c i e n c e s , Wormley, S u r r e y , GU8 5UB, England
ABSTRACT Infra-red,
C o a s t a l Zone Colour Scanner and S y n t h e t i c A p e r t u r e
Radar images have been used t o i d e n t i f y s e a s u r f a c e s t r u c t u r e s i n t h e B i s c a y , C e l t i c Sea and E n g l i s h Channel r e g i o n s .
Attention
h a s been f o c u s s e d on s h e l f - b r e a k c o o l i n g , s h e l f - b r e a k c h l o r o p h y l l ' a ' , Biscay e d d i e s , i n t e r n a l waves and t u r b i d i t y s t r u c t u r e s i n t h e E n g l i s h Channel and extended where p o s s i b l e w i t h examples drawn from work a t s e a .
INTRODUCTION
One of t h e most i m p o r t a n t c o n t r i b u t i o n s of remote s e n s i n g t o oceanographic and s h e l f s t u d i e s i n t h e B i s c a y , C e l t i c Sea and E n g l i s h Channel h a s been t o p r o v i d e c l e a r i l l u s t r a t i o n s of a v a r i e t y of p h y s i c a l and b i o l o g i c a l phenomena.
With a c l e a r
p i c t u r e i n mind of t h e p r o c e s s and i t s g e o g r a p h i c a l limits it becomes a r e l a t i v e l y s i m p l e m a t t e r t o i n v e s t i g a t e t h e p r o c e s s e s f u r t h e r w i t h measurements a t s e a .
For example t h e r e were no
r e p o r t s of t h e e x t e n s i v e s h e l f - b r e a k c o o l i n g i n t h i s a r e a u n t i l it had been f i r s t n o t e d i n t h e i n f r a - r e d s a t e l l i t e imagery.
The
widespread o c c u r r e n c e and p e r s i s t e n c e of t h e s h e l f - b r e a k c o o l i n g s t i m u l a t e d models of b o t h t h e M2 b a r o t r o p i c t i d a l c u r r e n t s and t h e i n t e r n a l t i d e f o r t h i s area.
I n t h i s paper some examples of
t h e k i n d s of s t r u c t u r e s t h a t can be observed u s i n g remote s e n s i n g t e c h n i q u e s a r e drawn from t h e B i s c a y , C e l t i c Sea and E n g l i s h Channel and i l l u s t r a t e s h e l f - b r e a k c o o l i n g and a s s o c i a t e d phytoplankton blooms, Biscay e d d i e s , s h e l f t i d a l f r o n t s , c o a s t a l u p w e l l i n g , f r o n t a l e d d i e s and i n s t a b i l i t i e s , i n t e r n a l waves and turbidity structures.
Some s u p p o r t i n g s e a t r u t h i s a l s o p r e s e n t e d
b u t it is clear t h a t r e a l i s t i c models f o r t h e s e p r o c e s s e s i s a s u b j e c t of f u t u r e r e s e a r c h .
288
F i g . 1. I n f r a - r e d s a t e l l i t e image (1339 GMT, 26 August 1 9 8 1 ) i l l u s t r a t i n g shelf-break cooling. The U s h a n t , S c i l l y I s l e s , Lands End, and C e l t i c S e a t i d a l f r o n t s c a n a l s o b e i d e n t i f i e d . In a d d i t i o n , c o a s t a l t i d a l f r o n t s o c c u r a l o n g t h e Armorican s h e l f . Cool w a t e r due t o p r e v i o u s wind i n d u c e d u p w e l l i n g a l s o o c c u r s o f f S o u t h w e s t I r e l a n d and t h e S p a n i s h C o a s t . High p r e s s u r e , calm wind c o n d i t i o n s e x i s t e d on 2 6 August 1981 and sea s u r f a c e t e m p e r a t u r e 'hot-spots' (see F i g . 4 ) a s s o c i a t e d w i t h w i n d l e s s h i g h p r e s s u r e c o n d i t i o n s c a n b e s e e n i n t h e C e l t i c S e a and w e s t e r n E n g l i s h Channel.
289
SURFACE TEMPERATURE STRUCTURE
1.
1.1.
She,lf T i d a l f r o n t s , banded s t r u c t u r e s and u p w e l l i n g f r o n t s
I n f r a - r e d s a t e l l i t e imagery h a s r e v e a l e d c l e a r l y t h e t i d a l f r o n t s i n t h e E n g l i s h Channel, C e l t i c Sea and Armorican s h e l f ( F i g s . 1 and 2 ) .
These t r a n s i t i o n s between t i d a l l y mixed and
s t r a t i f i e d w a t e r (Simpson and Hunter, 1 9 7 4 ; P i n g r e e and G r i f f i t h s , 1978) p e r s i s t f o r about
%
1 0 0 days o v e r t h e summer months J u n e ,
J u l y , August. The b o u n d a r i e s between mixed and s t r a t i f i e d w a t e r s appear t o be u n s t a b l e and a r e c h a r a c t e r i s e d by i r r e g u l a r e d d i e s
Fig. 2. A s k e t c h of some f e a t u r e s observed i n t h e i n f r a - r e d s a t e l l i t e imagery ( d o t t e d l i n e s ) . Also shown a r e some c u r r e n t measurements (see t e x t f o r e x p l a n a t i o n s ) . A d o t s i g n i f i e s t h e p o s i t i o n of t h e c u r r e n t meter mooring and on t h e s h e l f o n l y t h e measurements r e f e r t o t h e upper p a r t of t h e w a t e r column. A wavy arrow r e p r e s e n t s flow i n f e r r e d from s a t e l l i t e images. An arrow w i t h o u t a d o t i n d i c a t e s t h e movement of a s u r f a c e d r i f t i n g buoy. The numbers used g i v e t h e mean speed i n c m s-l. ~ l s o shown a r e t h e 1 0 0 fm and 1 0 0 0 fm c o n t o u r s .
290
which o c c a s i o n a l l y show a tendency t o be c y c l o n i c ( P i n g r e e e t a l . , 1 9 7 9 ) , as e x e m p l i f i e d by t h e Lands End f r o n t a l zone ( F i g . 1).
The
i n s t a b i l i t i e s have time s c a l e s of o r d e r 1 day and l e n g t h s c a l e s of a b o u t 2 0 km.
Long ( 3 0 km) i n t r u s i v e f i n g e r s w i t h s e p a r a t i o n s of
10-20 km c a n a l s o b e observed on t h e Ushant f r o n t and t h e s e f e a t u r e s t e n d t o be p a r t i c u l a r l y conspicuous i n September a s t h e mixed r e g i o n i n c r e a s e s i n a r e a and e x t e n d s a c r o s s t h e mouth of t h e E n g l i s h Channel.
A t t h i s t i m e of y e a r t h e Ushant t i d a l f r o n t can
show a marked s p r i n g - n e a p v a r i a t i o n i n g e o g r a p h i c a l e x t e n t . F r o n t a l i n s t a b i l i t i e s a r e t h o u g h t t o r e p r e s e n t an i m p o r t a n t agency i n t h e c r o s s f r o n t a l t r a n s f e r of w a t e r p r o p e r t i e s . T w o i n t e r e s t i n g f e a t u r e s t h a t have a l s o shown up w i t h i n f r a - r e d
imagery (which s t i l l r e q u i r e s e a t r u t h t o confirm t h a t t h e y a r e indeed r e a l f e a t u r e s of t h e sea s u r f a c e t e m p e r a t u r e s r a t h e r t h a n a t m o s p h e r i c e f f e c t s ) a r e t h e bands of a p p a r e n t l y c o l d w a t e r t h a t a p p e a r o n - s h e l f e x t e n d i n g a p p r o x i m a t e l y normal t o t h e s h e l f - b r e a k i n May and June and p a r a l l e l t o t h e s h e l f - b r e a k i n J u l y and August. The normal bands e x t e n d f o r
Q,
100-200 lan and have a wavelength
2.
15 km and p r o b a b l y r e s u l t from mixing o r i n t e r n a l t i d e s a s s o c i a t e d w i t h t h e l i n e a r t i d a l sand r i d g e s t h a t o c c u r i n t h e C e l t i c Sea on similar scales
Q,
1 5 km, f i g . 3 ( a ) .
The p a r a l l e l bands have
wavelengths of 20-30 km and a r e a l s o r e l a t i v e l y s t a t i o n a r y .
It
might be argued t h a t t h e y occur where t h e c u r r e n t s of t h e b a r o t r o p i c t i d e a r e i n phase w i t h t h e c u r r e n t s of a p r o g r e s s i v e i n t e r n a l t i d e r e s u l t i n g i n i n c r e a s e d l o c a l mixing. I n f r a - r e d s a t e l l i t e imagery h a s a l s o i d e n t i f i e d c l e a r l y t h e marked s e a s o n a l u p w e l l i n g t h a t o c c u r s a l o n g t h e Spanish and P o r t u g u e s e c o a s t and shown t h a t u p w e l l i n g a l s o o c c u r s o c c a s i o n a l l y o f f Southwest I r e l a n d and on t h a t p a r t of t h e French c o a s t which
i s a d j a c e n t t o t h e Armorican and A q u i t a i n e s h e l f . 1.2.
S h e l f Break c o o l i n g and Biscay mesoscale e d d i e s
W h i l s t some of t h e g r o s s f e a t u r e s l i s t e d above w e r e known b e f o r e t h e widespread u s e of i n f r a - r e d s a t e l l i t e imagery t h e r e
w e r e few r e p o r t s of s h e l f - b r e a k c o o l i n g and none showing t h e c h a r a c t e r i s t i c a l l y deep Biscay eddy s t r u c t u r e .
Shelf-break cooling
e x t e n d s t y p i c a l l y f o r 300 km a l o n g t h e s h e l f - b r e a k and s l o p e r e g i o n s and p e r s i s t s from l a t e May t o l a t e September and l i k e t h e t i d a l f r o n t s i s c h a r a c t e r i s e d by i r r e g u l a r s m a l l e r s c a l e structures.
S h e l f - b r e a k c o o l i n g h a s n o t been observed i n w i n t e r
291
F i g . 3 ( a ) . I n f r a - r e d s a t e l l i t e image ( 2 7 . 5 . 8 3 ) showing a p p a r e n t l y c o o l b a n d s e x t e n d i n g a p p r o x i m a t e l y normal t o t h e s h e l f - b r e a k and a s s o c i a t e d w i t h t h e s a n d r i d g e s which h a v e s i m i l a r t r a n s v e r s e wavelengths 10-15 km. The w h i t e r f e a t u r e s a r e c l o u d s and s h o u l d be i g n o r e d . The i s l a n d on t h e l o w e r r i g h t i s Ushant and t h e s t a r t of t h e Ushant f r o n t i s i n d i c a t e d by t h e w i s p y f e a t u r e s s t r e t c h i n g from t h e F r e n c h c o a s t . The p a s s c o r r e s p o n d s t o s p r i n g t i d e s and h i g h p r e s s u r e a t m o s p h e r i c c o n d i t i o n s and s e a m i s t may p e r h a p s s e r v e t o a c c e n t u a t e some of t h e s e f e a t u r e s .
t h o u g h s h e l f - b r e a k warming h a s b e e n n o t e d i n J a n u a r y 1 9 7 9 and 1982 ( a s f a r n o r t h as 4 7 O N ) and p r e s u m a b l y a s s o c i a t e d w i t h a d v e c t i o n and s p r e a d i n g of w a r m w a t e r from t h e S p a n i s h c o a s t .
Shelf-break
c o o l i n g h a s b e e n a t t r i b u t e d t o r e s u l t from m i x i n g by i n t e r n a l t i d e s simply because i t s p o s i t i o n corresponds approximately t o t h e r e g i o n where t h e M2 t i d a l c u r r e n t s have maximum v a l u e s and t h i s a s p e c t i s d i s c u s s e d i n more d e t a i l l a t e r .
