Ocean Circulation and Climate: Observing and Modelling the Global Ocean [1 ed.] 9780126413519, 0-12-641351-7

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Ocean Circulation and Climate Observing and Modelling the Global Ocean

This is Volume 77 in the INTERNATIONAL GEOPHYSICS SERIES A series of monographs and textbooks Edited by RENATA DMOWSKA, JAMES R. HOLTON and H. THOMAS ROSSBY A complete list of books in this series appears at the end of this volume.

Ocean Circulation and Climate Observing and Modelling the Global Ocean Edited by

Gerold Siedler

Institut für Meereskunde Universität Kiel Kiel, Germany and Instituto Canario de Ciencias Marinas Telde, Spain

John Church

Antarctic CRC and CSIRO Marine Research Hobart, Australia

John Gould

Southampton Oceanography Centre Southampton, UK

San Diego San Francisco New York Boston London Sydney Tokyo

This book is printed on acid-free paper. Copyright © 2001 by ACADEMIC PRESS All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Explicit permission from Academic Press is not required to reproduce a maximum of two figures or tables from an Academic Press chapter in another scientific or research publication provided that the material has not been credited to another source and that full credit to the Academic Press chapter is given.

Academic Press A Harcourt Science and Technology Company Harcourt Place, 32 Jamestown Road, London NW1 7BY, UK http://www.academicpress.com Academic Press A Harcourt Science and Technology Company 525 B Street, Suite 1900, San Diego, California 92101-4495, USA http://www.academicpress.com ISBN 0-12-641351-7 Library of Congress Catalog Number: 00-106870 A catalogue record for this book is available from the British Library

Typeset by Newgen Imaging Systems (P) Ltd, Chennai, India Printed and bound in Spain by Grafos SA Arte Sobre Papel, Barcelona 01 02 03 04 05 06 GF 9 8 7 6 5 4 3 2 1

Contents Contributors Foreword Preface Acknowledgment

xiii xvii xviii xx

Section 1: The Ocean and Climate Section 1 plates appear between pages 44 and 45 1.1 1.1.1 1.1.2 1.1.3 1.1.4 1.1.5 1.1.6 1.1.7 1.2 1.2.1 1.2.2 1.2.3 1.2.4 1.2.5 1.2.6 1.2.7 1.2.8 1.3 1.3.1 1.3.2 1.3.3 1.3.4 1.3.5 1.3.6

Climate and Oceans Hartmut Grassl WOCE and the World Climate Research Programme The scientific approach to the complex climate system Ocean–atmosphere interaction and climate Rapid changes related to the oceans Cryosphere and the oceans Anthropogenic climate change and the oceans Future climate research and ocean observing systems Ocean Processes and Climate Phenomena Allyn Clarke, John Church and John Gould A global perspective Air–sea fluxes Ocean storage of heat and fresh water Ocean circulation Ocean transport of heat, fresh water and carbon Climatic and oceanic variability Impacts of ocean climate Conclusion The Origins, Development and Conduct of WOCE B. J. Thompson, J. Crease and John Gould Introduction Large-scale oceanography in the 1960s and 1970s Ocean research and climate Implementation of WOCE (SSG initiatives) Implementation and oversight Was WOCE a success and what is its legacy?

3 3 4 5 6 7 7 8 11 11 12 17 17 23 24 27 30 31 31 31 32 36 41 43

Section 2: Observations and Models Section 2 plates appear between pages 76 and 77 2.1 2.1.1 2.1.2 2.1.3 2.1.4 2.1.5

Global Problems and Global Observations Carl Wunsch Different views of the ocean The origins of WOCE What do we know? The need for global-scale observations Where do we go from here?

