Oceanography and Marine Biology, An Annual Review, Volume 40: An Annual Review: Volume 40 (Oceanography and Marine Biology)

OCEANOGRAPHY and

M A R I N E B I OL OGY AN ANNUAL REVIEW

Volume 40

HH

OCEANOGRAPHY and M A RI N E B IO L O G Y A N A N N UA L RE V I E W Volume 40 Editors R. N. Gibson and Margaret Barnes The Dunstaffnage Marine Laboratory Oban, Argyll, Scotland e-mail: [email protected]

R. J. A. Atkinson University Marine Biological Station Millport, Isle of Cumbrae, Scotland e-mail: [email protected] Founded by Harold Barnes

First published 2002 by Taylor & Francis 11 New Fetter Lane, London EC4P 4EE Simultaneously published in the USA and Canada by Taylor & Francis Inc, 29 West 35th Street, New York, NY 10001 Taylor & Francis is an imprint of the Taylor & Francis Group This edition published in the Taylor & Francis e-Library, 2004. © 2002 R. N. Gibson, Margaret Barnes & R. J. A. Atkinson All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers. Every effort has been made to ensure that the advice and information in this book is true and accurate at the time of going to press. However, neither the publisher nor the authors can accept any legal responsibility or liability for any errors or omissions that may be made. In the case of drug administration, any medical procedure or the use of technical equipment mentioned within this book, you are strongly advised to consult the manufacturer’s guidelines. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging in Publication Data A catalog record for this book has been requested ISBN 0-203-18059-3 Master e-book ISBN

ISBN 0-203-23179-1 (Adobe eReader Format) ISBN 0-415-25462-0 (Print Edition)

CONTENTS

Preface

vii

Erratum to Vol. 39

viii

A review of sea-level research from tide gauges during the World Ocean Circulation Experiment

1

P. L. Woodworth, C. Le Provost, L. J. Rickards, G. T. Mitchum & M. Merrifield

Coastal and shelf-sea modelling in the European context

37

J. E. Jones

Biogeochemistry of Antarctic sea ice

143

David N. Thomas & Gerhard S. Dieckmann

Accumulation and fate of phytodetritus on the sea floor

171

Stace E. Beaulieu

Impact of changes in flow of freshwater on the Estuarine and open coastal habitats and the associated organisms

233

Bronwyn M. Gillanders & Michael J. Kingsford

A riot of species in an environmental calm: the paradox of the species-rich deep-sea floor

311

Paul V. R. Snelgrove & Craig R. Smith

Status and management of world sea urchin fisheries

343

N. L. Andrew, Y. Agatsuma, E. Ballesteros, A. G. Bazhin, E. P. Creaser, D. K. A. Barnes, L. W. Botsford, A. Bradbury, A. Campbell, J. D. Dixon, S. Einarsson, P. K. Gerring, K. Hebert, M. Hunter, S. B. Hur, C. R. Johnson, M. A. Juinio-Meñez, P. Kalvass, R. J. Miller, C. A. Moreno, J. S. Palleiro, D. Rivas, S. M. L. Robinson, S. C. Schroeter, R. S. Steneck, R. L. Vadas, D. A. Woodby & Z. Xiaoqi

Temporal and spatial large-scale effects of eutrophication and oxygen deficiency on benthic fauna in Scandinavian waters – a review Karin Karlson, Rutger Rosenberg & Erik Bonsdorff

427

Mammals in intertidal and maritime ecosystems: interactions, impacts and implications

491

P. G. Moore v

CONTENTS

Author index

609

Systematic index

671

Subject index

680

HH

vi

PREFACE

The fortieth volume of this series contains nine reviews written by an international array of authors that, as usual, range widely in subject and taxonomic and geographic coverage. The majority of articles were solicited but the editors always welcome suggestions from potential authors for topics they consider could form the basis of appropriate contributions. Because an annual publication schedule necessarily places constraints on the timetable for submission, evaluation and acceptance of manuscripts, potential contributors are advised to make contact at an early stage of preparation so that the delay between submission and publication is minimised. The appearance of this volume is a milestone in two senses. First, it represents an affirmation of the success of the series that has appeared annually for the last forty years and its continued appearance is a tribute to the vision of its founder, the late Harold Barnes. Secondly, it is the last volume with which Margaret Barnes will be associated as Editor. The series was first published in 1963 and initially she was involved with the “Review” in an unofficial capacity but following Harold’s untimely death in 1978, she ensured the uninterrupted continuation of the series by assuming the editorship herself. Since then a further 23 volumes have appeared under her guidance, latterly in collaboration with an expanded editorial team. The many authors with whom she has corresponded over the years will acknowledge her eye for consistency and detail as well as her courtesy in her dealings with them. The world of marine science is greatly in her debt for her longstanding contribution. The editors again gratefully acknowledge the willingness and speed with which authors complied with the editors’ suggestions, requests and questions and the efficiency of the copy editor and publishers in ensuring the regular annual appearance of each volume.

vii

ERRATUM TO VOL. 39

Kupriyanova, E. K., Nishi, E., ten Hove, H. A. & Rzhavsky, A. V. 2000. Life-history patterns in serpulimorph polychaetes: ecological and evolutionary perspectives. Oceanography and Marine Biology: an Annual Review, 39, 1–101. Figure 13 published in the above article was incorrectly printed and should be replaced by the one below. HH

viii

Oceanography and Marine Biology: anFRO Annual 2002, 40, SEA- LEVEL RESEARCH M Review TIDE G AUG E S1–35 © R. N. Gibson, Margaret Barnes and R. J. A. Atkinson, Editors Taylor & Francis

A REVIEW OF SEA-LEVEL RESEARCH FROM TIDE GAUGES DURING THE WORLD OCEAN CIRCULATION EXPERIMENT P. L. WOODWORTH 1 , C. LE PROVOST 2 , L. J. RICKARDS 3 , G. T. MITCHUM 4 & M. MERRIFIELD 5 1 Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead CH43 7RA, UK e-mail: [email protected] 2 Laboratoire d’Océanographie et de Géophysique Spatiale, GRGS/Observatoire Midi Pyrenées, 14 Avenue Edouard Belin, 31400 Toulouse, France 3 British Oceanographic Data Centre, Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead CH43 7RA, UK 4 Department of Marine Sciences, University of South Florida, 140 Seventh Avenue South, St Petersburg, Florida 33701, USA 5 University of Hawaii Sea Level Center, Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, Hawaii 96822, USA

Abstract This paper reviews the developments in tide gauge networks during the World Ocean Circulation Experiment (WOCE) and provides an overview of the resulting contributions to the scientific aims of the programme. The 1990s saw the rapid development of the satellite radar altimetry technique (results from which have been reviewed elsewhere), which played the major role in the measurement of ocean circulation variability during WOCE. This paper describes the complementary roles of altimetric and conventional in situ methods of sea-level recording by gauges which have evolved during the programme. In addition, it highlights those areas of research in which tide gauges (or bottom pressure recorders) have played a particularly important role. A final section looks to the future “age of altimetry” wherein the sea level and ocean circulation community must strive to construct an efficient, unified, global tide gaugeplus-altimetry system for application to a range of scientific objectives.

Introduction The World Ocean Circulation Experiment (WOCE) of the World Climate Research Programme (WCRP) has been the largest, international oceanographic experiment conducted to date. Its proposal during the mid-1980s was constructed to take into consideration the newly-developed capability for satellites to make near-global measurements of the ocean. In addition, agreements between almost 30 nations resulted in the most comprehensive set of deep-ocean hydrographic measurements so far, together with enhanced global capabilities for monitoring conventional in situ physical and chemical parameters. The first phase of WOCE, its field programme, lasted from 1990–97 and results from the first phase have already provided a wealth of new insight into the ocean (Siedler et al. 2001). 1

P . L. WOODW ORTH, C. LE PROVOST , L . J. R I C K A R D S , E T A L .

One of the most innovative WOCE measurement types was undoubtedly sea level observed from satellite altimetry. The spectacular success of the satellite radar altimetry technique and, in particular, of the TOPEX/POSEIDON (T/P) mission, in providing precise, nearglobal and routine measurements of ocean topography assured the success of the WOCE field programme itself. Altimetry is now an established technique of centimetric accuracy and complements perfectly the conventional in situ measurements of sea level by the global network of tide gauges. The role of altimetry in WOCE has recently been reviewed in an excellent paper by Fu (2001). The purpose of the present paper is to describe the measurements of sea level by gauges during WOCE, their particular scientific importance and their complementary value to sea-level measurements by altimetry. Many of the papers referred to will have a publication date after 1997. This is a reflection of the fact that WOCE is continuing in its second phase (called AIMS, see p. 27) and that the datasets collected during the field programme will continue to provide an invaluable resource to ocean circulation studies for many years. The paper is organised as follows. First, the background to the requirement for sea-level data during the WOCE programme is outlined and this background is followed by a description of the evolution and present status of the WOCE sea-level network. The next two sections review the developing confidence in the agreement between sea-level data from tide gauges and altimetry and from model sea-level information that took place during the 1990s. They also describe the resulting construction of methods for ongoing calibration of altimetry using (mostly island) tide gauges. The particular WOCE interest in the study of flows through the “choke points” of the Antarctic Circumpolar Current (ACC) by means of bottom pressure and coastal tide gauge data is then outlined. Other papers concerned with ocean variability on typically seasonal and longer timescales that have been published during WOCE and which are based, at least in part, on tide gauge information are then reviewed. The subsequent section reviews developments in deep ocean tide models, most of which were enabled by advances in altimetry, but which have required tide gauge data for validation. In addition, this section points to the extension of global tide model accuracy into shallow water areas through assimilation of coastal gauge data. The penultimate section describes the dramatic advances in new geodetic techniques for application to a range of ocean and sea-level studies. Finally, the future role of the global tide gauge network for ocean circulation and climate studies is discussed.