However u p w e l l i n g
p r o c e s s e s , m i x i n g by t r a p p e d waves and i n e r t i a l c u r r e n t s a r e a l l thought t o play a contributing r o l e i n shelf-break cooling. C o o l e r w a t e r from t h e s l o p e s h a s been o b s e r v e d s p r e a d i n g o n t o t h e shelf f o r limited distances and Ode1 canyons.
(%
1 5 km) from Penmarch, G u i l v i n e c
292
F i g . 3b. I n f r a - r e d s a t e l l i t e image showing a n e v o l v i n g B i s c a y vortex pair. The lower images ( 3 . 6 . 3 2 and 3 0 . 6 . 2 2 ) f i t i n t o t h e B i s c a y r e g i o n i n t h e same manner a s t h e u p p e r images ( 2 0 . 4 . 8 2 and 25.5.82).
I n common w i t h most o c e a n i c r e g i o n s , t h e d e e p B i s c a y shows l a r g e ( 1 0 0 km) i n t e r c o n n e c t e d e d d i e s t h a t are g e n e r a l l y c o n f i n e d t o t h e a b y s s a l p l a i n by t h e s l o p e s .
I n d i v i d u a l e x a m p l e s of s u c h
e d d i e s from i n f r a - r e d images h a v e b e e n g i v e n by F r o u i n ( 1 9 8 1 ) and D i c k s o n and Hughes ( 1 9 8 1 ) .
A t
sea t h e y h a v e b e e n s t u d i e d u s i n g
293
d r i f t i n g buoys (Madelain and K e r u t , 1978) and i n t h e T o u r b i l l o n e x p e r i m e n t ( L e Groupe T o u r b i l l o n ,
1983).
They have t h e
c h a r a c t e r i s t i c s t r u c t u r e s k e t c h e d i n F i g . 2 and can l a s t f o r a week o r more.
The a n t i c y c l o n i c s t r u c t u r e of t h e v o r t e x p a i r
i l l u s t r a t e d i n F i g . 3 ( b ) a p p e a r e d t o p e r s i s t i n one form o r a n o t h e r o v e r a p e r i o d of two months a p p a r e n t l y f e d by ( o r drawing i n ) a c o o l s t r e a m of water f l o w i n g a l o n g t h e b a s e of t h e s l o p e s . Although t h i s v o r t e x p a i r a p p e a r s i n t h e c e n t r a l B i s c a y p o i n t of view of t h e 3 0 0 0 - 4 0 0 0
from t h e
m topography it i s p r e s s e d up
a g a i n s t t h e b a s e of t h e s l o p e i n t h e S . E .
c o r n e r of t h e Biscay
and seems t o s u b s e q u e n t l y p u t some c o o l e r w a t e r up on t h e S p a n i s h slope.
B i s c a y e d d i e s a l s o a p p e a r t o be a b l e t o draw w a t e r o f f
t h e s h e l f and c o o l plumes can sometimes be o b s e r v e d e x t e n d i n g from t h e r e g i o n of s h e l f - b r e a k c o o l i n g f o r d i s t a n c e s of 100-200km, Fig.
2.
I n addition t o t h e interconnecting vortex p a i r s t r u c t u r e s , c y c l o n i c e d d i e s w i t h wave l e n g t h s of 1 0 0 km have been observed i n the S.E.
Biscay which a p p e a r t o be c o n f i n e d t o t h e lower p a r t of
t h e s l o p e s and a r e a l s o s k e t c h e d i n F i g . 2 . 1.3.
Hot s p o t s
Cloud-free i n f r a - r e d
s a t e l l i t e images g e n e r a l l y o c c u r under
high atmospheric p r e s s u r e conditions. show " h o t - s p o t s ' '
The daytime p a s s e s t h e n
a s t h e sea s u r f a c e warms up i n l o c a l i s e d p l a c e s
where c o n d i t i o n s a r e r e l a t i v e l y w i n d l e s s .
Measurements a t sea
and from d r i f t i n g buoys have shown t h a t under such w i n d l e s s conditions
( < F o r c e 2 ) t h e t o p m e t r e can w a r m up by
(Fig. 4 ) .
Such e f f e c t s have a l l o w e d s t r u c t u r e s t o be observed i n
2-3OC
t h e g e n e r a l l y t i d a l mixed c o n d i t i o n s of t h e c e n t r a l and e a s t e r n r e g i o n s of t h e E n g l i s h Channel, f o r example, f r e s h e r w a t e r s p r e a d i n g from t h e Bay of S e i n e r e g i o n p a r t i c u l a r l y a t neap t i d e s , and t h e e f f e c t s of t i d a l mixing and topography i n t h e Channel
Isles a r e a .
2. 2.1.
SURFACE CHLOROPHYLL STRUCTURES S h e l f - b r e a k f r o n t a l F l u o r e s c e n c e and R e f l e c t a n c e Measurements a t sea have shown t h a t b o t h t h e Ushant f r o n t and
t h e s h e l f - b r e a k c o o l i n g r e g i o n c a n show i n c r e a s e s o f c h l o r o p h y l l ' a ' a t t h e s u r f a c e (Pingree e t a l . ,
1982).
T h i s i s t h o u g h t t o be
294
201 18
OC 16
1 , 1 , , 1 19821 , , , , , , , 198
211
F i g . 4. S u r f a c e t e m p e r a t u r e r e c o r d from s u r f a c e d r i f t i n g buoy (which f o l l o w e d t h e 2 0 0 0 m c o n t o u r northwestward i n t h e v i c i n i t y of 9OW a t about 5 c m s - 1 ) showing marked d i u r n a l t e m p e r a t u r e variations.
due t o t h e f a v o u r a b l e n u t r i e n t and l i g h t regime a f f o r d e d by t h e p h y s i c a l mixing p r o c e s s e s .
I n June a band of i n o r g a n i c n u t r i e n t s
occurs along t h e shelf-break with n i t r a t e values t y p i c a l l y 2,
1 ug a t 1-1 N-NO3
(Fig. 5 ) .
I n J u l y , August i s o l a t e d , h i g h e r
t h a n background, n i t r a t e - n i t r o g e n
p a t c h e s occur w i t h g e n e r a l l y
c o o l e r w a t e r showing t h a t t h e r e i s , i n d e e d , on o c c a s i o n s , a n i t r a t e s o u r c e a t t h e s u r f a c e t h a t can be u t i l i s e d by phytoplankton p h y t o p l a n k t o n growing n e a r t h e s u r f a c e ( F i g . 6 ) .
Whilst t h e
v a l u e s of f l u o r e s c e n c e a t t h e s h e l f - b r e a k a r e v e r y v a r i a b l e w i t h e x c e p t i o n a l l y h i g h v a l u e s a s s o c i a t e d w i t h some nannoplankton communities ( f o r example t h e Prasinophycean f l a g e l l a t e Micromonas s p (1-211 d i a m e t e r ) t o g e t h e r w i t h t h e Chrysophycean f l a g e l l a t e
P s e u d o p e d i n e l l a s p ( 6 d~i a m e t e r ) ) , t h e c h l o r o p h y l l ' a ' v a l u e s a r e t y p i c a l l y only
Q ,
1 mg c h l ' a ' m-3,
an o r d e r of magnitude l o w e r
t h a n t h e v a l u e s t h a t a r e commonly a s s o c i a t e d w i t h blooms i n t h e v i c i n i t y of t h e s h e l f - t i d a l f r o n t s o r which o c c u r d u r i n g t h e s p r i n g bloom i n t h e C e l t i c Sea.
However mackerel eggs o c c u r i n
maximum number a t t h e s h e l f - b r e a k i n May-June
(Coombs e t a l . ,
1981) and it may be t h e l a r g e g e o g r a p h i c a l e x t e n t of t h i s r e g i o n
of i n c r e a s e d l e v e l s of s u r f a c e c h l o r o p h y l l ' a ' and t h e a s s o c i a t e d
296
49
N
4; 45
N
4;
F i g . 5. ( a ) S u r f a c e t e m p e r a t u r e (OC); ( b ) s a l i n i t y (o/oo); ( c ) c h l o r o p h y l l ' a ' (mg m-3) and ( d ) i n o r g a n i c n i t r a t e (pM) (3-6 J u n e 1 9 8 3 ) . 200 m c o n t o u r shown by d o t t e d l i n e .
p r o t r a c t e d p r o d u c t i v e s e a s o n of b o t h primary and secondary p r o d u c t i o n which p r o v i d e t h e e c o l o g i c a l a d v a n t a g e s t h a t f a v o u r t h i s spawning a r e a . The C o a s t a l Zone Colour Scanner (C.Z.C.S.)
imagery h a s shown
more c l e a r l y t h a n e v e r b e f o r e t h e g e o g r a p h i c s c a l e and p e r s i s t e n c e of t h e s h e l f - b r e a k blooms ( F i g . 7 ) .
Chlorophyll
a b s o r b s more s t r o n g l y a t t h e b l u e end of t h e v i s i b l e spectrum t h a n i n t h e y e l l o w p a r t and i n broad t e r m s a measure of t h e c h l o r o p h y l l from C.Z.C.S.
d a t a c a n be o b t a i n e d from t h e r a t i o of t h e
r e f l e c t a n c e s from c h a n n e l 1 ( b l u e , 443 nm) o r c h a n n e l 2 ( g r e e n , 5 2 0 nm) t o c h a n n e l 3 ( y e l l o w , 550 nm) a f t e r a p p l y i n g an a t m o s p h e r i c c o r r e c t i o n t o e a c h u s i n g c h a n n e l 4 ( r e d , 670 nm). Some of t h e s p e c t a c u l a r blooms t h a t have been observed a t t h e s h e l f - b r e a k a r e comprised mainly of c o c c o l i t h o p h o r e s ( H o l l i g a n
et al.,
1983) which g i v e a c h a r a c t e r i s t i c milky appearance t o t h e
water.
The c a l c i t e p l a t e s of t h e c o c c o l i t h o p h o r e s a r e s t r o n g l y
r e f l e c t i n g and t h e s t r u c t u r e of t h e s e blooms can b e s e e n i n t h e raw c h a n n e l 3 d a t a . C.Z.C.S.
I n common w i t h t h e i n f r a - r e d
imagery t h e
imagery h a s shown t h i n plumes e x t e n d i n g o u t from t h e
296
0 4
a
Temperatureloc)
,
CI
I
I
(0.4
48O
'ablr 48"
-
\
3 0
50 47O
Chlorophyll 'a'
(mg m?)
5"
7'
8-
.
6"
6. ( a ) S u r f a c e t e m p e r a t u r e ( O C ) and s h i p ' s t r a c k : s a l i n i t y (O/oO): ( c ) c h l o r o p h y l l ' a ' (mg m-3) and i n o r g a n i c n i t r a t e ( p M ) (August 1 9 8 0 ) . Bottom topography i s g i v e n i n metres.
s h e l f - b r e a k and e d d i e s i n t h e d e e p e r Biscay r e g i o n s .
The plumes
of p h y t o p l a n k t o n drawn o f f from t h e s h e l f - b r e a k s l o p e r e g i o n a r e f u r t h e r e v i d e n c e of p h y s i c a l p r o c e s s e s ( i n t h i s c a s e B i s c a y e d d i e s ) and may be i m p o r t a n t i n t h e development and s u b s e q u e n t decay o f s h e l f - b r e a k blooms.
2.2.