47 47 48 51 52 56

CONTENTS

vi 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.3 2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 2.3.6 2.3.7 2.3.8

High-Resolution Modelling of the Thermohaline and Wind-Driven Circulation ClausW. Böning and Albert J. Semtner The improving realism of ocean models Historical perspective Basic model design considerations: equilibrium versus non-equilibrium solutions Examples of model behaviour in different dynamical regimes Concluding remarks Coupled Ocean–Atmosphere Models Richard A. Wood and Frank O. Bryan Why coupled models? Formulation of coupled models Model drift and flux adjustment Initialization of coupled models Coupled model simulation of present and past climates Coupled model simulation of future climates Climate models, WOCE and future observations Summary and future developments

59 59 60 62 64 77 79 79 79 84 86 87 93 94 95

Section 3: New Ways of Observing the Ocean Section 3 plates appear between pages 172 and 173 3.1 3.1.1 3.1.2 3.1.3 3.1.4 3.1.5 3.2 3.2.1 3.2.2 3.2.3 3.2.4 3.2.5 3.3 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5

Shipboard Observations during WOCE B. A. King, E. Firing and T. M. Joyce The role of hydrographic measurements CTD and sample measurements Current measurements in the shipboard hydrographic programme Shipboard meteorology Summary and conclusions Subsurface Lagrangian Observations during the 1990s Russ E. Davis and Walter Zenk Determining currents in the ocean Historical aspects: Stommel’s vision to the WOCE Float Programme The WOCE Float Programme WOCE float observations The future Ocean Circulation and Variability from Satellite Altimetry Lee-Lueng Fu Altimeter observations The ocean general circulation Large-scale sea-level variability Currents and eddies Concluding discussions

99 99 102 111 120 121 123 123 123 127 129 137 141 141 143 148 162 170

CONTENTS 3.4 3.4.1 3.4.2 3.4.3 3.4.4 3.4.5 3.4.6

Air–Sea Fluxes from Satellite Data W. Timothy Liu and Kristina B. Katsaros Forcing the ocean Bulk parameterization Wind forcing Thermal forcing Hydrologic forcing Future prospects

3.5

Developing the WOCE Global Data System Eric J. Lindstrom and David M. Legler 3.5.1 Organization and planning for WOCE data systems 3.5.2 Elements of the WOCE Data System 3.5.3 The WOCE Global Data Set and future developments

vii 173 173 173 174 177 179 179 181 181 185 189

Section 4: The Global Flow Field Section 4 plates appear between pages 300 and 301 4.1 4.1.1 4.1.2 4.1.3 4.1.4 4.1.5 4.2 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.3 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.4 4.4.1 4.4.2 4.4.3 4.4.4

The World Ocean Surface Circulation Peter Niiler Background Methodology The global mean velocity and velocity variance The wind-driven Ekman currents Future global circulation observations

193

The Interior Circulation of the Ocean D. J. Webb and N. Suginohara Processes in the ocean interior Observational evidence Theory of gyre-scale circulation The abyssal circulation Conclusions

205

The Tropical Ocean Circulation J. S. Godfrey, G. C. Johnson, M. J. McPhaden, G. Reverdin and Susan E. Wijffels Flow and water mass transformation patterns Equatorial phenomena in the Pacific Ocean Equatorial Atlantic Near-equatorial circulation in the Indian Ocean Overall conclusions Tropical–Extratropical Oceanic Exchange Pathways Zhengyu Liu and S. G. H. Philander The role of diffusion and advection Tropical–subtropical exchanges of thermocline waters Tropical–subpolar exchange of Intermediate Waters Summary and further issues

193 195 198 201 203

205 206 209 211 213 215 215 216 226 233 245 247 247 248 252 254

CONTENTS

viii 4.5 4.5.1 4.5.2 4.5.3 4.5.4 4.6 4.6.1 4.6.2 4.6.3 4.6.4 4.6.5 4.6.6 4.7 4.7.1 4.7.2 4.7.3 4.7.4 4.7.5

Quantification of the Deep Circulation Nelson G. Hogg Deep circulation in the framework of WOCE Deep Western Boundary Currents The interior: The Deep Basin Experiment Summary The Antarctic Circumpolar Current System Stephen R. Rintoul, Chris W. Hughes and Dirk Olbers Flow in the zonally unbounded ocean Observations of the Antarctic Circumpolar Current Dynamics of the ACC Water mass formation and conversion The Southern Ocean and the global overturning circulations Conclusions Interocean Exchange Arnold L. Gordon Interocean links Bering Strait Indonesian Seas The Agulhas Retroflection Discussion