The WOCE programme and its sea-level requirements The field phase of WOCE had two goals as agreed at the WOCE International Conference in 1988 (WOCE 1989). The first goal was to develop models useful for predicting climate change and to collect the data necessary to test them, with an expectation that by the end of the field programme computers would be capable of running the global eddy-resolving models required for climate studies. The second goal was to determine the representativeness of the specific WOCE datasets for describing the long-term behaviour of the ocean, and to find methods for determining long-term changes in the ocean circulation. WOCE planning included a strategy for achieving both goals in terms of three core projects which were elaborated upon in the WOCE Implementation Plan (WOCE 1988a,b). 2

SEA- LEVEL RESEARCH FRO M T I D E G A U G E S

The three core projects (CPs) were: CP-1 The Global Description; CP-2 The Southern Ocean; CP-3 The Gyre-Dynamics Experiment. The first two core projects provided the main justifications for sea-level measurements during WOCE. The necessity for providing a global description of ocean circulation variability in CP-1 resulted in the Implementation Plan’s call for a quasi-global sea-level network using altimetry and gauges in combination. Sea-level measurements at island tide gauges were to be used, in effect, as satellite tracking data, to provide long wavelength corrections to the relatively poorly determined satellite orbits of the time. Consequently, considerable discussion took place on how many gauges would be “enough”. While it was generally agreed that gauges would also be required at special locations for ocean circulation measurement such as “choke points”, there seemed at the end of the 1980s to be an emphasis in the WOCE community (but not necessarily in the WOCE sea-level community) that a few 10s of island gauges would be sufficient for scientific requirements, given the availability of altimetry. However, aside from their use in one or two feasibility studies of large-scale sea surface variability and ocean circulation (e.g. Wunsch 1991a,b), island tide gauge data were never employed as tracking data as originally intended. The radical improvement in parameterisation of the Earth’s gravity field, uncertainties in which were the main source of errors in satellite orbit computations, resulted in the disappearance of “tilt and bias” and other ad hoc schemes (such as the use of island gauges) to reduce orbit error in altimeter data (Tapley et al. 1994). An altimeter satellite such as T/P became essentially a “tide gauge in space”, with a sea surface height accuracy of approximately 4 cm (Fu et al. 1996). Tide gauge data became employed primarily as a source of validation information for time series of altimeter seasurface heights, as a special source of sea-level data at ocean boundaries and straits where the spatial-temporal sampling of altimetry is not optimal. Most recently, as the T/P time series has extended beyond 8 yr, they have been used as a source of long-term calibration information to the overall altimetric measurement system. CP-2 had special importance for WOCE given the Southern Ocean’s influence on the water masses of the entire world ocean through the inter-ocean linkage of the ACC with a transport of approximately 130 Sv (1 Sv = 106 m3 s−1), and given its importance to deep water formation and other ventilation processes that affect global climate. A recent special issue of the Journal of Geophysical Research (Vol. 106, C2) contains a number of papers discussing aspects of Southern Ocean and ACC science studied during WOCE. An updated review of the ACC system is given by Rintoul et al. (2001). CP-2 demonstrated the advantages of conventional coastal or deep sea gauges (of which the latter are usually referred to as bottom pressure recorders, BPRs) in particular situations. The first was with regard to measurements at the “choke points” of the ACC which include the Drake Passage, southern Indian Ocean (e.g. Amsterdam–Kerguelen) and Australia– Antarctica. In the case of choke points or straits, pairs of coastal gauges or BPRs are a better choice for circulation monitoring than altimetry, with its long “return periods” of several days to weeks. A second advantage comes from the need for measurements in ocean areas which might either be ice-covered for a large part of the year or which, in the case of T/P, might be at latitudes higher than the inclination of the satellite. The operation of gauges in “environmentally hostile regions” became the subject of international working groups (IOC 1988, 1991, 1992) with the result that the Antarctic component of the global tide gauge network was enhanced significantly. Meanwhile, technical developments in the use of BPRs for WOCE resulted in the provision of instruments that could be deployed safely in deep waters for up to 5 yr (Spencer & Vassie 1997). 3

P . L. WOODW ORTH, C. LE PROVOST , L . J. R I C K A R D S , E T A L .

Sea-level measurements for WOCE Altimeter and tide gauge plans for WOCE The potential for global measurements of sea-level changes by means of satellite radar altimetry had been demonstrated prior to the start of WOCE by the Skylab (launch 1973), GEOS-3 (1975), Seasat (1978) and Geosat (1985) missions. All of these projects were funded by US agencies with data made available freely to the international community (although with a delay of several years in the case of the Geosat Geodetic Mission). This spirit of international co-operation within a national (or bi-lateral) programme reached new heights with the US/French T/P (1992) project, the planning for which benefited considerably from the establishment of an international Science Working Team (SWT) several years prior to launch. The relatively high-altitude T/P altimeter satellite was the first to be designed specifically for the study of large-scale ocean circulation and ocean tides. Collaboration also played a part in planning for the much-delayed European ERS-1 (1991) mission, although not to the same extent as for T/P. A consequence of these activities was that by the effective start of WOCE in the early 1990s, an international community of oceanographic researchers was in place ready to exploit the upcoming altimetric sea-level datasets, with which researchers were provided essentially free of charge. T/P and ERS-1, replaced eventually by ERS-2 (1995), performed excellently through the 1990s and continue to provide high-quality data to the present day (Fu 2001, Fu & Cazenave 2001). The Geosat Follow-On (1998) has not so far produced the anticipated datasets. At the time of writing, we expect the US/French JASON-1 satellite to be launched in December 2001 and to eventually replace T/P, while the European Envisat mission will be launched in March 2002 to continue the ERS time series. Further missions are anticipated for launch throughout the next decade. To many researchers, the apparent ease with which the delivery of altimeter data could be made to researchers by space agencies contrasted with the many difficulties experienced by the international group of tide gauge operators in supplying complementary in situ information. Why, one was sometimes asked, cannot a suitable near-global set of tide gauge data be collected without major effort and with a fraction of the cost of the satellites? Surely such data must be routinely available? This criticism (however understandable) stemmed from a lack of knowledge of the way in which tide gauge data were acquired on a global basis, and of the fact that the “global network” was constructed from a set of disparate national gauge sites, data from which were pooled by voluntary national contributions to international databanks. Given that funds (however small) for the in situ component were not forthcoming from the space agencies or the WOCE programme itself, the tide gauge network for WOCE had to be constructed around that which had been assembled for earlier, regional programmes. This included, in particular, the network developed by the University of Hawaii for the Tropical Ocean Global Atmosphere (TOGA) programme (Kilonsky & Caldwell 1991, McPhaden et al. 1998) and the Integrated Global Ocean Services System (IGOSS) sea level project (Kilonsky et al. 1997). Other elements included gauges at islands in the South Atlantic operated by the Proudman Oceanographic Laboratory (POL) (Spencer et al. 1993), and in the southern Indian Ocean by French groups (Le Provost et al. 1995b). The responsibility

4

SEA- LEVEL RESEARCH FRO M T I D E G A U G E S

for data flow from these sites to researchers was meanwhile delegated to two centres at the University of Hawaii Sea Level Center (UHSLC) and the British Oceanographic Data Centre (BODC) at POL, a task-sharing arrangement which has functioned well for most of the programme (Kilonsky et al. 1999). Sea-level data collection from tide gauges remains essentially multinational in character. There is still no one large source of funding with which to operate a coherent global in situ network. Nevertheless, experience during WOCE, and within the wider context of the Intergovernmental Oceanographic Commission (IOC) Global Sea Level Observing System (GLOSS) programme (p. 14), has been valuable in convincing individual national tide gauge agencies of the international importance of their data and thereby of the necessity to maintain and upgrade tide gauge installations.

The WOCE Tide Gauge Network The WOCE Tide Gauge Network has evolved during the 1990s into that described by Figure 1 and Tables 1, 2 and 3. The network currently comprises 160 stations reporting in “delayed mode” and 127 in “fast mode”. With a few exceptions, all of the fast mode data are incorporated into the delayed mode dataset. However, the delayed mode dataset includes additional data that are not available in real or near-real time. The data are usually supplied as hourly values, but in some cases may be more frequent (e.g. 6 min and 15 min). During the WOCE period, efforts have been made by several countries to install new tide gauges, especially in the polar regions. For example, Denmark has installed several gauges around the coast of Greenland, and Australia, France and the UK have installed gauges on islands in the Southern Ocean and around the coast of Antarctica. These installations have improved the distribution of gauges and enhanced the global sea-level dataset. In parallel, the number of gauges delivering their data in near-real time has increased. The WOCE Data Handbook (WOCE, 1994) stated that hourly, or more frequent, observations of sea level are required and, for locations at mid or high latitudes, the tide gauge measurements should be supplemented, wherever possible, by sea-level atmospheric pressure data. The strategy for tide gauges in WOCE was to take advantage of the existing extensive regional networks such as those provided by the GLOSS and TOGA programmes and to extend them in accordance with the following needs: (1) (2) (3)

to complement altimetric measurements in oceans with sparse island distributions; to instrument the high latitude Southern Ocean, both to complement altimetric measurements and as an independent measure of variability in a poorly observed region; to instrument straits and channels which can be monitored by surface elevation measurements and through which there is considerable transport (e.g. Drake Passage).