F l u o r e s c e n c e Along S h e l f t i d a l f r o n t s
I n c r e a s e s i n r e f l e c t a n c e a l s o o c c u r a l o n g t h e Ushant t i d a l f r o n t ( F i g . 7 ) where p h y s i c a l mixing p r o c e s s e s a g a i n c o n t r o l t h e
291
F i g . 7. C . Z . C . S . ( C o a s t a l Zone C o l o u r S c a n n e r ) image (22 J u n e 1 9 8 1 ) showing r e g i o n s of r e l a t i v e l y h i g h s u r f a c e c h l o r o p h y l l i n t h e v i c i n i t y of t h e s h e l f - b r e a k and t o t h e s t r a t i f i e d s i d e of t h e Ushant f r o n t .
a v a i l a b i l i t y of n u t r i e n t s and l i g h t .
A s t h e season progresses
t h e p h y t o p l a n k t o n c o m p o s i t i o n c h a n g e s from a dominance of d i a t o m s t o a dominance of d i n o f l a g e l l a t e s which t e n d t o o c c u r i n t h e s t r a t i f i e d waters adjacent t o t h e t i d a l f r o n t s .
Spectacular
s u r f a c e blooms of d i n o f l a g e l l a t e s (Gyrodinium a u r e o l u m ) h a v e b e e n o b s e r v e d i n 1975, 1 9 7 6 , 1978, 1981 whicn e x t e n d from t h e f r o n t a l boundary w e l l a c r o s s i n t o s u r f a c e w a t e r s of t h e s h a l l o w thermocline
(%
2 0 m ) r e g i o n s of t h e w e s t e r n E n g l i s h Channel where
v a l u e s of c h l o r o p h y l l ' a ' a s h i g h a s recorded.
'~r
1 0 0 mg c h l ' a ' m-3
h a v e been
The p r e c i s e r o l e of w a t e r movement, n u t r i e n t f l u x e s and
v e r t i c a l m i g r a t i o n of t h e d i n o f l a g e l l a t e s i n m a i n t a i n i n g t h e s e
298
surface d i s t r i b u t i o n s i n t h e shallow s t r a t i f i e d waters adjacent to
t h e Ushant t i d a l f r o n t i s a s u b j e c t of c o n t i n u i n g r e s e a r c h . INTERNAL WAVES AND TIDES
3.
(i)
Surface radar s t r u c t u r e s .
Whilst s h e l f - b r e a k c o o l i n g may
be c o n s i d e r e d a s p o s s i b l e i n d i r e c t e v i d e n c e f o r i n t e r n a l t i d e s , i n f r a - r e d s a t e l l i t e imagery h a s n o t y e t p r o v i d e d c l e a r examples o f internal tides.
T h i s i s h a r d l y s u r p r i s i n g s i n c e measurements a t
t h e s h e l f - b r e a k n e a r 48ON have shown t h a t a l t h o u g h t h e t h e r m o c l i n e may o s c i l l a t e by more t h a n 50 m a t s p r i n g t i d e s ( F i g . 8 ) t h e r e may b e no s u r f a c e t e m p e r a t u r e e x p r e s s i o n of t h e internal tide.
The s y n t h e t i c a p e r t u r e r a d a r (S.A.R.) on board t h e
SEASAT on t h e o t h e r hand h a s p r o v i d e d s t r i k i n g examples of i n t e r n a l waves i n t h e Biscay r e g i o n and a l l o w e d an e s t i m a t e t o be made f o r t h e phase speed f o r t h e i n t e r n a l t i d e p r o p a g a t i n g
HOURS 6
12
E
r I-
n W
n
F i g . 8. I s o t h e r m s ( O C ) o b t a i n e d from r e p e a t e d S.T.D. p r o f i l e s ( e v e r y 1 0 mins) n e a r t h e s h e l f - b r e a k i n about 250-350 m d e p t h f o l l o w i n g a d r i f t i n g dahn whose approximate mean p o s i t i o n was 48O08'N 8O11'W. The h i g h e r f r e q u e n c y o s c i l l a t i o n s of a b o u t 15 min p e r i o d c o u l d b e c o n t o u r e d w i t h o u t a l i a s i n g u s i n g t h e echo sounder t o monitor t h e a c o u s t i c s c a t t e r i n g l a y e r s i n t h e thermocline. The t i m e of maximum o f f - s h e l f t i d a l s t r e a m i n g is i n d i c a t e d by an arrow. The t i d a l o s c i l l a t i o n s of t h e t h e r m o c l i n e (14°C c o n t o u r ) i n t h i s r e g i o n have a peak t o t r o u g h d i s p l a c e m e n t o f a b o u t 50 m a t s p r i n g t i d e s . There i s a l s o a marked second b a r o c l i n e mode. The 1 6 O C c o n t o u r n e a r t h e s u r f a c e i s n o t shown.
299
on-shelf. V a r i a t i o n s i n sea s n r f a c e roughness due t o t h e i n t e r a c t i o n of t h e i n t e r n a l t i d a l c u r r e n t s w i t h t h e s u r f a c e waves a l l o w s t h e r a d a r t o r e v e a l t h e i n t e r n a l waves c l e a r l y ( F i g . 9 ) . Such waves c a n , i n f a c t , be observed a t s e a u s i n g s h i p ' s r a d a r
(see f o r example Haury e t a l . ,
1983) o r even v i s u a l l y a s a r e s u l t
of t h e i n c r e a s e d number of b r e a k i n g waves a s s o c i a t e d w i t h t h e i n t e r n a l wave t r a i n s .
On o c c a s i o n s p a r a l l e l " w a l l s o f white''
w a t e r ( b r e a k i n g s u r f a c e waves) s e p a r a t e d by a b o u t 1 k m can be s e e n s t r e t c h i n g f o r s e v e r a l m i l e s i n d i c a t i n g t h e p r e s e n c e of l a r g e i n t e r n a l waves p r o p a g a t i n g o n - s h e l f . image f o r t h i s r e g i o n ( P i n g r e e and M a r d e l l , 1981)
The S . A . R .
i n d i c a t e s t h a t a l t h o u g h t h e r e a r e many s o u r c e s f o r t h e i n t e r n a l waves t h e y mainly o r i g i n a t e a t t h e s h e l f - b r e a k from l o c a l i s e d sources.
I n t e r n a l waves appear t o move on-shelf
about
Q
30 km i n
what i s assumed t o be a t i d a l p e r i o d g i v i n g a phase speed of 6 7 c m s-1.
They a l s o p r o p a g a t e o f f - s l o p e and o u t i n t o t h e B i s c a y .
The i n t e r n a l waves i l l u s t r a t e d i n F i g . 9 have wavelengths of o r d e r 1 km and n o n - l i n e a r e f f e c t s a r e i m p o r t a n t i n t h e i r g e n e r a t i o n and s u b s e q u e n t p r o p a g a t i o n . Such images have s t i m u l a t e d t h e development of n u m e r i c a l models and measurements of t h e i n t e r n a l t i d e u s i n g t h e r m i s t o r c h a i n s and c u r r e n t meter moorings.
S i n c e t h e i n t e r n a l t i d e s a r e t h o u g h t t o r e p r e s e n t one
of t h e main c a n d i d a t e s c a u s i n g s h e l f - b r e a k c o o l i n g and t h e a s s o c i a t e d s h e l f - b r e a k f l u o r e s c e n c e it i s of i n t e r e s t t o c o n s i d e r some of t h e p o s s i b l e c h a r a c t e r i s t i c s of t h e i n t e r n a l t i d e s i n t h i s region. (ii) Numerical models
The f o l l o w i n g s i m p l e n u m e r i c a l model n e g l e c t s r o t a t i o n , assumes t h e s h e l f - s l o p e r e g i o n h a s a r e g u l a r geometry and i s o n l y v a l i d f o r long waves (so lee wave f o r m a t i o n where n o n - h y d r o s t a t i c p r e s s u r e becomes i m p o r t a n t i s n o t t a k e n i n t o a c c o u n t ) .
Although
i n i t s p r e s e n t form t h e model may n o t b e v e r y r e a l i s t i c it d o e s show t h a t long wave i n t e r n a l t i d e s might be f o r c e d by t h e b a r o t r o p i c t i d e a s t h e t i d a l c u r r e n t s move up and down t h e s l o p e t h e r e b y c a u s i n g o s c i l l a t i o n s of t h e t h e r m o c l i n e .
I n t h i s model
a c r e s t i s formed n e a r t h e s h e l f - b r e a k j u s t a f t e r on-shelf s t r e a m i n g , whereas a t r o u g h forms j u s t a f t e r o f f - s h e l f streaming.
tidal
tidal
The crests and t r o u g h s d i v i d e i n t h e s l o p e r e g i o n
n e a r t h e s h e l f - b r e a k and p r o p a g a t e a s f r e e waves b o t h on-shelf and o f f - s h e l f
towards t h e ocean.
S i n c e t h e t r o u g h formed d u r i n g
300
Fig. 9 . A d i g i t a l l y p r o c e s s e d p o r t i o n of t h e s y n t h e t i c a p e r t u r e r a d a r (S.A.R.) p a s s on 2 0 August 1978 showing i n t e r n a l waves i n t h e s h e l f - b r e a k r e g i o n w i t h w a v e l e n g t h s t y p i c a l l y o f o r d e r 1 km. The image c e n t r e i s l o c a t e d a t 46°51'36"N, 5O9'58"W. The p a s s c o r r e s p o n d s t o t i d a l c o n d i t i o n s 1 h o u r a f t e r maximum o f f - s h e l f t i d a l streaming a t spring t i d e s .
off-shelf
t i d a l streaming i s propagating on-shelf
against the
t i d a l c u r r e n t and a marked s t e e p e n i n g of t h e i n t e r n a l wave p r o f i l e o c c u r s which p r o p a g a t e s a s an i n t e r n a l t i d a l b o r e . The model i s a v e r t i c a l s e c t i o n normal t o t h e s h e l f - b r e a k spanning oceanic ( 4 0 0 0 m ) ,
s l o p e and s h e l f r e g i o n s ( 2 0 0 m ) .
The
t h e r m o c l i n e i s r e p r e s e n t e d by an upper l a y e r h ' of d e n s i t y p ' and The x a x i s i s c h o s e n p o s i t i v e a l o w e r l a y e r , h " , of d e n s i t y p " .
301
200 m
50 km
Fig. 1 0 . Schematic r e p r e s e n t a t i o n of t h e model showing t h e r m o c l i n e spanning o c e a n i c , s l o p e and s h e l f r e g i o n s . The g r i d s c a l e i s 500m.
i n t h e on-shelf
d i r e c t i o n and t h e g r i d s c a l e i s 500 m ( F i g . 1 0 ) .
F o r s i m p l i c i t y c o n d i t i o n s a r e t a k e n as uniform i n t h e a l o n g - s h e l f sense.
The i n t e r n a l t i d e U i s d e f i n e d by U = u'
- u"
where u ' i s
t h e c u r r e n t i n t h e upper l a y e r and u" i s t h e c u r r e n t i n t h e lower layer.
The b a r o t r o p i c t i d e , o r v e r t i c a l l y i n t e g r a t e d t i d a l
c u r r e n t , u, i s assumed t o be unmodified by t h e i n t e r n a l t i d e and i s s p e c i f i e d i n advance i n a c c o r d a n c e w i t h s h e l f - s l o p e geometry.
The e q u a t i o n of c o n t i n u i t y f o r t h e upper l a y e r can be transformed i n t o a n equation f o r t h e i n t e r n a l o s c i l l a t i o n
n
against t i m e t , t o give
a ax
(h'u)
a ( h ' h "7 + ax U) = a at
where H = h '
+
h" = h l
+
h2
and uH = u ' h '
+
u"h"
The f i r s t t e r m on t h e l e f t hand s i d e of e q u a t i o n (1) i s t h e s o u r c e t e r m f o r t h e i n t e r n a l d i s p l a c e m e n t of t h e t h e r m o c l i n e
n.
I t a l s o a l l o w s t h e b a r o t r o p i c t i d e t o move t h e i n t e r n a l t i d e back
and f o r t h on t h e s h e l f s i n c e h ' = h l
-
q.