259 259 260 266 269 271 271 274 280 291 296 300 303 303 306 307 310 313

Section 5: Formation and Transport of Water Masses Section 5 plates appear between pages 428 and 429 5.1 5.1.1 5.1.2 5.1.3 5.1.4 5.1.5 5.1.6 5.2 5.2.1 5.2.2 5.2.3 5.2.4 5.2.5 5.2.6 5.2.7 5.3 5.3.1 5.3.2 5.3.3 5.3.4 5.3.5

Ocean Surface Water Mass Transformation William G. Large and A. J. George Nurser The problem Theory of surface water mass transformation Ocean surface temperature, salinity and density Surface fluxes of heat, fresh water and density Surface water mass transformation and formation Summary

317

Mixing and Stirring in the Ocean Interior John M. Toole and Trevor J. McDougall Scales of mixing and stirring Background The Temporal-Residual-Mean circulation Lateral dispersion between the mesoscale and the microscale Diapycnal mixing in and above the main thermocline Mixing in the abyss Discussion

337

Subduction James F. Price A little of the background on oceanic subduction Surface-layer dynamics and thermodynamics of the subduction process Development of steady, continuous models: Application to numerical model analysis and observations Transient response of the thermocline to decadal variability Summary and outlook

317 318 321 326 332 335

337 338 340 345 346 352 354 357 357 360 361 365 370

CONTENTS 5.4 5.4.1 5.4.2 5.4.3 5.4.4 5.4.5 5.5 5.5.1 5.5.2 5.5.3 5.5.4 5.5.5 5.6 5.6.1 5.6.2 5.6.3 5.6.4 5.6.5 5.6.6 5.6.7 5.6.8 5.7 5.7.1 5.7.2 5.7.3 5.7.4 5.8

5.8.1 5.8.2 5.8.3 5.8.4

ix

Mode Waters Kimio Hanawa and Lynne D. Talley Ventilation and mode water generation Definition, detection and general characteristics of mode waters Geographical distribution of mixed-layer depth and mode waters in the world’s oceans Temporal variability of mode water properties and distribution Summary

373

Deep Convection John Lazier, Robert Pickart and Peter Rhines Convection and spreading Plumes – the mixing agent Temperature and salinity variability Restratification Summary and discussion

387

The Dense Northern Overflows Peter M. Saunders The sources Overflow paths Observed transport means and variability Processes in the overflows Analytical models of the overflow Numerical models of the overflow Overflow variability What have we learnt in WOCE? Mediterranean Water and Global Circulation Julio Candela Marginal seas Formation of Mediterranean Water Outflow of Mediterranean Water at the Strait of Gibraltar The effect of Mediterranean Water outflow on the circulation of the North Atlantic and the World Oceans Transformation and Age of Water Masses P. Schlosser, J. L. Bullister, R. Fine, W. J. Jenkins, R. Key, J. Lupton, W. Roether and W. M. Smethie, Jr Background Tracer methodology and techniques Exemplary results Outlook

373 374 376 384 386

387 391 393 396 398 401 401 402 404 411 412 414 416 416 419 419 421 422 427 431

431 432 433 450

Section 6: Large-Scale Ocean Transports Section 6 plates appear between pages 492 and 493 6.1 6.1.1 6.1.2 6.1.3 6.1.4 6.1.5 6.1.6 6.1.7 6.1.8

Ocean Heat Transport Harry L. Bryden and Shiro Imawaki The global heat balance Bulk formula estimates of ocean heat transport Residual method estimates of ocean heat transport Direct estimates of ocean heat transport Discussion Challenges Summary Outlook for direct estimates of ocean heat transport

455 455 456 458 459 466 470 473 474

CONTENTS

x 6.2 6.2.1 6.2.2 6.2.3 6.2.4 6.2.5 6.2.6 6.2.7 6.2.8 6.3

6.3.1 6.3.2 6.3.3 6.3.4 6.3.5

Ocean Transport of Fresh Water Susan E. Wijffels The importance of freshwater transport Indirect estimates of oceanic freshwater transport Impacts of uncertainties on model development Direct ocean estimates of freshwater transport Comparison of direct and indirect flux estimates Mechanisms of oceanic freshwater transport Global budgets Summary Storage and Transport of Excess CO2 in the Oceans: The JGOFS/WOCE Global CO2 Survey Douglas W. R. Wallace Introduction Background The JGOFS/WOCE Global CO2 Survey Synthesis of Global CO2 Survey data: Review Conclusions and outlook