Under the WOCE data management system, data flows from the data collectors to Data Assembly Centres (DACs) and Special Analysis Centres (SACs). The data are quality assured and then distributed to end-users and archived. The DACs and SACs ensure the direct involvement of research groups in the management of WOCE datasets. When WOCE

5

Figure 1

The WOCE Tide Gauge Network. Fast and Delayed Mode stations are indicated by circles and triangles, respectively.

P . L. WOODW ORTH, C. LE PROVOST , L . J. R I C K A R D S , E T A L .

6

SEA- LEVEL RESEARCH FRO M T I D E G A U G E S

Table 1 Station Name

Ammassalik, Greenland Ascension, United Kingdom Atlantic City, NJ, USA Basques, Canada Bermuda, St Georges Is., UK Cape Hatteras, North Carolina, USA Ceuta, Spanish N Africa Charleston, South Carolina, USA Churchill, Canada Dakar, Senegal Diego Ramirez, Chile Duck, North Carolina, USA Edinburgh, Tristan da Cunha Esperanza, Argentina (Antartica) Exuma, USA Faraday, Argentine Is. Fortaleza, Brazil Fort Pulaski, Georgia, USA Galveston, Texas, USA Gibraltar

WOCE Atlantic Ocean sea-level stations.

GLOSS No. 228 263 220 221

UHSLC No.

Latitude (N +ve)

Longitude (E +ve)

Fast-mode data

Delayed-mode data 1990–1996 1983–1998

291

65.50 −7.92

−37.00 −14.42

1993–2000

264 273 259

39.35 47.57 32.37

−74.42 −59.13 −64.70

1985–2001 1997–2001 1985–1999

35.14

−75.03

9006 249 9039

261

35.90 32.78

−5.32 −79.93

1985–2001

9056 253

274 223

58.78 14.67

−94.02 −17.04

1985–2001 1994–2001

180 219 266 185

599 260

−56.52 36.18 −37.50 −63.40

−68.72 −75.07 −12.03 −56.98

1993–1998 1985–2001

23.77 −65.25

−76.01 −64.27

283 752

−3.43 32.03

−38.47 −80.90

2000 1985–2001

217 248

775

29.31 36.12

−94.79 −5.04

1985–2001

222 265

275

44.67 −20.50

−63.58 −29.03

1985–2001

601

12 188

289

1961–1997 1982–1989, 1992–1995 1991 1979–1997 1984–1995

1992–1993 1959–1971, 1984–1997

9019 216

242

69.22 24.55

−51.01 −81.81

1985–2001

Lerwick, United Kingdom

236

293

60.15

−1.13

1993–2001

Little Cornwallis, Canada Lome, Togo Miami, Haulover Pier, USA Newport, Rhode Is., USA Newlyn, United Kingdom Nuuk/Godthaab, Greenland Palmeira, Cape Verde

153 224

75.23 6.13 25.90

−96.57 1.28 −80.12

1989–1993

1961–1990, 1993–1996 1920–1997 1974–1975, 1993 1992–1996 1926–1954, 1969–1995, 1997 1959–1978, 1980–1999 1986–1994 1982–1992

290

253

41.51

−71.33

1985–2001

241 225

294

50.10 64.17

−5.06 −51.73

1985–2001

235

16.75

−22.98

2000

7

1971–1991 1985–1996

1996–1998

Halifax, Canada Ilha da Trindade, Trinidad and Tobago Ilulissat, Greenland Key West, Florida, USA

218

1968–1989, 1991–1995 1992–1997

1915–1999 1985–1995

P . L. WOODW ORTH, C. LE PROVOST , L . J. R I C K A R D S , E T A L .

Station Name

Penedo Sao Pedro e Paulo, Brazil Pensacola, Florida, USA Ponta Delgada, Azores

Port Stanley, United Kingdom

GLOSS No.

Table 1

continued

UHSLC No.

Latitude (N +ve)

199

Longitude (E +ve)

0.92

−29.34

Fast-mode data

1982–1985

288 245

762 211

30.44 37.74

−87.21 −25.67

1985–2001 1994–2001

305

290

−51.75

−57.93

1992–2000

726 245 225

16.87 −54.93 60.72 64.15 −3.85 18.46 0.17

−24.98 −67.62 −46.33 −21.93 −33.82 −66.12 6.51

1999–2000 1985–2001 1988–1996

Porto Grande, Cape Verde Puerto Williams, Chile Qaqortoq, Greenland Reykjavik, Iceland Rocas, Atol Das, Brazil San Juan II, Puerto Rico Sao Tome, Sao Tome

254 9003 9020 229

Settlement Point, USA Siboney, Cuba Signy, South Orkney Is. Sisimiut, Greenland South Caicos, UK St. Croix, Virgin Is.

211 215 306 9021 296 9011

257

26.77 23.92 −60.70 66.93 22.00 17.70

−79.00 −82.47 −45.06 −53.67 −72.00 −64.77

1985–2001

264 223 238

292 276 295

−15.97 47.57 58.22

−5.07 −52.72 −6.38

1993–2001 1993–2001 1985–2001

237 181 9023

600

62.00 −54.80 25.73

−6.77 −68.03 −80.16

1996–2001

St. Helena, UK St. John’s, Canada Stornoway, UK Torshavn, Faroe Islands Ushuaia, Argentina Virginia Key, Biscayne Bay

206 260

Delayed-mode data

1978–1980, 1982–1991, 1993–1995, 1998 1964–1969, 1974, 1988–1989, 1991–1998 1990, 1993 1964–1998 1991–1996 1984–1997 1985–1998 1985–1986, 1988 1985–1993 1990 1988–1996 1991–1996 1991–1992 1991–1993, 1996–1997 1986–1998 1961–1996 1974–1983, 1985–1999 1985–1995 1996–1998 1994–1997

recognised the need for in situ sea-level data, it was only natural to take advantage of the experience that already existed at both UHSLC and BODC, and request that they become WOCE DACs. The UHSLC was established as the “fast mode” DAC and tasked with the assembly, quality control and distribution of all sea-level data from WOCE gauges delivered by satellite or other near-real time systems. The data were to be made available to investigators in a time frame of 1–3 months after data collection. The BODC, as the “delayed mode” DAC, was to assemble and supply sea-level data from the WOCE network to the full extent of quality control. Distribution was to be within 18–24 months after data collection. BODC was also tasked to ensure archival of the sea-level data as a WOCE dataset in the World Data Centre (WDC) system by the end of the WOCE experiment. 8

SEA- LEVEL RESEARCH FRO M T I D E G A U G E S

Table 2 Station Name

WOCE Indian Ocean sea level stations.

GLOSS No.

UHSLC No.

Latitude (N +ve)

Longitude (E +ve)

−10.42

105.67

Fast-mode data

Delayed-mode data 1986–1987, 1990–1991 1985–2000

Christmas Is., Australia

47

Cocos Is., (Keeling), Australia Crozet Island, France Darwin, Australia Diego Garcia, UK

46

171

−12.12

96.09

1985–2001

21 62 26

178 168 104

−46.43 −12.47 −7.29

51.87 130.09 72.39

1995–2001 1985–2001 1988–2000

13 54 27 9041

176 109 117

−29.87 −33.87 −0.69 6.77

31.52 121.09 73.15 73.17

1985–2001 1987–2001 1991–2001

− 49.35 4.23 4.18

70.02 100.62 73.53

Durban, South Africa Esperance, Australia Gan, Republic of Maldives Hanimaadhoo, Republic of Maldives Kerguelen Island, France Lumut, Malaysia Male (Hulhule), Republic of Maldives Mawson, Antarctica Mombasa, Kenya Point La Rue, Seychelles Port Elizabeth, South Africa Port Louis Harbour, Mauritius Port Victoria, Hodoul Is., Seychelles Rodrigues, Mauritius Salalah, Oman Simonstown, South Africa St. Paul Island, France Zanzibar, Tanzania

23 43 28

180 108

1993–2001 1989–2001

22 8 9042 76

101 121

−67.60 −4.70 − 4.67 −33.96

62.88 39.66 55.53 25.06

1986–2000 1993–2001

18

103

−20.16

57.05

1986–2001

−4.62

55.46

−19.67 16.94 −34.19 −38.72 −6.16

63.42 54.07 18.04 77.58 39.02

273 19 4 268 24 297

105 114 179 151

1986–2001 1989–2001 1994 –2001 1985–2001

1993–1997 1984–1998 1969, 1988–1998 1970–1997 1990–1998 1987–1998 1991–1998 1993–1998 1984–1995 1988–1998 1991–1997 1986–1998 1993–1998 1973, 1978–1997 1942–1947, 1964–1965, 1986–1998 1977–1982, 1986–1992 1986–1997 1989–1998 1958–1996 1994–1998 1984–1998

The “fast mode” sea level DAC The creation of the WOCE DAC necessitated a major new initiative for the UHSLC. Before it was established, sea-level data were collected, processed, and distributed within 1–2 yr after the calendar year of the data collection. The WOCE “fast mode” DAC, on the other hand, was to provide information needed to check the altimeter data against the more traditional and well-understood sea-level data from the tide gauges. The altimetry data were available within a month or so of collection. Thus, the UHSLC had to process data from a globally distributed set of stations and make the in situ sea-level data available to users on a comparable timescale. The turn around time for this dataset is much faster than for the TOGA dataset, and the geographical extent of the dataset exceeds that of the earlier TOGA and IGOSS datasets. Fortunately, the UHSLC had access to a large fraction of the open ocean 9

P . L. WOODW ORTH, C. LE PROVOST , L . J. R I C K A R D S , E T A L .