Variations i n surface
e l e v a t i o n are neglected with r e s p e c t t o t h e i n t e r n a l o s c i l l a t i o n . The b a r o t r o p i c t i d e , u , i s p r e s c r i b e d a c c o r d i n g t o t h e non-divergent equation
302
a ax (Hu) =
0
Thus the source term for the internal tide varies as -('/HI
2
3
ax
and has a maximum value just at the top of the slopes when ax
is constant.
A simplified momentum equation is obtained by subtracting the momentum equations for the upper and lower layers to give
= (uu) = at + ax a
B - an + K V U 2
- dlP
where B = g(l
(3)
ax
is the reduced gravity and g is the "
acceleration due to gravity. More complete forms for the term arising from advection a (uU) gave qualitatively similar results and in this simple treatment 2 are not further discussed. The term KV U represents attenuation by diffusion, with coefficient K, and also assists with numerical stability. The mean depth of the upper layer was taken as hl = 30 m and the slope region had a uniform gradient of 1 in 10 from H = 200 m to H = 4000 m in 38 km. Thus in a linear model the phase speed is
f
%
50 cm
s",
with B
%
1 cm sec-2 , and h2 = 170 m,
considerably less than that suggested by the S . A . R . image. The 2n corresponding wavelength X = -2T I;- for M2 tidal frequency, u = T is X = 23 km. K was chosen such that Kk2 % 2 / r so free waves in a linear model would decay to l/e of their amplitude after a time T and T was set T = 4T where T is the M2 tidal period, thus 2 2 -1 K % 1.4 x 10 m s The amplitude of the oscillating barotropic tide was taken as 75 cm s-l which represents a peak spring tide condition for a % 100 km stretch along the shelf-break in the Celtic Sea Armorican Shelf region. (iii) Long waves without rotation. The structure of the internal tide is illustrated by hourly sequences of the displacement of the thermocline. The linear model where all non-linear terms were neglected is shown in Fig. 11 and the results of the non-linear model using the full equations (l), (2) and (3) is shown in Fig. 12. In both models a trough occurs at the shelf-break just after maximum of€-shelf tidal streaming.
.
-40 m
6
12
5
11
3
9
2
a
303
7
F i g . 11. I n t e r n a l t i d a l d i s p l a c e m e n t s of t h e t h e r m o c l i n e e v e r y l u n a r hour u s i n g l i n e a r i s e d e q u a t i o n s . The s l o p e r e g i o n e x t e n d s from 0 t o S where S i s t h e s h e l f - b r e a k ( 2 0 0 m ) and 0 i s t h e s t a r t of t h e o c e a n i c r e g i o n ( 4 0 0 0 m ) . D i s an on-shelf A vertical p r o p a g a t i n g t r o u g h and C i s an ocean-going t r o u g h . s c a l e of 4 0 m i s shown a t hour 3 . Hour 3 c o r r e s p o n d s w i t h maximum o n - s h e l f t i d a l s t r e a m i n g and maximum o f f - s h e l f t i d a l s t r e a m i n g o c c u r s a t h o u r 9 ( d e p i c t e d by a r r o w ) .
304
1 2 q
5
4
IA
1
YA
2
8
F i g . 1 2 . I n t e r n a l t i d a l d i s p l a c e m e n t of t h e t h e r m o c l i n e e v e r y l u n a r hour using t h e f u l l y non-linear e q u a t i o n s . The s l o p e r e g i o n e x t e n d s from 0 t o S where S i s t h e s h e l f - b r e a k ( 2 0 0 m ) a n d 0 i s t h e s t a r t of t h e o c e a n i c r e g i o n ( 4 0 0 0 m ) . B i s an o c e a n g o i n g t r o u g h and A i s a n o n - s h e l f p r o p a g a t i n g t r o u g h . Hour 3 c o r r e s p o n d s w i t h maximum o n - s h e l f t i d a l s t r e a m i n g .
305
However i n t h e n o n - l i n e a r model t h e l e a d i n g edge o f t h e on-shelf p r o p a g a t i n g t r o u g h i s u n a b l e t o move o n - s h e l f
against the
b a r o t r o p i c t i d a l c u r r e n t s u n t i l t h e t i d a l streams s l a c k e n .
This
r e s u l t s i n a v e r y d i s t o r t e d and s t e e p e n e d t r o u g h f o r t h e i n t e r n a l t i d e which s u b s e q u e n t l y p r o p a g a t e s r a p i d l y a c r o s s t h e s h e l f when t h e t i d a l streams are o n - s h e l f
and i s h a l t e d and momentarily
r e v e r s e d i n d i r e c t i o n d u r i n g maximum o f f - s h e l f (iv)
E f f e c t due t o r o t a t i o n .
t i d a l streaming.
When r o t a t i o n i s t a k e n i n t o
a c c o u n t and c o n d i t i o n s are a g a i n uniform i n t h e a l o n g - s l o p e s e n s e r o t a t i o n i n c r e a s e s t h e p r o p a g a t i o n speed.
In addition r e l a t i v e l y
more energy i s a s s o c i a t e d w i t h t h e c u r r e n t s r a t h e r t h a n t h e i n t e r n a l d i s p l a c e m e n t s of t h e t h e r m o c l i n e .
L i n e a r t h e o r y and
n u m e r i c a l model g i v e t h e p r o p a g a t i o n speed f o r long waves w i t h h o r i z o n t a l crests a s
c
2
h h
p' - -$iH
= g(l
1 2)
(1
-
fL u2
--)-I
(4)
hl+h2
where f i s t h e C o r i o l i s p a r a m e t e r and a i s t h e t i d a l f r e q u e n c y . For f/o
%
0.77 appropriate for these l a t i t u d e s , t h i s w i l l r e s u l t
i n a n i n c r e a s e i n p h a s e s p e e d and wavelength f o r t h e p r o g r e s s i v e i n t e r n a l t i d e of a b o u t x 1 . 6 . The waves a r e now d i s p e r s i v e and t h e group v e l o c i t y , c l i n e a r long waves i s d e f i n e d a s ao
%==-
-
c(1
and w i t h f / o
%
-
g'
for
2 2 f /u ) 0.77,
a s b e f o r e , t h i s g i v e s a group v e l o c i t y of
a b o u t 1 . 6 t i m e s s m a l l e r t h a n t h e p h a s e speed of waves i n t h e a b s e n c e of r o t a t i o n .
T h i s i m p l i e s t h a t any m o d u l a t i o n of t h e
long-wave i n t e r n a l t i d a l s i g n a l a t t h e s h e l f - b r e a k due t o t h e s p r i n g - n e a p c y c l e of t h e b a r o t r o p i c t i d e w i l l t r a v e l o n l y s l o w l y on s h e l f o r o f f s h e l f .
t e r m s i n e q u a t i o n s (1) and ( 3 ) w e r e i n c l u d e d t h e d i s t o r t i o n s of t h e i n t e r n a l t i d e on t h e s h e l f w e r e When t h e n o n - l i n e a r
no l o n g e r a s i n d i c a t e d i n F i g . 1 2 b u t had d e e p l y p e n e t r a t i n g t r o u g h s and t h e wave p r o f i l e on t h e s h e l f a l s o t e n d e d t o be symmetric w i t h r e s p e c t t o t h e t r o u g h s . (v)
S h o r t e r wavelengths.
The waves so f a r c o n s i d e r e d assume
t h a t t h e i n t e r n a l t i d a l c u r r e n t s a r e uniform i n t o p and bottom layers.
C l e a r l y t h i s i s not v a l i d f o r s h o r t e r wavelengths a s
306 e x e m p l i f i e d by t h e S.A.R.
image which i n d i c a t e s n o n - l i n e a r
i n t e r n a l wave t r a i n s or p a c k e t s of i n t e r n a l s o l i t o n s .
More
r e a l i s t i c models would have t o make allowance f o r t h e v e r t i c a l s t r u c t u r e of t h e c u r r e n t s i n t h e upper and lower l a y e r f o r t h e h i g h e r wave numbers a s i n t h e Korteweg and d e V r i e s (1895) f i r s t approximation.
S o l i t a r y waves and s o l i t o n s t r a v e l a t s p e e d s i n
e x c e s s of t h a t g i v e n by s m a l l a m p l i t u d e l i n e a r t h e o r y .
Their
f r a c t i o n a l i n c r e a s e i n phase speed i s v e r y a p p r o x i m a t e l y
%
f n/h,
( A l p e r s and S a l u s t i , 1983) and s o t h e f i n i t e a m p l i t u d e of s h o r t e r waves may produce i n c r e a s e s i n phase speed t h a t c o u l d match t h e Thus nonv a l u e ( % 67 c m s-l) i n f e r r e d from t h e S.A.R. image. l i n e a r e f f e c t s of f i n i t e a m p l i t u d e f o r t h e s h o r t e r waves o r t h e l i n e a r e f f e c t s o f r o t a t i o n f o r t h e l o n g e r waves s i g n i f i c a n t l y i n c r e a s e s t h e p r o p a g a t i o n s p e e d s of t h e i n t e r n a l waves. ( v i ) Measurements a t sea. An extreme example of t h e s t r u c t u r e of t h e i n t e r n a l t i d e a t s p r i n g t i d e s o b t a i n e d from a t h e r m i s t o r c h a i n mooring p l a c e d on t h e s h e l f i n t h e r e g i o n of maximum M2 t i d a l c u r r e n t s a t 47°40.0'N
6O19.1'W
from t h e s h e l f - b r e a k ) i s shown i n F i g . 13
.
( a b o u t 2 0 km
A t spring t i d e s t h e
b a r o t r o p i c t i d a l c u r r e n t s r e a c h a l m o s t 2 knot a t t h i s p o s i t i o n and a l t h o u g h t h e t i d a l c u r r e n t s a r e reduced a t t h e s h e l f - b r e a k t h e y a r e s t i l l comparable w i t h t h e phase speed of t h e i n t e r n a l tide.
The t r o u g h formed d u r i n g o f f - s h e l f
p r o p a g a t i n g on-shelf
t i d a l streaming i s t h u s
a g a i n s t t h e t i d a l c u r r e n t and a t s p r i n g
t i d e s t h i s w i l l r e s u l t i n a marked s t e e p e n i n g of t h e i n t e r n a l tide. Measurement made from f i x e d moorings w i l l need c o r r e c t i n g f o r t h e d i s t o r t i o n s t h a t occur a s t h e t i d a l c u r r e n t s a d v e c t t h e i n t e r n a l t i d e p a s t t h e mooring.
C u r r e n t measurements made n e a r
t h e t h e r m i s t o r c h a i n mooring showed t h a t t h e l e a d i n g edge of t h e t r o u g h of t h e i n t e r n a l t i d e p a s s e d t h e t h e r m i s t o r c h a i n mooring when t h e on-shelf
t i d a l c u r r e n t was a b o u t 1 . 5 k n o t ( a b o u t 1 . 0
h o u r s a f t e r maximum on-shelf
t i d a l streaming).
Thus some of t h e
s t e e p e n i n g a s s o c i a t e d w i t h t h e t r o u g h of t h e i n t e r n a l t i d e i s a p p a r e n t and due t o making measurements a t a f i x e d p o i n t r a t h e r t h a n f o l l o w i n g t h e o s c i l l a t i n g b a r o t r o p i c t i d a l flow.
The
14O-15OC i s o t h e r m s descend below 50 m f o r a b o u t 2 0 % of t h e wave
period.
During t h i s t i m e a l o c a l w a t e r column would move
on-shelf
about 1 - 2 k m which i s o n l y a s m a l l f r a c t i o n
t h e wavelength of t h e i n t e r n a l t i d e
(%
30 k m ) .
(%
5 % ) of
So it a p p e a r s
307
F i g . 13. I s o t h e r m s (OC) from t h e t h e r m i s t o r c h a i n mooring 0 6 9 (47O41.8" 6018.2'W). The measured s t r u c t u r e of t h e i n t e r n a l t i d e p r o p a g a t i n g on-shelf i s h i g h l y d i s t o r t e d w i t h d e e p l y p e n e t r a t i n g t r o u g h s . There i s a l s o a n o t i c e a b l e second b a r o c l i n i c mode. Some smoothing of t h e d a t a was n e c e s s a r y t o produce a c l e a r e r i l l u s t r a t i o n . The p e r i o d i l l u s t r a t e d corresponds t o s p r i n g - t i d e conditions with semi-diurnal c u r r e n t s typically 80-90 c m s-l ( v e r t i c a l l y i n t e g r a t e d ) . Q
t h a t a t spring t i d e s , a t l e a s t , t h e i n t e r n a l t i d e is d i s t o r t e d i n t h i s p a r t i c u l a r r e g i o n w i t h more d e e p l y p e n e t r a t i n g t r o u g h s . A c l o s e r i n s p e c t i o n of Fig.
1 3 shows t h a t t h e t r o u g h s a r e
g e n e r a l l y composed of two l a r g e a m p l i t u d e waves a t t h i s s i t e . A t some p l a c e s n e a r t h e s h e l f - b r e a k t h e i n t e r n a l t i d a l s i g n a l
t a k e s on t h e form of a g r o u p of s h o r t wavelength i n t e r n a l waves p r o p a g a t i n g o n - s h e l f .
< 1
(%
km)
An example i s i l l u s t r a t e d
i n P i g . 1 4 which shows a l a r g e a m p l i t u d e wave f o l l o w e d by s m a l l e r waves and such waves a r e b e l i e v e d t o c a u s e t h e s u r f a c e f e a t u r e s s e e n i n t h e S.A.R.
image ( F i g . 9 ) .
C u r r e n t measurements have a l s o been made a t t h e s h e l f - b r e a k n e a r 47O30'N t o see whether t h e s h e a r produced by t h e i n t e r n a l t i d e is s u f f i c i e n t t o c a u s e mixing i n t h e t h e r m o c l i n e and c o n t r i b u t e t o t h e s h e l f - b r e a k c o o l i n g observed i n t h e i n f r a - r e d s a t e l l i t e imagery. G r a d i e n t Richardson numbers of measured by c u r r e n t meters s e p a r a t e d v e r t i c a l l y by
%
1 have been
%
84 m w i t h
t e m p e r a t u r e d i f f e r e n c e s a c r o s s t h e b a s e of t h e t h e r m o c l i n e of It i s hard not t o Q l 0 C f o r p e r i o d s of % 1 hour a t s p r i n g t i d e s . draw t h e c o n c l u s i o n t h a t a c l o s e r s e p a r a t i o n of c u r r e n t meters
308
' > i g . 14. Ecno sound.er t r a c e from 47052.5" 6 O 2 9 ' W ( 2 0 n.m from t n e s h e l z - b r e a k ( 2 0 0 m c o n t o u r ) on 2 7 . 7 . 8 3 ) showing l a r g e a m p l i t u d e waves on t h e t h e r m o c l i n e p r o p a g a t i n g o n - s h e l f which p r o d u c e t h e f e a t u r e s s e e n i n t h e S.A.R. image ( F i g . 9 ) . (Near v e r t i c a l l i n e s show C . T . D . d i p s ) .
would p r o d u c e e v e n lower R i c h a r d s o n number v a l u e s . p r o f i l e s h a v e a l s o shown small-scale
(
.02 a W
g -.02 0
-.06 cn I-
z W z .06 0 IL
W
a
a H 0 0
0
z
5
-.06 3
a
(3
a K
.O2kzZzd
w -.02
650
700
750
WAVELENGTH (nm)
F i g . 9 . R e c o n s t r u c t i o n of an o b s e r v e d r e f l e c t a n c e s p e c t r u m ( f o r a d i a t o m p o p u l a t i o n ) t h r o u g h t h e u s e o f G a u s s i a n s h a p e d components. The b o t t o m p a n e l shows t h e d i f f e r e n c e between t h e measured and reconstructed spectra.
329
1.0 *
0.8 cl
9
lx
w
V)
m
0
I
I
I
I
I
I
I
I
I
I
I
-
-
0.4 0.2 0.6
0W
0
u)
a
g
z
IZ
l0
0
W -I LL
w
a
g
0.3 0.2 0.1
0
*
z
o
a
ijj -0.1 cn 3 a -0.2 0
a 0 a a
0.2
0
W
0.2 650
700
750
WAVELENGTH ( n m )
F i g . 1 0 . R e c o n s t r u c t i o n of an o b s e r v e d r e f l e c t a n c e s p e c t r u m ( f o r a v i s u a l l y d i s c o l o u r e d bloom of Mesodinium rubrum) t h r o u g h t h e u s e of Gaussian shaped components. The lower p a n e l i l l u s t r a t e s t h e d i f f e r e n c e between t h e measured and r e c o n s t r u c t e d s p e c t r a .
330 f l a g e l l a t e s o r Plesodinium rubrum. V7e f i n d t h a t e a c h d i f f e r e n t a l g a l g r o u p h a s d i f f e r e n t o p t i c a l
properties.
F o r example, even though t h e e x t r a c t e d c h l o r o p h y l l
c o n t e n t i s t h e same f o r two g r o u p s , t h e most s i g n i f i c a n t f l u o r e s c e n c e l i n e may b e l o c a t e d a t d i f f e r e n t w a v e l e n g t h s . i l l u s t r a t e s t h i s point.
Table 2
The c h l o r o p h y l l 5 c o n c e n t r a t i o n was 6 . 0
mg/m3 a t b o t h s t a t i o n s 1 0 and 34.
However, f o r s t a t i o n 1 0 , t h e
main f l u o r e s c e n c e l i n e i s a t 682 nm, w h i l e f o r s t a t i o n 3 4 , it i s l o c a t e d a t 6 9 2 nm.
TABLE 2
Comparison o f G a u s s i a n a m p l i t u d e s f o r s t a t i o n s h a v i n g sin?ilar e x t r a c t e d c h l o r o p h y l l 5 concentration b u t d i f f e r e n t phytoplankton.
Dominant PhytoChlorophyll 5 A 6 8 2 ( ~l o 3 ) Station Plankton concentration 10 34 3
33 2
38
diatoms Mesodinium flage 1l a t e s Mesodinium dinoflagellates Mesodinium
A
~
~
l o~ 3 )(
Ax7 1 0 ( ~
6.0 6.0
1.26 0.23
0.33 0.94
-0.05 0.49
14.1
1.46
1.28
0.33
14.0
0.19
1.01
0.54
2.1
0.43
2.8
0.29
-0.6
0.25
lo3)
-0.51 -0.60
F i g u r e 11 i l l u s t r a t e s t h e r e l a t i o n s h i p between t h e a m p l i t u d e s a t 7 1 0 nm and 682 nm f o r t h e f o u r g r o u p s .
F o r a l l b u t t h e Mesodinium
s p e c t r a t h e a m p l i t u d e a t 710 nm i n c r e a s e s a t a b o u t one h a l f t h e r a t e of t h e 682 nm a m p l i t u d e .
Note t h a t t h e a r t i f i c i a l l i n e a r b a s e l i n e
i s r e s p o n s i b l e f o r t h e n e g a t i v e 7 1 0 nm a m p l i t u d e s .
W e are not
i n t e r p r e t i n g t h e s e a s a b s o r p t i o n and v i s u a l i n s p e c t i o n and t h e l i t e r a t u r e supports t h i s . S i m i l a r l y , t h e c h o i c e of a l i n e a r b a s e l i n e c o l o u r s o u r i n t e r p r e t a t i o n o f t h e a p p a r e n t s l o p e o f t h e Mesod i n i u m d a t a i n F i g u r e 11.
I n s p e c t i o n of t h e s p e c t r a from t h e
v i s u a l l y d i s c o l o u r e d s t a t i o n s ( F i g . 3) shows t h a t t h e s h o r t wavel e n g t h s e n d of t h e b a s e l i n e i s b e i n g l i f t e d by p h y c o e r y t h r i n f l u o r e s c e n c e n e a r 6 0 0 nm.
W e c o n c l u d e t h a t Mesodinium rubrum h a s
o n l y a s m a l l and c o n s t a n t amount of c h l o r o p h y l l 5 f l u o r e s c e n c e a t 682 nm ( a b s o l u t e c o n c e n t r a t i o n s are d i f f i c u l t t o c a l c u l a t e a t t h i s
331 T h i s is i n t e r e s t i n g from a b i o l o g i c a l
s t a g e i n our a n a l y s i s ) .
p o i n t o f view b e c a u s e a l t h o u g h Mesodinium rubrum i s c a p a b l e of p h o t o s y n t h e s i s i t i s n o t a p l a n t , b u t a p r o t o z o a n c o n t a i n i n g what a r e r e g a r d e d as " i n c o m p l e t e s y m b i o n t s " - e s s e n t i a l l y j u s t c h l o r o p l a s t s ( T a y l o r , B l a c k b o u r n and B l a c k b o u r n , 1 9 7 8 ) .
The a b s e n c e of
f l u o r e s c e n c e a t 6 8 2 nm p r e s u m a b l y means t h a t t h i s o r g a n i s m l a c k s t h e form o f c h l o r o p h y l l 5 which n o r m a l l y c o n s t i t u t e s 80% of t h e c e l l t o t a l i n o t h e r k i n d s of p h y t o p l a n k t o n
(Prgzelin, 1981).
The many
forms of c h l o r o p h y l l 5 (which a r e n o t d i f f e r e n t i a t e d i n t h e r o u t i n e e x t r a c t i v e p r o c e d u r e s u s e d by o c e a n o g r a p h e r s ) a r e u n e q u a l l y d i s t r i b u t e d w i t h i n t h e p l a n t p h o t o s y n t h e t i c mechanism ( G o v i n d j e e and B r a u n , 1 9 7 4 ) and t h e r e a r e some i n d i c a t i o n s t h a t t h e r e l a t i v e amounts o f
f l u o r e s c e n c e c h a n g e s between a l g a l t y p e s ( G o e d h e e r , 1 9 7 2 ) o r i n a g e i n g o r l o w l i g h t a d a p t e d c e l l s (Brown, 1 9 6 7 ) .
3 .O
0
Mesodinium
A
dinof lagellotes
2.0
I .o
J
a
0
5
a - I .o
-0 5
0
0 5
10
15
A M P L I T U D E 68Znrn ( I O - I~ F i g . 11. The r e l a t i o n s h i p between t h e G a u s s i a n a m p l i t u d e s a t 6 8 2 nm and 712 nm i n 39 r e f l e c t a n c e s p e c t r a from B r Y t i s h Columbia c o a s t a l waters. S p e c t r a t r o m Mesodinium rubrum p o p u l a t i o n s show a s i g n i f i c a n t l y d i f f e r e n t signature, thus allowing t h i s species t o be r e m o t e l y d i f f e r e n t i a t e d from o t h e r forms of p h y t o p l a n k t o n w i t h i n t h e s t i p p l e d zone ( e x t r a c t e d c h l o r o p h y l l g c o n c e n t r a t i o n 4 199 mg/m3).
-
332 ESTIMATION OF CHLOROPHYLL TON POPULATIONS
A
CONCENTRATION FOR DIFFERENT PHYTOPLANK-
W e a r e c u r r e n t l y i n v e s t i g a t i n g w h e t h e r it i s p o s s i b l e t o u s e t h e
methods d e s c r i b e d h e r e t o r e m o t e l y d e t e c t e i t h e r g r o s s t a x o n o m i c o r p h y s i o l o g i c a l changes i n a p h y t o p l a n k t o n p o p u l a t i o n .