Table 3

WOCE Pacific Ocean sea-level stations.

Station Name

GLOSS No.

UHSLC No.

Latitude (N +ve)

Longitude (E +ve)

Fast-mode data

Delayed-mode data

Aburatsu, Japan Adak Island, Alaska, USA Apra Harbor, Guam Arica, Chile Balboa, Panama Baltra, Galapagos Is.

82 302 149 9001 168 169

354 40 53 83 302 3

31.57 51.86 13.43 −18.47 0.96 −0.44

131.42 −176.06 144.07 −70.33 −79.57 −90.03

1985–2001 1985–2001 1985–2001 1985–1999 1985–2001 1985–2001

1961–1998 1950–1997 1948–1997 1982–1998

113

2

1.37

172.93

1988–2001

79

−46.60 −24.83 22.88 −27.67 −12.50 −67.57 −43.95

168.04 152.04 −109.91 −70.83 −77.02 −68.13 −176.57

9058 103 146

47 11

23.88 27.10 1.98

121.38 142.18 −157.47

1985–2001 1985–2001

9043

556

41.75

−124.18

1985–2001

57 158

41 333 551

7.83 53.90 −33.85 37.81

125.63 −166.05 151.23 −122.05

1992–2001 1985–2001 1985–2001

1985–1998 1901–1996

107

14

23.87

−166.03

1985–2001

1974–1998

121 88

25 364

−8.53 41.78

179.22 140.73

1985–2001 1985–2001

287

60

19.73

−155.67

1985–2001

−42.88

147.33 159.96 −157.87 121.58 124.02 168.62 −169.52 −78.83

1977–1998 1967, 1969–1998 1927–1932, 1946–1997 1985, 1987–1995 1974–1998 1905–1997 1984–1993 1975–1998

1999–2001 1985–2001 1990–1998

−156.47 −171.72

1985–2001

Betio, Tarawa, Gilbert Is., Kiribati Bluff Harbour, New Zealand Bundaberg, Australia Cabo San Lucas, Mexico Caldera, Chile Callao, Peru Cendering, Malaysia Chatham Island, New Zealand Chen Kung, Taiwan, China Chichijima, Japan Christmas, Line Is., Kiribati

Crescent City, California, USA Davao, Philippines Dutch Harbor, Alaska, USA Fort Denison, Australia Fort Point (San Francisco), California, USA French Frigate Shoals, Hawaii, USA Funafuti, Ellice Is., Tuvalu Hakodate, Japan Hilo, Hawaii, USA

129 59 161 9002 173 293 128

332 34 88 93

71

Hobart, Australia

9018

Honiara, Solomon Islands Honolulu, Hawaii, USA Hualien, Taiwan, China Ishigaki, Japan Jackson Bay, New Zealand Johnston Island, USA Trust Juan Fernandez, Chile

66 108 9014 9015 109 176

403 52 21

−9.43 21.31 23.97 24.33 −43.98 16.75 −33.62

Kahului, Hawaii, USA Kanton, Phoenix Is., Kiribati

9025 145

13

20.90 −2.81

9 57

10

1985–2001 1985–2001 1985–2000 1985–2001

1968–1977, 1985–1998 1974–1998 1984–1989 1997–1998 1973–1998 1980–1998 1985–2000 1990–1995

2000–2001

1985–2001 1985–2001

1993–1994 1975–1997 1955–1963, 1965–1972, 1974–1998 1951–1984, 1996–1997 1984–1990

1947–1997 1977–1979, 1981–1988, 1990–1998 1950–1997 1949–1967, 1972–1998

SEA- LEVEL RESEARCH FRO M T I D E G A U G E S

Station Name Kapingamaringi, Caroline Is, Fd. St. Micronesia Ketchikan, Alaska, USA Kodiak Island, Alaska, USA Kushimoto, Japan Kushiro, Japan

GLOSS No.

Table 3

continued

UHSLC No.

Latitude (N +ve)

Longitude (E +ve)

Fast-mode data

Delayed-mode data

117

29

1.98

154.78

1985–2001

1978–1998

9046 9047 85 89

571 39 353 350

55.33 57.73 33.47 42.97

−131.63 −152.52 135.78 144.38

1985–2001 1985–2001 1985–2001 1985–2001

Kwajalein, Marshall Island

111

55

8.73

167.73

1985–2001

La Jolla (San Diego), California, USA La Libertad, Ecuador Lautoko, Fiji Lobos De Afuera, Peru Lombrum, Papua New Guinea Macquarie Is., Australia Majuro, Marshall Islands

159

569

32.72

−117.17

1985–2001

1949–1997 1975–1997 1961–1997 1963, 1965–1998 1946–1995, 1997 1906–1997

172 9048 9027

91 402 84 400

−2.20 −17.60 −6.93 −2.33

−80.92 177.43 −80.72 147.37

1985–2001 1992–2001 1985–1999 1994–2001

1982–1994 1994–1998

130 112

5

−54.48 7.10

158.97 171.37

1985–2001

134.46 125.19 −104.33

1985–2001

Malakal, Belau Manado (Bitung), Indonesia Manzanillo, Mexico

Mera, Japan

120 69 163

7 395

7.33 1.44 19.50

86

352

34.92

139.83

1985–2001

1992–2001

Midway Island, Hawaii, USA Moturiki, New Zealand Nagasaki, Japan

106

50

28.22

−177.37

1985–2001

9022 83

362

−37.65 32.73

176.18 129.87

1985–2001

Naha, Japan Naos Island, Panama Nauru, Gilbert Is., Kiribati Nawiliwili, Hawaii, USA Naze, Japan

81 9049 114 9024 9016

355 300 4

26.22 8.92 −0.53 21.97 28.38

127.67 −79.53 166.09 −159.04 129.05

1985–2001 1991–1998 1985–2001

Neah Bay, Washington, USA Nishinoomote, Japan Noumea, New Caledonia Nuku Hiva, Marquesas Is.

9068

558

48.37

−124.62

1985–2001

9017 123 142

19 31

30.73 −22.29 −8.93

130.10 166.44 −140.82

1985–2001 1985–1998

Nuku’alofa, Tonga Ofunato, Japan Pago Pago, American Samoa Papeete, Tahiti Pascua (Easter) Island, Chile

9051 87 144 140 137

38 351 56 15 22

−21.13 39.67 −14.28 −17.53 −27.15

−175.17 141.72 −170.68 −149.57 −109.05

1990–2001 1985–2001 1985–2001 1985–2001 1985–2001

11

1993–1997 1968–1972, 1974–1998 1969–1998 1986–1990 1953–1959, 1961–1982, 1992–1998 1965, 1967–1997 1947–1997 1995 1964, 1968–1998 1966–1998 1991–1997 1974–1998 1954–1997 1965–1974, 1976–1998 1934–1997 1965–1998 1967–1998 1982, 1986–1997 1990–1998 1965–1998 1948–1997 1969–1999 1957–1958, 1962–1963, 1977–1998

P . L. WOODW ORTH, C. LE PROVOST , L . J. R I C K A R D S , E T A L .

Station Name

GLOSS No.

Table 3

continued

UHSLC No.

Latitude (N +ve)

Longitude (E +ve)

Fast-mode data

Delayed-mode data

143

24

−8.98

−158.53

1985 –2001

1977–1998

115

1

6.98

158.03

1985–2001

9026

46

−17.77

168.03

1993–2001

Prince Rupert, Canada

155

540

54.32

−130.33

1985–2001

Provideniya, Russia Prudhoe Bay, Alaska, USA

309 151

579

64.40 70.20

−173.02 −148.03

1994–2001

Quepos, Costa Rica

167

87

9.40

−84.17

1985–1995

65

10

−4.20

152.02

1985–2001

139 138 118 177

23 16 28 35

−21.21 −23.13 15.23 −26.28

−159.08 −134.95 145.74 −80.13

1985–2001 1985–2000 1985–2001 1992–1997

9053 150 154 162 157 56 9057 122 9054 156 60 116

30 560 559 90 592 335

−0.75 60.17 57.05 18.73 44.63 −42.55 24.59 −18.13 −4.58 49.15 −19.25 7.45

−90.31 −149.43 −135.34 −111.17 −124.04 147.93 121.87 178.43 −81.28 −125.92 146.83 151.09

1985–2001 1996–2001 1994–2001 1992–1997 1996–2001 1985–2001

1969–1971, 1974–1998 1977–1982, 1993–1998 1909–1922, 1963–1995 1977–1989 1992–1993, 1995–1997 1961–1965, 1971–1994 1966–1971, 1974–1997 1977–1998 1969–1998 1978–1998 1987–1993, 1996–1997 1978–1998

Penrhyn, Cook Islands Pohnpei, Caroline Is., Fd St. Micronesia Port Vila, Vanuatu