For t h i s d a t a
s e t w e c a n d i f f e r e n t i a t e Mesodinium rubrum p o p u l a t i o n s from t h o s e of d i n o f l a g e l l a t e s , d i a t o m s and f l a g e l l a t e s on t h e b a s i s o f t h e 7 1 0 nm e m i s s i o n . Where t h e a m p l i t u d e of t h e 7 1 0 nm G a u s s i a n i s nega t i v e o r t h e 6 8 2 nm G a u s s i a n i s g r e a t e r t h a n 0 . 0 0 0 5
(unstippled area
i n F i g . 11) w e c a n c a l c u l a t e t h e c h l o r o p h y l l c o n c e n t r a t i o n a c c o r d i n g t o t h e formula: mgChl a/m3 = 0 . 4
+
(28.34A682
+
77.66Asg2
-
16.48A710) x
lo2
(1)
F i g u r e 1 2 i l l u s t r a t e s t h e . a g r e e m e n t between t h i s c a l c u l a t i o n and e x t r a c t e d c h l o r o p h y l l g c o n c e n t r a t i o n f o r p o p u l a t i o n s dominated by d i n o f l a g e l l a t e s , diatoms o r f l a g e l l a t e s .
The c o r r e l a t i o n c o e f f i c -
i e n t i s 0 . 9 6 w h i l e t h e s c a t t e r a b o u t t h e 1:l l i n e i s a b o u t 2 1 . 0 mg/m3.
This i s b e t t e r than using t h e eigenvector a n a l y s i s
(Fig. 7 ) .
15
10 R= 0.963 AC = .f I MG/M3
5
0
EXCLUDING DISCOLOURED STATIONS
I . 5
10
15
20
EXTRACTED CHLOROPHYLL g (MG/M3)
F i g . 1 2 . The a g r e e m e n t between t h e e x t r a c t e d c h l o r o p h y l l a concent r a t i o n and t h a t c a l c u l a t e d on t h e b a s i s of t h r e e G a u s s i a n s i g n a l s a t 682 nm, 692 nm and 7 1 0 nm.
333 Where t h e a m p l i t u d e o f t h e 710 nm G a u s s i a n i s p o s i t i v e and t h e 6 8 2 nm G a u s s i a n less t h a n 0 . 0 0 0 5
( s t i p p l e d area i n Fig.
l l ) , w e can
r e m o t e l y c l a s s i f y t h e dominant o r g a n i s m as Mesodinium a n d c a l c u l a t e t h e c o n c e n t r a t i o n of c h l o r o p h y l l mgChl g/m3 = 5.19
+
( 5 . 6 5 A710)
2 from t h e 710 nm a m p l i t u d e .
x 1 02
(2)
F i g u r e 1 3 i l l u s t r a t e s t h e a g r e e m e n t between t h i s c a l c u l a t i o n and e x t r a c t e d c h l o r o p h y l l o v e r t h e r a n g e 4 t o 199 mg c h l o r o p h y l l a / m 3 . The c o r r e l a t i o n c o e f f i c i e n t i n t h i s case i s 0.95,
while t h e s c a t t e r
a b o u t t h e l i n e i s a s g r e a t a s f 4 mg/m3.
200 r
/
/@
'IONS
AMPLITUDE 710nm ( x 100) F i g . 1 3 . The r e l a t i o n s h i p between t h e a m p l i t u d e of t h e 710 nm G a u s s i a n e m i s s i o n and t h e e x t r a c t e d c h l o r o p h y l l g c o n c e n t r a t i o n f o r s e v e n s p e c t r a o b t a i n e d from v i s u a l l y d i s c o l o u r e d blooms o f M z d i n i u m rubrum. R e g r e s s i o n a n d c o r r e l a t i o n c o e f f i c i e n t s do n o t include highest point. ~~
334 SUMMARY AND CONCLUSIONS
The f l u o r e s c e n c e l i n e h e i g h t (FLH) method c a n b e s u c c e s s f u l l y employed t o r e m o t e l y measure t h e c h l o r o p h y l l g c o n c e n t r a t i o n i n many o c e a n i c a r e a s , however, where l a r g e blooms o f t h e c i l i a t e Mesodinium rubrum a r e e n c o u n t e r e d , t h e a c c u r a c y o f t h e FLH c a l c u l a t i o n i s s i g n i f i c a n t l y a f f e c t e d by an a p p a r e n t s h i f t i n t h e emission wavelength. From an a n a l y s i s of 56 r e f l e c t a n c e s p e c t r a o b t a i n e d i n c o a s t a l B r i t i s h Columbia w a t e r s , w e f i n d a t l e a s t t h r e e p r i n c i p a l G a u s s i a n s h a p e d f l u o r e s c e n c e l i n e s , l o c a t e d a t 682 nm, 6 9 2 nm and 7 1 0 nm. I n t h i s d a t a s e t s p e c t r a from v i s i b l y d i s c o l o u r e d blooms of Mesodinium rubrum c o u l d b e s t b e m o d e l l e d by a s s u m i n g l a r g e e m i s s i o n s a t 7 1 0 nm and 692 nm, w i t h v e r y s m a l l e m i s s i o n a t 682 nm. The c o n c e n t r a t i o n o f e x t r a c t a b l e c h l o r o p h y l l 5 ( a l l f o r m s ) f o r t h e s e p o p u l a t i o n s c o u l d b e a c c u r a t e l y e s t i m a t e d from t h e h e i g h t of t h e 7 1 0 nm G a u s s i a n .
F o r a l l o t h e r s p e c t r a , where t h e a p p a r e n t
f l u o r e s c e n c e l i n e i s l o c a t e d n e a r 685 nm, t h e e x t r a c t a b l e c h l o r o phyll
a
i s e s t i m a t e d e i t h e r b y t h e FLH method o r an e q u a t i o n
employing t h e a m p l i t u d e s a t 682 nm, 6 9 2 nm and 7 1 0 nm.
335 REFERENCES and Gower, J . F . R . , 1 9 8 1 . A i r b o r n e B o r s t a d , G . A . , Brown, R . M . , r e m o t e s e n s i n g o f sea s u r f a c e c h l o r o p h y l l and t e m p e r a t u r e a l o n g t h e o u t e r B r i t i s h Columbia c o a s t . P r o c e e d i n g s of t h e 6 t h Canadian Symposium on Remote S e n s i n g , H a l i f a x , N . S . , pp. 541-541. B o r s t a d , G.A. and Gower, J . F . R . , 1983. A s h i p and a i r c r a f t s u r v e y of phytoplankton c h l o r o p h y l l d i s t r i b u t i o n i n t h e e a s t e r n A r c t i c ( i n press). Canadian A r c t i c . Brown, J . S . , 1967. F l u o r o m e t r i c e v i d e n c e f o r t h e p a r t i c i p a t i o n of c h l o r o p h y l l 5 - 695 i n s y s t e m s of p h o t o s y n t h e s i s . Biochem. Biophys. A c t a . , 143:391-398. Clark, D.K., 1981. P h y t o p l a n k t o n a l g o r i t h m s f o r t h e Nimbus-7 CZCS. I n : J . F . R . Gower ( E d i t o r ) , Oceanography From S p a c e , Plenum P r e s s , N e w York, pp. 227-228. Doerf f e r , R. , 1 9 81. F a c t o r a n a l y s i s i n ocean c o l o u r i n t e r p r e t a t i o n . I n : J.F.R. Gower ( E d i t o r ) , Oceanography From S p a c e , Plenum P r e s s , N e w York, pp. 339-345. 1972. Fluorescence i n r e l a t i o n t o photosynthesis. Goedheer, J . C . , Ann. Rev. P l a n t P h y s i o l . , 23:87-112. Clark, D.K., Brown, J.W., Brown, O.B. , E v a n s , R . H . , Gordon, H . R . , Broenkow, W.W., 1983. P h y t o p l a n k t o n pigment c o n c e n t r a t i o n s i n t h e Middle A t l a n t i c B i g h t : a comparison of s h i p d e t e r m i n a t i o n s and CZCS e s t i m a t e s . Appl. O p t i c s , 2 2 :20-36. Gordon, H . R . , C l a r k , D . K . , M u e l l e r , J . L . and H o v i s , W . A . , 1980. P h y t o p l a n k t o n p i g m e n t s from t h e Nimbus-7 C o a s t a l Zone C o l o u r S c a n n e r : Comparisons w i t h s u r f a c e measurements. Science, 2 1 0 :6 3-6 6 . Gower, J . F . R . , 1980. O b s e r v a t i o n s o f i n s i t u f l u o r e s c e n c e of Boundary L a y e r M e t e o r o l o g y , chlorophyll 5 i n Saanich I n l e t . 1 8 : 235-245. 1981. U s e o f i n v i v o f l u o r e s c e n c e Gower, J.F.R. a n d B o r s t a d , G . A . , l i n e a t 6 8 5 nm f o r remote s e n s i n g s u r v e y s of s u r f a c e c h l o r o p h y l l a. I n : J . F . R . Gower ( E d i t o r ) , Oceanography From S p a c e , Plenum Fress, N e w York, pp. 329-338. Gower, J . F . R . , L i n , S . , and B o r s t a d , G . A . , 1983. The i n f o r m a t i o n cont e n t of d i f f e r e n t o p t i c a l s p e c t r a l r a n g e s f o r r e m o t e c h l o r o p h y l l estimation i n c o a s t a l waters. I n t . J . Remote S e n s i n g ( i n p r e s s ) . G o v i n d j e e and B r a u n , B . Z . , 1 9 7 4 . L i g h t a b s o r p t i o n , e m i s s i o n and S t e w a r t ( E d i t o r ) , A l g a l Physiology photosynthesis. I n : W.D.P. and B i o c h e m i s t r y , Univ. C a l i f o r n i a P r e s s , B e r k e l e y , pp. 346-390. 1979. G o v i n d j e e , Wong, D . , P r e z e l i n , B.B. and Sweeney, B.M., C h l o r o p h y l l 5 f l u o r e s c e n c e of G o n y l a u l a x p o l y e d r a grown on a Photochem. l i g h t - dark c y c l e a f t e r t r a n s f e r t o c o n s t a n t l i g h t . P h o t o b i o l . , 30:405-411. M u e l l e r , J . L . , 1913. The i n f l u e n c e of p h y t o p l a n k t o n on ocean colour spectra. PhD. T h e s i s , Oregon S t a t e U n i v e r s i t y , C o r v a l -
lis.
N e v i l l e , R.A. and Gower, J . F . R . , 1 9 7 7 . P a s s i v e remote s e n s i n g Of phytoplankton v i a c h l o r o p h y l l fluorescence. J . Geophys. R e s . , 82 :3487-3493. P r g z e l i n , B.B., 1981. Light reactions irl photosynthesis. In: T. P l a t t ( E d i t o r ) , P h y s i o l o g i c a l b a s e s of p h y t o p l a n k t o n e c o l o g y . Can. B u l l . F i s h . Aquat. S c i . , 2 1 0 :1 - 4 2 . S m i t h , R . C . and B a k e r , K.S., 1982. O c e a n i c c h l o r o p h y l l c o n c e n t r a t i o n s a s d e t e r m i n e d by s a t e l l i t e (Nimbus-7 C o a s t a l Zone C o l o u r S c a n n e r ) . M a r . B i o l . , 66:269-280.
336 Taylor, J.F.R., B l a c k b o u r n , D . J . a n d B l a c k b o u r n , J . , 1 9 7 1 . The red-water c i l i a t e Mesodinium r u b r u m a n d i t s “ i n c o m p l e t e symb i o n t s ” : a review i n c l u d i n g new u l t r a s t r u c t u r a l o b s e r v a t i o n s . J . Fish. R e s . B d . C a n a d a , 28:391-407.
337
SATELLITE REPRESENTATION OF FEATURES OF OCEAN CIRCULATION INDICATED BY CZCS COLORIMETRY C.S. YENTSCH Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, Main 04575, U.S.A.
ABSTRACT Coastal Zone Color Scanner (CZCS) images have been used to demonstrate that the major factors which influence the patterns of ocean color and hence the abundance of phytoplankton are associated with the density discontinuities of large scale ocean currents. This argues that variations in color in large scale patterns are reflecting phytoplankton growth. Pigment patterns, therefore, are not passive tracers of surface water movement.
INTRODUCTION There is now a considerable number of CZCS images which allow the biological oceanographer to visually see patterns of phytoplankton pigments over large regions of the earth's oceans. In examining these images, one's first impression is that the ocean is characterized by highly diverse patterns of pigment concentrations. It is also evident that the spatial magnitude of these patterns differ. Immediately we can ask : "Are these patterns the result of spatial movements of phytoplankton ? " "Can phytoplankton be considered a conservative tracer of the water masses, thereby producing patterns similarly seen by addina cream to a teacup ? " Or, "Are these patterns explainedinterms of factors other thanhorizontal transport, specifically those factors which we believe regulate the growth and abundance of phytoplankton in the oceans ? " These questions are important to oceanographers since the distribution of phytoplankton in time and space, and the mechanisms controlling this distribution, have been obtained largely by one-dimensional shipboard observations which are limited in coverage of both time and space : the need for remote sensing is driven by the desire to view the
enormity of ocean space in synoptic fashion and to test wether or not we have not biased our impressions by quasi-synoptic observation on ships. This paper has two main goals. First, in a general sense, to acquaint the uninitiated reader with some of the factors affecting large scale distribution of phytoplankton in the oceans and to demonstrate how the spatial distributions are viewed from space. The second goal of the paper is to demonstrate and interpret the large scale patterns of phytoplankton in terms of the major planetary inertial forces that are operatinq on water masses. I will argue that the spatial patterns are reflecting the degree of buoyant forces in the water mass. That is, spatial changes that one observes in these images are regulated by the intensity of vertical mixing throughout the water column. If correct, then the large scale patterns, and perhaps the small scale patterns as well, are reflecting the net growth of phytoplankton. In other words, the distribution of phytoplankton pigment abundance observed in the surface waters of the oceans is representing growth processes and not merely the redistribution of abundance. T o give substance to these goals, I will utilize satellite images and conceptual models as well as water column observations. I have specifically chosen regions of the oceans where fluid forces favor the destruction of buoyancy in the water column, thereby
promoting vertical effects associated the rotary motions the interaction of
mixing. These forces are derived from the shear with major frontal regions of ocean currents, of mesoscale ocean eddies, and friction from tidal flow across shallow waters.
Large scale features of phytoplankton distribution associated with general circulation. For phytoplankton, the extremes of poverty and luxury are defined by the oligotrophic central gyres of the ocean on one hand, and the eutrophic waters that lie adjacent to major continents on the other hand. Between these extremes are sharp gradients of phytoplankton abundance which are correlated with water masses which have an extreme baroclinicity and intense horizontal advection. The effects of ocean currents were seen from space first in satellite thermal images. However, more recently, CZCS colorimetry has demonstrated the sharp color discontinuities associated with ocean currents, thus delineating marked gradients in phytoplankton abundance. The question is : How does large scale flow change the
339
distribution of phytoplankton ? The density field of large ocean currents are "baroclinic analogues" of upwelling and represent the largest, perhaps most important, mechanism in the world's ocean of vertical transport of nutrients (Yentsch, 1974). Some of the best examples of these extensive color fronts representing discontinuities in phytoplankton abundance are found in regions occupied by the western boundary current systems. Figure 1 is CZCS Orbit 0 2 6 4 6 that features the Gulf Stream system from Cape Hatteras to Yarmouth, Nova Scotia. The reader's attention is called to the delineation by color of slope and Gulf Stream waters.
Fig. 1. CZCS Orbit 0 2 6 4 6 featuring the Gulf Stream system from Cape Hatteras to Yarmouth, Nova Scotia. The process which is responsible for the delineation of color concerns augmentation of phytoplankton growth which is directly associated with the geostrophic flow. The first fluid dynamic model of the Gulf Stream was produced by Carl Rossby. He considered the Stream a major jet driving into a non-rotating stratified fluid (Fig. 2 ) . When the earth's rotation (C ) was considered as a f
balance to the pressure gradient (P ) , the Rossby model predicted g that secondary cross-stream flow would be associated with the horizontal advection. This cross-stream flow was transported alonq lines of equal density from the Sargasso Sea into the slope and coastal waters off New England. The important aspect of this model
. ;~-** ~;~-
yi~a-2~.
'-;ria%
b m n m l s t r axes
Ti1ad-L
'uy isvpy c n a l
transport, nutrients necessary for growth traverse great distances
340
Fig. 2 . The Rossby (1936) model of the Gulf Stream system off New England. horizontally and vertically - that is, from the deep waters of the Sargasso Sea to the surface waters of the euphotic zone in slope waters off New England. Examples of the effect of this transport can be seen by comparing density structure across the Gulf Stream (Fig. 3 ) with the distribution of a limiting nutrient such as nitrate-nitrogen in the same section (Fig. 4 ) . Facing into the picture, one sees line of equal density intersecting at station 9, which is referred to as the "cold wall", since cooler, deeper waters are elevated to that side of the Gulf Stream. The enrichment process is signaled by the fact that lines of equal density are mirrored by lines of equal distributions of nitrate which, as mentioned above, is the limiting nutrient for phytoplankton growth in these waters. It should be noted that the fluid dynamics behind movements of the water along the isopycnals, is still not well understood and the resultant magnitude of vertical transport is not well known. In general, the cause of the movement along isopycnals can be considered an imbalance between the pressure gradient (P ) and the Coriolis forces (C,) associated with the g mass transport of the Gulf Stream itself. Regardless of the cause, the fertility of the waters lying adjacent to the main thrust of the Gulf Stream (in the cold wall) can be traced along lines of equal density from the north central Sargasso Sea to the cold wall of the Gulf Stream (Fig. 5). The enriched water entering the eupho-
341
tic zone in the cold wall causes a marked discontinuity in the spatial abundance of phytoplancton chlorophyll. It is this variation that one clearly sees from space by way of CZCS colorimetry as a marked difference in water color which distinguishes the slope waters from that in the Gulf Stream. Other examples of phytoplankton augmentation associated with major ocean currents can be observed in CZCS images of Florida and the western Gulf of Mexico. In this region, the thermal loop current forms a front which is due to the entry of equatorial water into the western Gulf of Mexico through the Straits of Yucatan (Fig. 6). The equatorial water penetrates as far north as
W
1
I
I-
w 300P
56.5 I 1
350-
I I
,’.-.
!
400-
’\
I
1
,
I
:
‘, ,
450-
2:5
1 ‘,
$
500550
\
1
‘.) I
I
\
I I
1
I
\!
,
\
1
,
’\ 1 ‘
’t
: I
600
Fig. 3. Distribution of density (ot) across the Gulf Stream. Section between Cape Cod and Bermuda. Chain 37 July 1963. (Yentsch, 1974)
N and essentially encompasses most of the region of thewestern Gulf of Mexico. The CZCS colorimetric pattern of this image correlates with the general thermal pattern shown in the infrared image (Fig. 6). This correlation shows that warm equatorial waters are associated with low concentrations of phytoplankton pigment and
27’
342
NO3 OMiler50 100 I50 200 250 300 123 4 56 7 8 9 10 I1 01.' ' ' 1 I 1 1 I
350
400 I
I
m
a W
500-
Fig. 4. Distribution of nitrate-nitrogen (pg-at/L) across the Gulf Stream. Section between Cape Cod and Bermuda. Chain 37 July 1963. (Yentsch, 1974). OMILES 50 1 2 3 4
100 5 6
I50
7
2M)
8
9
250 10
. . .. . . . .
300 I1
350
400
12
.
.I Fig. 5. Distribution of chlorophyll (pg-at/L) across the Gulf Stream. Section between Cape Cod and Bermuda. Chain 37 July 1963. (Yentsch, 1974).
343
Fig. 6 . NIMBUS-7 imagery from Orbit 1965 on 15 March 1979. CZCS Channel 1 ( 4 4 3 nm) image where light tone denotes high attenuation of blue due to phytoplankton chlorophyll. Channel 6 images of sea-surface temperature variation in which the dark tone depicts cold water. cooler waters with high levels of phytoplankton pigment. The nutrient enrichment process and increased phytoplankton abundance are a combination of the flow of the loop current and the constraints placed on that current by the shape of the Florida peninsula continental platform. The position of the pigment fronts outlined from the image follow the trend of the isobaths along both coasts of the Florida peninsual (Fig. 7). The general position of these fronts is interpreted to be associated with the mass transport on
344
Fig. 7. NIMBUS-7 CZCS image from Orbit 30 on 2 November 1980, of the Florida region showing chlorophyll concentration (dark) on the coastal shelf (upper) and the major bathymetric features of the region (lower).
345
either side of the peninsula. This is confirmed by comparing the dynamic topography on the western side of the Gulf (Fig. 8). The distributisn of sea level height shows that channel constraints of the mass flow by the Florida escarpment augments the horizontal velocity of the flow. Along with this augmentation of flow, an imbalance between the Coriolis and pressure forces create the isopycnal flow which causes the enrichment of the waters adjacent to the peninsula. Therefore, it is through these processes that
30
25'
20'
I
I
95O
90'
~,
I
85-
800
Fig. 8 . Dynamic topography of the Gulf of Mexico region (Nowlin and McLellan, 1967). we can account for the color outline of the general path of the current. In summary, satellite imagery shows a correspondence between pigment-ocean color and major ocean currents. This demonstrates that the aegeostrophic forces associated with major ocean currents markedly influence phytoplankton growth and hence, their distribution. These large scale processes in effect, dictate the major patterns of growth and abundance of phytoplankton in the oceans. Mesoscale eddies associated with western boundary flow Mesoscale eddies are common features of the Gulf Stream system especially in the region north of Cape Hatteras. These eddies or Gulf Stream rings, as they are often called, form from extensive meanders of the Gulf Stream system (Fig. 9). Such meanders at
346
40'
-
35'-
i 30'
Fig. 9. Chart of the depth, in hundreds of meters, of the isothermal surface, showing the Gulf Stream, nine cyclonic rings, and three anticyclonic rings. Contours based on data obtained between 16 March and 9 July 1975.(From Richardson et al., 1978). t i m e s c l o s e ( p i n c h o f f ) p o r t i o n s o f w a t e r masses o n e i t h e r s i d e o f t h e G u l f S t r e a m . The e d d i e s f o r m e d b y " p i n c h i n q o f f " a w a r m c o r e
of S a r g a s s o S e a w a t e r a r e r e f e r r e d t o a s w a r m c o r e r i n s s a n d r e s i d e i n t h e s l o p e w a t e r t o t h e w e s t o f t h e Gulf S t r e a m ( F i p . 1 0 ) . Cold
c o r e r i n g s a r e t h e r e v e r s e i n t h a t by t h e " p i n c h i n a o f f " p r o c e s s , s l o p e w a t e r i s e n t r a i n e d i n t h e c e n t e r . These c o l d core r i n g s g e n e r a l l y move i n t o t h e S a r g a s s o S e a
( F i g . 1 1 ) . T h e s e rinus were
Fig. 10. Warrr core ring (center) and new ring forming on right.