Rabaul, Papau New Guinea Rarotonga, Cook Islands Rikitea, Gambier Saipan, Mariana Islands San Felix, Chile Santa Cruz, Galapagos Is. Seward, Alaska, USA Sitka, Alaska, USA Socorro Is., Mexico South Beach, Oregon, USA Spring Bay, Australia Suao, Taiwan, China Suva, Fiji Talara, Peru Tofino Townsville, Australia Truk Atoll, Caroline Is., Fd St. Micronesia Valparaiso, Chile

Wake Island, Marshal Is. Yakutat Bay, Alaska, USA Yap, Caroline Is, Fd. St. Micronesia

18 92 334

1985–2001 1992–1996 1985–2001

175

81

−33.33

−71.63

1985–2000

105

51

19.28

166.62

1985–2001

9055 119

570 8

59.55 9.51

−139.73 138.13

1985–2001 1985–2001

1991–1998 1981–1992 1972–1998 1950–1965 1963–1996 1985–1998 1963–1995 1944–1974, 1977–1978, 1982–1998 1950–1967, 1969–1997 1961–1997 1951–1952, 1969–1970, 1973–1998

gauges that were capable of reporting sea-level data quickly enough to meet the near-real time requirement. The UHSLC also had experience with distributing data and data products on the required timescale via the IGOSS sea level project. The UHSLC in situ sea-level stations employ a robust design that emphasises redundancy of measurements including an automated switch that produces reference level information (Mitchum et al. 1994). These stations provide long-term sea-level monitoring accurately 12

SEA- LEVEL RESEARCH FRO M T I D E G A U G E S

related to a datum at the millimetre level using inexpensive float-operated as well as acoustic gauges. They upload data to the UHSLC via the National Oceanic and Atmospheric Administration (NOAA) Geostationary Operational Environmental Satellite (GOES) Data Collection System (DCS), Japan’s Geostationary Meteorological Satellite (GMS) DCS, and the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Meteorological Satellite (METEOSAT) DCS, and provide the nucleus of the WOCE “fast mode” dataset. The UHSLC has also expanded upon the contacts made with other national agencies contributing sea-level data to the TOGA and IGOSS projects. Through these activities, described more fully by Kilonsky et al. (1997), the UHSLC has significantly augmented their collection of near-real time sea-level data.

The “delayed mode” sea-level DAC BODC’s role as the “delayed mode” DAC is to assemble, distribute and supply sea-level data to the full extent of quality control possible covering all of the gauges in the network. WOCE requirements are that the elevations should be accurate to 1 cm, the timing to 2 min and the atmospheric pressure measurements to 1 mbar. Operation of the “delayed mode” sea level DAC meant an expansion of BODC’s previous sea-level activities to complement the “fast mode” centre activities described above. Data collected by the UHSLC as part of the TOGA dataset provided a significant element of the initial dataset but contacts were also initiated with some 25 organisations around the globe requesting their sea-level data for WOCE. Data are requested annually, with atmospheric pressure data requested in addition to sea level, although these have not proved easy to obtain. Special effort has gone into obtaining data from gauges which not readily accessible in near-real time. In addition to the sea-level data collected from standard tide gauges, BODC also has the responsibility for data collected by bottom pressure recorders and inverted echo sounders. Most of the contribution for this part of the dataset comes from the POL’s ACCLAIM (Antarctic Circumpolar Current Levels by Altimetry and Island Measurements) network in the South Atlantic.

WOCE tide gauge datasets and products The major product from the two WOCE Sea Level DACs is a quality controlled tide gauge dataset (Rickards & Dowell 1992, Dowell & Rickards 1993). Both organisations carry out their own quality control, and although there are some differences in details, the essential elements are the same. These centre on tidal analysis of the data, and comparisons of data between neighbouring sites, to resolve any problems of timing drifts and to maintain reference level stability. Obvious spikes in the data are removed and gaps are documented. Any remaining questionable fluctuations are carefully checked to determine whether the fluctuation is a real event or perhaps an indication of a mechanical problem with the tide gauge. Documentation is assembled describing the tide gauge and its site, benchmarks, levelling and datum history, peculiar characteristics of the tide gauge (e.g. complex local bathymetry, seiching, silting up of the harbour, river mouths) and summarising the data completeness and quality. Both DACs place a strong emphasis on discussion of the data with the supplier to resolve any problems and to improve data quality. 13

P . L. WOODW ORTH, C. LE PROVOST , L . J. R I C K A R D S , E T A L .

The UHSLC is presently collecting data from 127 stations for the “fast mode” component. These include contributions from the Laboratoire d’Océanographie et de Géophysique Spatiale (LEGOS) in France (5 stations), the Australian National Tidal Facility (ANTF) (19 stations), Canada (5 stations), Brazil (2 stations), BODC in the UK (6 stations), the US National Ocean Service (31 stations), and, building upon their contribution to the IGOSS maps, the Japanese Meteorological Agency (8 stations along the coast of Japan). For 82 “fast mode” stations, the existing time series have been extended backward to 1985 in order to connect Geosat and T/P datasets. The “fast mode” holdings now include over 1650 site-yr of hourly data. The BODC is at present collating data from approximately 160 tide gauge sites worldwide for the WOCE “delayed mode” DAC. Data from these have been supplied from 20 countries. All historical data from these tide gauges have been requested in addition to the data collected during the WOCE period. Several sites have data extending back over 80 yr and approximately 35 extend over 40 yr. The total volume of data received so far is over 3550 site-yr. Most data are supplied in digital form, but occasionally historical manuscript data has been received and digitised, for example from Gibraltar. Included in the “delayed mode” dataset are data from the ACCLAIM network in the South Atlantic (seven island and Antarctic tide gauges plus bottom pressure gauge data). These latter are from deployments at about 20 locations. At present 50 records, each usually of approximately 1 yr in length, have been collected. Data from both WOCE DACs are in high demand. Regular requests for sea-level data from WOCE scientists are received. However, since the establishment of the public directories (i.e. ftp sites), and web sites at the DACs, most requests are serviced by the scientists themselves, and a record of them logged. Since the inception of WOCE, advances in web and internet technologies have enabled the UHSLC DAC to present an increasing volume of near-real time data online, and to participate in the development of data portals and hubs, such as the Pacific Marine Environmental Laboratory (PMEL) Climate Data Portal and the National Oceanographic Partnership Program (NOPP) Virtual Ocean Data Hub (Soreide et al. 2001). For further information, the UHSLC and BODC web sites (http://uhslc.soest.hawaii.edu and http://www.bodc.ac.uk, respectively) may be consulted. The first set of WOCE CD-ROMs was released to coincide with the WOCE Scientific Conference in May 1998 (WOCE 1998). These were distributed to all scientists registered for the Conference and the Sea Level DACs distributed further copies to their data suppliers. Version 2.0 of the WOCE Global Data CD-ROMs (WOCE 2000) were published in September 2000, and have been widely distributed to the WOCE Community. Again, copies have been forwarded to the sea-level data suppliers, GLOSS contacts and other sea-level scientists. In addition to the datasets from the WOCE Sea Level DACs, the CD-ROM includes a tidal constituents dataset produced by LEGOS, Toulouse, the Permanent Service for Mean Sea Level (PSMSL) monthly and annual mean dataset and the GLOSS Station Handbook. There are also links back to the web pages of each DAC so that recently acquired data can be accessed. A final version of the WOCE Global Data CD-ROM set is due to be published in 2002.

The Global Sea Level Observing System (GLOSS) programme The WOCE sea-level activities took place alongside those of the GLOSS programme of IOC. GLOSS was proposed in the mid-1980s by Dr David Pugh (of what is now the Proudman Oceanographic Laboratory) and Professor Klaus Wyrtki (of the University of 14

SEA- LEVEL RESEARCH FRO M T I D E G A U G E S

Hawaii) as a means of ensuring the long-term provision of worldwide sea-level information from tide gauges to the PSMSL and to international oceanographic programmes such as WOCE (IOC 1990, Woodworth 1991, 2000). The major component of the programme is the GLOSS Core Network (GCN), the island subset of which corresponds closely to that of the WOCE network, and which is now over two-thirds complete. During the 1990s, a number of important developments took place in the sea-level field, including the provision of precise altimetric and geodetic information and requirements for real time sea-level data, with the result that GLOSS priorities had to be re-assessed and a new Implementation Plan constructed (IOC 1998). The programme now contains particular components called GLOSS-OC, for the monitoring of aspects of the ocean circulation, and GLOSS-ALT, for the ongoing calibration of altimeter missions (p. 16), both of which owe their origins to experiences during WOCE. A further component called GLOSS-LTT is concerned with the provision of long-term sea-level change information for climate studies, with particular overlap with the objectives of the PSMSL. GLOSS developments have complemented well those of the WOCE programme. For example, GLOSS initiated a special Southern Ocean sea-level centre at the ANTF, based in Adelaide, which works closely with the two WOCE DACs (http://www.ntf.flinders.edu.au/). GLOSS has played major roles in the training of scientists and technicians from many countries and in disseminating advice on requirements for modern sea-level measurements. Most new GLOSS-related tide gauges are now recommended to be based on the acoustic (Porter & Shih 1996) or pressure (Woodworth et al. 1996c) techniques rather than the conventional stilling well method. In addition, near real-time recording is becoming a priority for several oceanographic applications, including assimilation of tide gauge and altimeter data together in ocean models. IOC (2000a) provides a review of technical developments in tide gauges.

Validations of tide gauge and altimetric sea-level data and numerical model simulations by data intercomparisons A large number of papers can be found in the literature which include comparisons of altimetric sea-level data with tide gauge information. These comparisons were usually performed in order to provide spot-confirmations of the precision of altimetric time series prior to further analysis of the altimetry for large-scale studies such as El Niño variability. A review of early comparisons, mostly in the tropical Pacific and using Geosat data, can be found in Picaut & Busalacchi (2001). Two which may be referred to include Mitchum (1994), who compared tide gauge and TOPEX altimetry leading to the calibration studies (p. 16), and Verstraete & Park (1995) who conducted one of the few studies in the Atlantic Ocean. Harangozo et al. (1993) performed comparisons between T/P data and monthly mean values of sea level from the PSMSL database, pointing towards the eventual use of altimeter data in studies of long-term sea-level change. Most studies tended to confirm the accuracy of a single altimeter sea surface height measurement as being typically 4 –5 cm (in the case of T/P, slightly worse for ERS), which was a level of accuracy anticipated from knowledge of the error budget of the altimetric system. A second valuable application of tide gauge sea-level data is in the validation of ocean model simulations related to ocean circulation and climate change studies, as outlined in 15

P . L. WOODW ORTH, C. LE PROVOST , L . J. R I C K A R D S , E T A L .

Section 5 of the GLOSS Implementation Plan (IOC 1998). Examples of such applications are more numerous now than at the start of the WOCE programme because ocean numerical models are more realistic, and ocean modellers consider sea-level fluctuations to be an important physical quantity of interest for their studies. The report of the Honolulu Sea Level Workshop (NOAA 1998) contains several examples of such applications, including the review report of Gornitz (1998) which refers to Tokmakian (1996) for global ocean model experiments and Enfield & Harris (1995) for tropical Pacific simulations including data assimilation. Koblinsky in NOAA (1998) also discusses another major set of validation and application to ENSO (El Niño Southern Oscilation) modelling and prediction. Related studies include those of Fukumori et al. (1998), Ji et al. (1995) and Xue et al. (2000). A number of other studies of the use of tide gauge data in model development can be found in the literature. An example is that of Ezer et al. (1995) who investigated the interpentadal variability of the mid-latitude North Atlantic Ocean, based on short-term model simulations but with high resolution, and simulated the state of the Atlantic during 1955–59 and 1970–74. Results agreed with earlier studies, indicating that the Gulf Stream was considerably weaker (by about 30 Sv) during the 1970s, and also suggested changes in poleward heat transport. Tide gauge data from 15 stations along the North American coast were used to validate the model information and to confirm that the modelled climate changes were realistic. A further point to make with regard to the symbiosis between models and data concerns the use of models for the design and improvements of sea-level gauge networks, making use of the physical insight provided through the models. The review report of Gornitz (1998), and discussion in the present paper, point to the fact that for monitoring of the global ocean circulation one cannot rely completely upon altimetry and that gauges are required in certain identifiable areas. Such areas can best be identified through model experiments. A second example can be taken from the related field of long-term sea-level changes caused by climate change. Although the results of General Circulation Models are not in full agreement, they all indicate that sea-level changes will not be the same everywhere over the global ocean, and that enhanced signals may occur in areas such as parts of the North Atlantic or around Antarctica (Warrick et al. 1996, Church et al. 2001). This points to the need to include in any new sea-level gauge implementation plan the installation of new stations in these areas. Such choices, involving perhaps considerable expenditure, have to be optimised using the best possible sources of model information. The encouraging correspondence between tide gauge and altimeter data and numerical model information led to increasing confidence in the ability of a subset of the global tide gauge network to provide an ongoing calibration system for altimetry, even the most enthusiastic supporters of which had come to realise contains systematic instrumental errors comparable to the ∼1 mm yr−1 signals of interest in studies of long-term sea-level change. This topic is discussed in the following section.

Altimeter calibration using WOCE tide gauges All altimeter missions require some form of calibration as a check on the performance of the radar and associated hardware. This is especially true for T/P and JASON-1, which have 16

SEA- LEVEL RESEARCH FRO M T I D E G A U G E S

the highest accuracy requirements for application to topics such as global sea-level change. This calibration is the analogue of what is known as datum control in tide gauge operations, and maintenance of the datum is the most critical factor in making long tide gauge records useful for climate studies. Similarly, maintaining the altimetric datum will be especially critical when one has access to a multi-decadal altimetric record obtained by combination of time series from a succession of altimeter missions. A first question is, which datasets are the most suitable for comparison with the altimetry to perform the calibration? These data will clearly take the form of height measurements of “targets” by both the altimeter and by in situ methods. Several types of target have been experimented with in the past including active devices such as radar transponders and passive targets such as large lakes monitored by water level recorders. However, it happens that the most convenient, direct and permanent calibrations of ocean altimetry can be performed using the surface of the open sea itself near to tide gauges. Mitchum (1998) showed that one can make powerful calibrations of altimeter data (called “relative” calibrations in altimeter terminology) by comparing simply the time series of altimeter information obtained near to a tide gauge with the time series of sea-level data obtained from the gauge itself, without concern for any overall unknown systematic error or bias in the differences between the time series. In the early years of the mission, the TOPEX altimeter data were found to possess a bias (the difference between sea surface height measured by the altimeter and in situ systems determined from “absolute” calibrations, see below) of the order of 17 cm, and relative calibrations of the type conducted by Mitchum (1998) suggested a time-dependent error in the altimeter measurements, such that unrealistic determinations of global and regional sea-level change were obtained. Subsequent analysis showed these findings to be due to an algorithm error in the software used to compute T/P altimeter range. After the error had been corrected, the absolute bias reduced essentially to zero (which had been the bias obtained for POSEIDON altimeter since the start of the mission) and, as a consequence of the relative calibration by means of tide gauges, estimates of global sea-level change for the mission were significantly reduced (Chelton et al. 2001, Nerem & Mitchum 2001). This demonstration of the continuing value of gauges was received with some amusement by sections of the tide gauge community and with embarrassment by altimetry overenthusiasts. However, in reality the experience provided confirmation of the need to pursue the development of a combined gauge-altimeter global sea-level monitoring system. Mitchum (2000) has since developed a more sophisticated version of his earlier method, which was based upon a repeat-track analysis of the altimeter information and subsequent comparison with gauge data, to include consideration of land movements at gauge sites. He showed that relatively ad hoc estimates of land movements are adequate for global sea-level change studies (resulting in a residual error of order 0.4 mm yr−1 for the T/P mission duration, which is considerably lower than uncertainties from other sources), although information on land movements from techniques such as the Global Positioning System (GPS) or the Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) system would clearly be desirable if possible. Relative calibrations of the Mitchum type have been conducted independently by other authors (Chambers et al. 1998, Moore et al. 2000). Murphy (1998) applied a similar method to time series obtained from missions with long or non-repeat ground tracks. An “absolute” calibration differs from the “relative” method in that one attempts to relate the absolute value of the altimeter measurement of sea surface height to tide gauge sea-level 17

P . L. WOODW ORTH, C. LE PROVOST , L . J. R I C K A R D S , E T A L .

data that have been located in the same geocentric reference frame using an advanced geodetic system such as GPS (see p. 25). The absolute calibration component of altimeter bias in most missions to date has been undertaken at dedicated sites such as Bermuda (for GEOS-3 and Seasat), Harvest and Lampedusa (T/P), Venice Tower (ERS-1), Newhaven/ Herstmonceux (T/P and ERS-1) which have a tide gauge, the data from which are located in the same geocentric reference frame as the altimeter data by means of laser ranging and GPS connections between laser and gauge. In these early examples, the laser had two functions. First, to determine the geocentric co-ordinates of the gauge, the GPS being useable for differential laser-gauge connections only; and second to enable short-arc precise orbit determination for the area of the test site, the global orbits provided by data centres being too poor for calibration purposes. However, nowadays GPS itself is capable of providing geocentric co-ordinates to the centimetre level, and the accuracy of the global orbits obtained from near-continuous satellite tracking by GPS and DORIS approaches that calculable by short-arc determinations (p. 25). Consequently, many gauges equipped with GPS at ocean islands and along continental coastlines could, in principle, be used for absolute calibration, as long as the distance between gauge and altimeter measurement point is not large (a few 10 s km being ideal) and as long as precise estimates of geoid-difference are available from a local gravity model. If continuous (or repeated) GPS measurements are made alongside the gauge, then vertical land movements due to geological processes will be automatically accounted for. An alternative form of absolute calibration, which has been investigated by several groups, is the use of an ocean buoy equipped with a GPS receiver (or at least a GPS antenna if the receiver is located on a boat separate from the buoy itself ), which then functions as both the gauge and GPS, and allows for the direct over-flight of the satellite without the need for geoid-difference corrections. However, while such GPS-buoys might be useful in dedicated experiments, they lack the critical quality of permanence that is provided by a conventional coastal tide gauge. One can imagine several different strategies for long-term calibration of a set of altimeter missions over many years. Those contrived by participants in the upcoming JASON-1 mission have been described by Haines & Ménard (2000). The ideal strategy is obviously ongoing absolute calibration. However, a more practical alternative is probably the use of individual, precise relative calibrations using gauges for the length of each mission, but with some form of absolute calibration performed at the start of, and to some extent throughout, the mission. This is the approach adopted by the community to date and which will probably be employed for the near future. Whichever particular approach is chosen, it is clear that a subset of the global network of gauges (a version of GLOSS-ALT) will continue to be a major component of the calibration system.