347
Fig. 11. Cold core ring (CCR) off Cape Hatteras. first observed by Fritz Fuglister using shipboard temperature measurements, however, both warm and cold core rinqs, because of their sharp thermal gradients, are easily identified in satellite thermal imagery. Satellite observation by CZCS colorimetry has demonstrated that both warm. core and cold core rinqs are also well defined in terms of their differences in color
:
The sharp thermal
gradient as seen by the satellite, are mirrored by gradients in phytoplankton pigment (Gordon et al., 1982). The question we can now ask is
:
Why is this so ?
The rotary motion of ocean eddies to phytoplankton growth concerns changes in the vertical distribution of the density field within the eddy. If we assume that phytoplankton growth is nutrient limited and distribution of nutrients is reflected by the density field, then the following concepts (Fig. 12) influence spatial patterns of growth throughout the eddy. N
+
and N- represent
two water masses of nutrient-rich, cold, dense and nutrient-poor, warm buoyant water, respectively. These are enclosed in a cylinder which simulates the dimensions of an oceanic eddy. The two water masses are separated by the density nutrient boundary layer (Nb) which for this discussion we can refer to as the thernocline. In the non-rotational stationary Rode, the boundary between the two water masses is horizontal across the cylinder. However, when the cylinder is rotated with velocities in the surface beina somewhat greater than at depth, the Coriolis and other inertial forces will be balanced by the pressure gradient created by the aeostrophic flow within the eddy. In the anti-cyclonic mode, sea surface level domes up around the axis (warm core) while in the cyclonic mode,
348
STATIONARY
WARM
COLD
CORE
CORE
Fig. 12. Geostrophic relationships in warm and cold core rings. Nb, nutrient boundary: Ze, euphotic zone: H and L are the high and low velocities.
it will be depressed in the axis (cold core). In the anti-cyclonic eddies, such as the warm core rina, the lighter water will accumulate at the center and the heavy water will be swept to the rim of the eddy. Assuming that the volume of the eddy is being maintained, the boundary dips downwardtowards the axis and upward towards the rim of the eddy. The reverse situation occurs in the cold core ring. If the eddy is illuminated from the surface, and the photic layer (Z ) resides at a comparable depth and boundary layer, we can see why productivity is enhanced due to the upward displacement of nutrient rich water. This upward displacement of the nutrient boundary layer allows vertical mixing to easily transport nutrients to the euphotic zone. Therefore, the spatial pattern of phytoplankton distribution reflects relative nutrient addition to the euphotic layer by the differences vertically in the level of the boundary between the two water masses. The explanation for the observed distribution of phytoplankton pigments in rings argues that geostrophic principles apply to these rings. Implicit to this nutrient enrichment hypothesis is the idea that the rotary motion induces nutrient transport along isopycnals and phytoplankton production occurs when these isopycnals intersect the euphotic layers. Coastal tidal processes Simpson and Hunter (1974), Pingree and Griffiths (1978), and Pingree et al. (1975) have pioneerd the approach of using satellite
349
imagery and modelling to the study of tidal frontal phenomena in the waters around Great Britain. Remote sensing was needed to obtain information on water mass structure and its pigment distribution and to obtain these parameters in a synoptic fashion over wide areas. In the final analysis, the concepts derived from either observation and/or numerical modelling were substantiated or reinforced by remote sensing capabilities. In this section, I will describe a similar study which essentially began in 1927 with a series of shipboard observations by H.B. Bigelow in the area of the Gulf of Maine and Georges Bank. His conclusions as the result of the observation are confirmed by satellite imagery taken in 1979. In the beginning, Bigelow measured the thermal structure of water masses of the Gulf of Maine and Georges Bank and computed the stability of these water masses to outline different regimes of vertical mixing. From this analysis, he concluded that the different regimes of temperature which outline the areas of vertical mixing were due to the intense tidal action throughout the area. More recently, Garrett et al. (1978) subjected this region (Fig. 13) to an analysis using a numerical model developed by Simpson and Hunter (1974). This model proposes that the difference between mixed and stratified waters is dictated by an index or ratio of the potential energy (required to thoroughly mix the water) to the rate of energy that is dissipated by the flow or tidal current across the bottom. The relevant parameter of index for separating mixed from stratified waters by tide is referred to as log H/U3, where the water, H, is divided by the tidal velocity frictional component, U3. Essentially, this numerical model (Fig. 13) confirmed Bigelow's original observations that tidally mixed areas were centered on Georges Bank and Nantucket Shoals. It also identified other tidal regions off Nova Scotia and in the Bay of Fundy. The question now asked is how real the model is and/or how accurate Bigelow's original observations are -this is where the satellite images can help us. Comparison of satellite thermal and colorimetric imagery (CZCS, Fig. 14) with the numerical model and Bigelow's observations, confirms that much of the mixing is tidally driven. In the thermal image, the light areas indicate warm water and the dark areas indicate colder waters. The region of Georges Bank and Nantucket Shoals clearly shows u p as well as the cold tidally mixed regions off Nova Scotia. Dark filamentous segments appear to be intrusions of either warm slope water of the Gulf Stream and
350
U S A
BOSTON
r'
Fig. 13. Numerical model of tidal mixing, log H/W3 (after Garrett et al., 1978); 1.5 indicates areas totally mixed by tides. other mixed areas that had not been identified by either observations or modelling. Satellite thermal imagery compared with the bathymetry of this area gives information with regard to the critical mixed depth for tidal activity. The mixed fronts around Georges Bank appear to center on the 60m isobath; this depth appears to be rather consistent for the entire region. The significance of tidal mixing on phytoplankton abundance is explained as follows : During the months when the water column is being heated in this region, the greatest buoyancy of surface waters tends to isolate the nutrient rich water from the euphotic zone. Therefore, the restoration of growth by vertical mixing of nutrients into the euphotic zone becomes crucial in regulating the rate of phytoplankton growth. The conceptual model of the density and nutrient distribution across Georges Bank explains why the Bank itself imparts color and temperature signatures on the water (Fig. 15). Nutrient rich water in deeper waters is brought up into the euphotic zone by the tidal action at the frontal edge on either side of the Bank. This water is mixed across the top of the Bank which is in the euphotic zone and promotes luxurious growth on top of the Bank. In summary, the CZCS colorimetry shows that high concentrations of phytoplankton pigment are located on the Bank and the other frontal regions which outline the areas of vertical mixing, such as Nantucket Shoals. The low phytoplankton pigment concentrations
351
Fig. 14. Orbit 3326 14 June 1 9 7 9 . Top image : sea surface temperature; dark, cold water; light area, warm water. Bottom image : phytoplankton pigment; dark, high pigment; light, low pigment. (Yentsch and Garfield, 1981). occur in the slope waters or in the central region of the Gulf of Maine where tidal mixing and bottom friction action is not effective. Passive tracer or growth There appear to be two obvious hypotheses to attribute to the patterns of ocean color. 1) Distribution of phytoplankton pigments are passive tracers to the movements of surface waters and, 2) Distribution of phytoplankton reflects the fluid aynamics of the
352
isotherms
Fig. 1 5 . Conceptual diagram of nutrient enrichment on Georges Bank. water masses which supplies nutrients for phytoplankton growth the hypothesis of nutrient enrichment for growth. The first hypothesis is unattractive because the satellite images that I have observed show a close correlation between the thermal signatures and the colorimetric signatures. If the low nutrient concentrations in the surface waters of the ocean are limitinq growth and hence, the abundance of phytoplankton, one would expect that the horizontal diffusion would progressively disperse t!ie phytoplankton. Any correlation between temperature and. c o l o r would come about almost by accident. The nutrient enrichment hypothesis argues that it is the vertical flux of nutrients which requlate phytoplankton abundance. It is through this process that one can account for the close correspondence between temperature and water color observed in satellite imagery. This hypothesis also argues that in order to have correspondence between temperature and color, growth must be in excess of that removed by grazing or sinking by the phytoplankton piqment, and is consistent with our concept of how productivity is requlatcd. In short, regulation of abundance is brouqht about by periodic injections of nutrients which change the growth rate in the surface waters of the ocean. The satellite imagery shown in this paper demonstrates tliat color chanaes are closely associated with vertical mixing. The CZCS color pigment patterns are not an undecipherable mix, but clearly reflect the role vertical mixing plays in nutrient supply. A paradox arises
:
the acquisition of buoyancy to water masses is the
antithesis to growth. But growth occurs throuqhout the oceans because
certain forces tend to override the buoyant forces.
These forces are largely associated with ocean currents and the vertical mixinq as a result of bottom friction, and/or the diffe-
353
rences between the density of the water masses. The definition that I have used here of large scale motion requires better definition. The scale of the motions I am discussing are those that are influenced by earth's rotation
-
that is
water motions whose Rossby number is characteristically very smallhence the large scale motions are comparatively slower than the velocity imposed by the earth's rotation. One imagines that in
-
water masses where the Rossby number is very large that is, the flow is large compared to the earth's rotation - color pigment relationships could probably be treated in a Lagrangian sense. Flow of this sort is uncharacteristic of the open ocean. Summary At the opening of this symposium, Jacques Nihoul stressed that remote sensing occupies a companion role with conceptual and numerical modelling. Both are the principle tools by which oceanographers can study their medium. The numerical models used in this text and reported elsewhere, document the interrelationship between modelling and remote sensing. In order for this approach to be successful the modeller has to acquire a mental picture of the pattern of events that will occur in the sea and have the capAbility of comparing these patterns to a satellite image. As more satell-iteimagery becomes available to the biological oceanographer, pattern recognition will become important. This recognition will depend on better measurements of motions in the ocean interior, as well as an appreciation of the size of these features
ACKNOWLEDGEMENTS The author greatly acknowledges the assistance of Pat Boisvert and Jim Rollins in preparing the manuscript. The work was funded by the National Aeronautics and Space Administration, the Office of Naval Research, the National Science Foundation and the State of Maine. REFERENCES
Garrett, C.J.R., Keeley, J.R. and Greenberg, D.A., 1978. Tidal mixing versus thermal stratification in the Bay of Fundy and the Gulf of Maine. Atmosphere-Ocean, 16: 403-423. Gordon, H.R. , Clark, D.K. , Brown, J.W. , Brown, O.B. and Evans, 1982. Satellite measurements of phytoplankton concenR.H.,
364
tration in the surface waters of a warm core Gulf Stream ring. J. Mar. Res., 40: 491-502. Nowlin, W.D., Jr. and McLellan, H.J., 1967. A characterization of Gulf of Mexico waters in winter. J. Mar. Res., 25(1): 29-59. Pingree, R.D. and Griffiths, D.K., 1978. Tidal fronts on the shelf seas around the British Isles. J. Geophys. Res. , 03: 4615-4622. Pingree, R.D., Pugh, P.R., Holligan, P.M. and Forster, G.R., 1975. Summer phytoplankton blooms and red tides along tidal fronts in the approaches to the English Channel. Nature, London, 250: 672-677. Richardson, P.L., Cheney, R.E. and Worthington, L.V., 1978. A census of Gulf Stream rings, Spring 1975. J. Geophys. Res., 83: 6136-6144. Rossby, C.G., 1936. Dynamics of steady ocean currents in light of experimental fluid mechanics. Papers in Phys. Oceanogr. and Meteorol. , 5(1) : 3. Simpson, J.H. and Hunter, J.R., 1974. Fronts in the Irish Sea. Nature, London, 250: 404-406. Yentsch, C.S., 1974. The influence of geostrophy on primary production. Tethys, 6(1-2): 111-118. Yentsch, C.S., 1983. Satellite observation of phytoplankton distribution associated with large scale oceanic circulation. NAFO Sci. Coun. Studies, 4: 53-59. Yentsch, C.S. and Garfield, N., 1981. Principal areas of vertical mixing in the waters of the Gulf of Maine, with reference to the total productivity of the area. In: J.F.R. Gower (Editor), Oceanography from Space, Plenum Publ. Corp. , pp. 303-312.