Bottom pressures and ACC choke points Bottom pressure recorders have been used for many years to measure ocean tides in the deep ocean and, more recently, non-tidal barotropic changes in the water column resulting from ocean circulation changes. Spencer & Vassie (1997) and Cartwright (1999) provide histories of the development of the technique.

18

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BPRs have an advantage over conventional coastal gauges in that they can be deployed in almost all parts of the ocean at all but the greatest depths (i.e. to depths of approximately 5000 m), including high latitude regions where conventional gauge operations may be difficult due to ice cover or other environmental conditions. They provide continuous deepocean measurements, without the aliasing problems of altimetry (Gille & Hughes 2001). On the other hand, a continuous BPR record cannot be obtained from a single deployment for more than a few years (4 yr being the longest to date from the POL MYRTLE device, Spencer & Foden 1996, Spencer & Vassie 1997), while the data sometimes contain a longterm instrumental drift (“transducer creep”) and cannot be related to a geodetic datum. It has to be recognised that bottom pressure is a different variable to surface sea level, because the latter can contain contributions from baroclinic changes in the water column. Bottom pressure is usually more useful for circulation studies because ocean modellers normally require pressure information rather than sea level. On the other hand, it means that a time series from a deep ocean BPR cannot be expected to correspond in all respects to a series of sea-level measurements by an altimeter. BPRs were considered for deployment at ACC “choke point” sections during WOCE CP-2 due to the success of Drake Passage bottom pressure measurements during the International Southern Ocean studies (ISOS) programme which spanned 1977–82 (Wearn & Baker 1980, Whitworth & Peterson 1985), and to the lack of established conventional coastal gauges in the region, a situation that is now improved as a result of GLOSS developments. From the ISOS BPR and current meter datasets, Whitworth & Peterson (1985) determined the rms variability in the transport through the Passage to be of the order of 10 Sv and to be primarily barotropic. They also recorded two examples of transport fluctuations approaching 50% of the mean (or approximately 50 Sv) over periods as short as two weeks. The 10 Sv corresponds to approximately 5 mbar (or 5 cm) pressure difference if barotropic flow is assumed, which means that the changes are at the limit of measurement by altimetry, aside from any considerations of sampling. From Whitworth & Peterson (1985) and from later modelling (Woodworth et al. 1996b, Hughes et al. 1999), we know that most of the pressure (or sea level) changes which are associated with the transport changes take place at the southern side of the Passage. (For other studies of monitoring Drake Passage flows using altimetry alone, see Challenor et al. 1996 and Challenor & Tokmakian 1999.) BPR measurements in the Drake Passage were recommenced during WOCE by UK groups (Spencer et al. 1993). The rms variability of Drake Passage transport was observed to fluctuate from year to year (Meredith et al. 1996). For example, rms of 5.3 Sv was recorded in 1993 compared with 8.9 Sv in 1990 (after application of a 10-day filter). All years 1989–94 demonstrated lower variability than observed by Whitworth & Peterson (1985) and no evidence was found for large fluctuations of several 10 s of Sv. Hughes et al. (1999) concluded that the transport variability on timescales of 10–220 days was consistent with that of a barotropic mode following f/H contours, with the south side of relatively greater importance for monitoring changes in transport. Gille et al. (2001) employed BPR, T/P, zonallyaveraged wind and numerical model information and concluded that barotropic transport and wind forcing are coherent over timescales of approximately 10–256 days at the Drake Passage, with barotropic transport lagging wind forcing by about 1/18 of a cycle for a broad range of frequency, suggesting the ocean response to wind is controlled by both a “tendency term” (an along-stream average current that balances wind stress fluctuations) and a frequencydependent viscous process.

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Woodworth et al. (1996b) demonstrated, by comparing data from coastal and deep-sea instruments, that the accuracy of T/P altimetry in the Drake Passage area is similar to that obtained for other parts of the world. In particular, the BPR and T/P data from the northern side of the Passage were found to be in close agreement. On the other hand, the southern side information indicated qualitative similarities to, but large quantitative differences between, altimetric SSP and in situ bottom pressure, a part of which was explainable from baroclinic variability around a known, topographically-trapped “cold core eddy” centred at approximately the location of the MYRTLE BPR. This emphasises that BPR deployments have to be chosen with care to optimise the measurement of the barotropic processes of interest, because baroclinic corrections to the pressure data can be large and imprecisely known. Rubython et al. (2001) provide a full discussion of the bottom temperature data from the MYRTLE position, and of the repeat hydrographic information obtained nearby, with particular emphasis on a 0.1°C cooling which occurred during 1994–95 and which has been linked to variability in Weddell Sea Deep Water. Baroclinic fluctuations in mid-Passage were previously studied using inverted echo sounders (IESs) by Meredith et al. (1997) with the same group of IESs deployed also to monitor Denmark Strait overflow (Dickson et al. 1999). In addition to the Drake Passage, deployments have taken place at the Africa–Antarctica and Australia–Antarctica choke points by US groups and Amsterdam–Kerguelen by UK/ French groups. An early analysis of the temporal variability of the ACC transport between Amsterdam and Kerguelen was made by Vassie et al. (1994). Combined analyses of all the separate choke point datasets are still in progress. However, it is clear that the use of “south side” gauges, whether BPRs or conventional gauges on the Antarctic coast, as a monitor of ACC transport variability is a potentially important research topic for both tide gauge specialists and ocean modellers. Woodworth et al. (1999) demonstrated the remarkable coherence of subsurface pressure (SSP), or “inverse barometer corrected sea level”, measured at locations around Antarctica for timescales of 10 days and longer, which suggests that a small number of high-quality Antarctic gauges, whether located at the “choke points” or not, might be adequate for providing the required ongoing information on the strength of the ACC which cannot be provided by altimetry. On longer timescales, gauge records are required in Antarctica to contribute to an understanding of the interesting ocean-atmosphere dynamics associated with the Antarctic Circumpolar Wave (Jacobs & Mitchell 1996) and other features. BPRs have also been employed during WOCE to study the spatial scales of the coherence of barotropic ocean variability, with bottom pressure change in ocean sub-basins found to be coherent over large areas (Luther et al. 1987, Filloux et al. 1991, Chave et al. 1992, Woodworth et al. 1995, Hughes & Smithson 1996). Knowledge of the spatial scales of the coherence is of particular importance to the measurement of changes in the distribution of ocean mass by forthcoming space gravity missions. It also opens up the possibility for future ocean circulation models to be constrained by sea-level information from altimetry at the surface, together with data on ocean mass changes integrated throughout the water column, arising from measurements of space gravity (NRC 1997, Wahr et al. 1998). All BPR data contributed to WOCE are included in the official CD-ROM sea-level products for the programme and can also be obtained from the Global Undersea Pressure (GLOUP) dataset managed by the PSMSL on behalf of the International Association for the Physical Sciences of the Ocean Commission on Mean Sea Level and Tides (IAPSO CMSLT). 20

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Other uses of tide gauge data during the WOCE period Sea-level data were employed during WOCE to monitor flows through straits at locations other than the Southern Ocean. One group of papers concerned the Indonesian ThroughFlow. Arief & Murray (1996) investigated the particular relationship between Cilicap sea level and flow through Lombok Strait, which accommodates approximately a quarter of the throughflow, and concluded that Cilicap could be used to predict flows with a small lead. Molcard et al. (2001) compared 20 m depth currents in Ombai Strait with upstream sea-level data from Benoa during 1995–96 and concluded that Benoa level might be useable (given better quality gauges) as a proxy for the current. Bray et al. (1996) conducted a wider-area study combining sea-level and thermocline information and concluded that interannual variability in the throughflow depends on changes in deep as well as surface flows. Potemra et al. (1997) studied the large-scale pressure gradient forcing of throughflow using T/P data (previously validated using tide gauges) and found that the throughflow is controlled by sealevel changes on the Pacific side on interannual timescales and by a combination of Indian and Pacific Ocean processes on seasonal to annual timescales. Susanto et al. (2000) found that the intraseasonal (1–2 months period) variability of sea level in the Makassar Strait was a response to remotely forced Kelvin waves from the Indian Ocean progressing through Lombok Strait, together with Rossby waves from the Pacific Ocean. These several examples clearly demonstrate the importance to the international oceanographic community of good quality tide gauges located at strategic positions to monitor the Indonesian Through-Flow. Several recent papers have been concerned with aspects of tidal and long-term exchanges through the Strait of Gibraltar (Ross et al. 2000, Tsimplis 2000, Tsimplis & Bryden 2000). One conclusion is that, while gauges can be used to monitor the Atlantic-inflow and Mediterranean-outflow exchanges to first order, other information is also needed on the position and slope of the interface between Mediterranean and Atlantic waters across the Strait in order to estimate transports reliably. In the Mediterranean itself, Tsimplis (1997) used sea-level data from each end of the Strait of Euripus to investigate tidal and non-tidal flows through the strait. Clarke & Ahmed (1999) used data from six locations along the South American coast from Peru to Chile to investigate intraseasonal oscillations of sea level, observing poleward propagation at these timescales. Flows along coastlines were also investigated by McClimans et al. (1999) who employed bottom pressure and coastal sea-level data to monitor a shelf edge current, in this case a predominantly barotropic current along the Norwegian continental slope. Sea-level changes and currents along the coast of India and their relationship to monsoon rainfall and subsequent reduction in salinity of coastal waters were studied by Shankar & Shetye (1999) and Shankar (2000), while the relationship of sea levels in Bangladesh to the Southern Oscillation was considered by Singh et al. (2001). WOCE data were also used to test aspects of large-scale ocean dynamics. Prior to the availability of T/P altimetry, Kawabe (1993, 1994) made extensive use of Pacific island tide gauge data to investigate the interannual variability of equatorial sea level related to El Niño with the role of equatorial Kelvin and Rossby waves parameterised within a two-layer reduced-gravity model. Also prior to the availability of copious, precise altimeter data, Unal & Ghil (1995) studied interannual and interdecadal oscillation patterns of sea-level change using 213 tide gauges, emphasising the worldwide distribution of quasi-biennial and lowerfrequency (period 4–5 yr) ENSO-related variability in the records, in addition to secular trends. Merrifield et al. (1999) employed regression analysis of tide gauge and T/P data in 21

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the Pacific for the period for which both data types were available to construct Empirical Orthogonal Function (EOF) patterns of temporal and spatial variability connected with the ENSO cycles. The EOF patterns of change were then applied to the longer historical record from gauges alone to assess island regions most at risk of anomalously high sea levels during peak El Niño and La Niña events. Johnston & Merrifield (2000) studied the time dependence of the North Equatorial Countercurrent and South Equatorial Current in the western Pacific during 1975–97, and its relation to ENSO events, using a combination of tide gauge and T/P data. Sturges et al. (1998) and Hong et al. (2000) successfully related the sea-level changes observed at Bermuda and along the US Atlantic coast to basin-scale changes in the wind field through parameterisations of gyre-scale dynamics. Sturges & Hong (2001) differenced sea levels measured on the US east coast by gauges from those calculated for the offshore side of the Gulf Stream by an ocean model and showed that the differences agreed well at low frequencies with measured transports. Ezer (2001) tested this hypothesis using a more sophisticated Atlantic model forced by observed surface data, showing that variations in sealevel difference between ocean and coast are indeed coherent with Gulf Stream variations for periods shorter than 1 yr or longer than 4–5 yr. In the Pacific, sea-level data from the coast of Japan were used extensively during WOCE to demonstrate and monitor the interannual and interdecadal variability of Kuroshio transports and meanders (Kawabe 1995 and references therein). Senjyu et al. (1999) discussed interannual variations throughout Japan with the first EOF of the variability representing coherent changes around the entire coast, and the second mode resulting from large Kuroshio meanders. Several authors including Ponte (1997), Gaspar & Ponte (1997, 1998), Ponte & Gaspar (1999), Woodworth et al. (1995) and Mathers & Woodworth (2001) investigated dynamical violations of the “inverse barometer model” throughout the world ocean on a range of timescales with the use of tide gauge and altimeter data and numerical models, finding closer agreement with the simple model at longer timescales, but pointing to dynamical violations forced by winds and air pressure changes on timescales of days to weeks. The study of long-term change in global sea level did not fall within the particular remit of the WOCE programme. Nevertheless, the study benefited in several ways from the development of the international sea level network during WOCE. A first example concerns the provision of calibration information to the first determinations of truly global sea-level change from altimetry (Nerem & Mitchum 2001). A second concerns the establishment or refurbishment of a number of island gauges in both hemispheres that have records one or two decades long. These will in time provide a special set of deep ocean secular trends in sea level, which may be more representative of oceanic change than trends measured at continental coastlines so far. A third derives from a fuller understanding of the ocean from WOCE in general and its role in climate and sea-level change (i.e. essentially WOCE Goal 1). Recent reviews of research into long-term sea-level changes and their causes, including the role of the ocean circulation, can be found in Raper et al. (2000) and Church et al. (2001). Similarly, the study of variability in sea level at different frequencies cannot strictly be considered as part of WOCE. Nevertheless, there is considerable overlap. Research in this field was reviewed by Woodworth (1993). Of more recent European research, Tsimplis & Baker (2000) and Tsimplis & Josey (2001) have studied changes in European and Mediterranean sea levels associated with the North Atlantic Oscillation (NAO), while Tsimplis et al. (1994) and O’Connor et al. (2000) have investigated causes of the North Sea “pole tide” 22

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including the role of the Atlantic wind field. Each of these can be related to forcings (e.g. wind stress) which in turn will relate to changes in regional ocean circulation. Tsimplis & Woodworth (1994) described the global distribution of the seasonal cycle of sea level using worldwide tide gauge data while Plag & Tsimplis (1999) investigated interannual variations in the northern Europe sea level seasonal cycles.

Deep ocean tide model developments during WOCE The development of a new generation of deep ocean tide models during WOCE was a major achievement of satellite altimetry, and in particular of the T/P mission (Le Provost et al. 1995a, Andersen et al. 1995, Woodworth et al. 1996a, Shum et al. 1997, Ray & Woodworth 1997, Le Provost 2000). In this section, we provide a brief overview of the way the gauge data have contributed to these model developments. Tide gauges have proved to be important in global tidal studies in two main ways. The first is in the provision of in situ information by which one can validate the altimetric information, including the provision of comprehensive accuracy assessments. The second stems from the fact that altimetric sampling is not well matched to the needs of tide model development near the coasts. One has to assimilate conventional gauge (or BPR) tidal data into the models along with the altimetry in order to provide the required densification of tidal information along shorelines. In 1995, the T/P SWT conducted a major assessment of a large number of new global ocean tide models which had become available largely as a result of the superb accuracy and coverage of T/P data and of progress in numerical modelling. The primary reason for the study was to provide the SWT with a recommendation on which models to use for future altimetric analyses. However, as ocean tides play a major role in many areas of geophysics, it was felt that such an assessment would also be useful to the wider community. That report was published as Shum et al. (1997). Before the T/P era, two main global ocean tide models had been used by researchers, those of Schwiderski (1980a,b) and Cartwright & Ray (1990). The Schwiderski model was constructed by means of a hydrodynamic interpolation scheme for the assimilation of the tidal constants dataset derived from the global collection of tide gauge data. That model, although now known to contain decimetric and larger errors, played a central role in oceanographic and geophysical research for more than a decade. Geosat in the late-1980s provided the first copious altimetric dataset for extended global tide studies and enabled the derivation of models of comparable or better accuracy than Schwiderski (e.g. Cartwright-Ray). However, it was not until 1995, when almost three years of T/P data had become available, that really significant improvements in this field started to emerge. The new models were computed in different ways. Some were purely empirical solutions derived from T/P data, somewhat analogous to Cartwright-Ray, although analysis details were different in each case. Others differed in being the results of different forms of sophisticated data assimilation into numerical models wherein hydrodynamic constraints effectively act as a filter on and an interpolator between the data. Preliminary evaluations and comparisons of all the new models showed that they were very similar, with barely a centimetre or two difference between them in most parts of the world. Therefore, a set of tests was constructed, one of the most important of which comprised comparisons of tidal parameters derived from the models to those obtained from pelagic and island tide gauge data. 23

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For the Shum et al. (1997) comparisons, 49 island and 53 BPR gauge records were employed. The tidal harmonic constants from these 102 sites were compared with those from the new models, and root-mean-square (rms) differences for each constituent were computed. Most new models were seen to have residual rms values of less than 2 cm for the main lunar component M2, and an overall root-sum-square (rss) derived from the 8 major constituents of the order of 2.5–3.0 cm. This was was a significant improvement on the Schwiderski and Cartwright-Ray models (4 cm and 3 cm rms for M2 and 4.8 cm and 4.6 cm overall rss, respectively). One of the primary objectives of this study was to select two of the best models for future reprocessing of T/P data, because future work could not be done with many models. One of the original specifications for the selection had been that one of the two models should be a pure hydrodynamic model and that the other should be based primarily on T/P data. However, in practice that choice was not possible as the nearest candidate pure hydrodynamic model FES94.1 (Le Provost et al. 1994) was not accurate enough. Consequently, after considerable discussion, the choice was made to select CSR3.0 (Eanes & Bettadpur 1996), as primarily a T/P derived model but with hydrodynamic model information content, and FES95.2 (Le Provost et al. 1998), as primarily a hydrodynamic model but with T/P information content. These two models could in some sense be said to approach an optimum model from different directions, and have since been used extensively by researchers of T/P data during the last 5 yr. Since then, after almost a decade of T/P and ERS observations, a new generation of models has been developed, which are incremental improvements on the ones produced in 1995. Taking advantage of the longer altimeter time series, the empirical models have gained in resolution in the frequency domain (better de-aliasing) and in space, mainly over shallow waters by using smaller bins for constructing the altimeter time series to be analysed (Ray 1999). Along-track tidal analysis has also become more reliable, and the results have been assimilated in hydrodynamic models (Matsumoto et al. 2000, Tierney et al. 2000). In addition, assimilation of tidal information from tide gauge data has been developed at the global (Lefèvre et al. 2000) and regional (Matsumoto et al. 2000) scales. These new solutions have improved upon those of the mid-1990s only marginally over the deep ocean (i.e. there has been little improvement on the deep-ocean 2 cm rms for M2 referred to above) but there has been significant improvement in shallow water areas. Dorandeau et al. (2000) demonstrated this improvement by means of a comparison of these solutions with a set of 739 tide gauges selected by Lefèvre et al. (2001). The residual rms values for M2 in the shallow water areas are reduced from approximately 17 cm for CSR3.0, to 14 cm for the GOT99 model (Ray 1999), to