475 475 475 476 478 483 486 487 488 489 489 489 495 503 520

Section 7: Insights for the Future Section 7 plates appear between pages 588 and 589 7.1 7.1.1 7.1.2 7.1.3 7.1.4 7.1.5 7.2 7.2.1 7.2.2 7.2.3 7.2.4 7.2.5 7.2.6 7.3

Towards a WOCE Synthesis Lynne D. Talley, Detlef Stammer and Ichiro Fukumori Exploiting the WOCE data set Data-based analyses Model evaluation and development Ocean state estimation Summary and outlook

525

Numerical Ocean Circulation Modelling: Present Status and Future Directions J. Willebrand and D. B. Haidvogel Remarks on the history of ocean modelling Space–time scales of ocean processes and models Modelling issues Atmospheric forcing and coupling Organization of model development Concluding remarks

547

The World during WOCE Bob Dickson, Jim Hurrell, Nathan Bindoff, Annie Wong, Brian Arbic, Breck Owens, Shiro Imawaki and Igor Yashayaev 7.3.1 Assessing the representativeness of the WOCE data set 7.3.2 The state of the atmosphere during WOCE 7.3.3 The analysis of decadal change in intermediate water masses of the World Ocean 7.3.4 Climatic warming of Atlantic Intermediate Waters 7.3.5 Spin-up of the North Atlantic gyre circulation 7.3.6 Altered patterns of exchange in Nordic Seas 7.3.7 System-wide changes in the Arctic Ocean 7.3.8 Interdecadal variability of Kuroshio transport 7.3.9 Evidence of water mass changes in the Pacific and Indian Oceans 7.3.10 Summary and Conclusions

525 526 535 535 542

547 548 549 553 554 556 557

557 558 563 565 567 569 571 573 576 580

CONTENTS 7.4 7.4.1 7.4.2 7.4.3 7.4.4 7.4.5 7.4.6 7.4.7 7.4.8

Ocean and Climate Prediction – the WOCE Legacy Neville Smith The long-term context Building from WOCE WOCE observations WOCE and climate prediction The mean state and long-term change Ocean variability and prediction: GODAE Institutionalizing the benefits of WOCE Conclusions

References Acronyms, abbreviations and terms Index

xi 585 585 588 589 592 595 597 600 601 603 686 693

This Page Intentionally Left Blank

Contributors Arbic Brian ([email protected]) Massachusetts Institute of Technology, Cambridge, MA 02139, USA, and Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Bindoff Nathan ([email protected]) Antarctic CRC, GPO Box 252-80, Hobart, Tasmania 7001, Australia Böning C. W. ([email protected]) Institut für Meereskunde, Universität Kiel, 24105 Kiel, Germany Bryan Frank O. ([email protected]) National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307, USA Bryden Harry L. ([email protected]) Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK Bullister John ([email protected]) NOAA/PMEL, R/E/PM Bin C15700, 7600 Sand Point Way NE Building 3, Seattle, WA 98115-0070, USA Candela Julio ([email protected]) Departamento de Oceanografía Física, Centro Investigación Científica y de Educación Superior de Ensenada (CICESE), Km. 107 Carretera Tijuana-Ensenada, 22860 Ensenada B. Cfa., Mexico Church John A. ([email protected]) Antarctic CRC and CSIRO Marine Research, GPO Box 1538, Hobart, Tasmania 7001, Australia

Clarke Allyn ([email protected]) Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth NS, B2Y 4A2, Canada Crease J. ([email protected]) College of Marine Studies, University of Delaware, Lewes, DE 19958-1298, USA Davis Russ E. ([email protected]) Scripps Institution of Oceanography, La Jolla, CA 92093-0230, USA Dickson Bob ([email protected]) Centre for Environment, Fisheries and Aquaculture Science, Lowestoft, Suffolk NR33 0HT, UK Fine R. ([email protected]) RSMAS, University of Miami, Miami, FL 33149-1098, USA Firing E. ([email protected]) JIMAR, University of Hawaii at Manoa, Honolulu, HI 96822, USA Fu Lee-Lueng ([email protected]) Jet Propulsion Laboratory 300-323, 4800 Oak Grove Drive, Pasadena, CA 91109-8099, USA Fukumori Ichiro ([email protected]) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099, USA Godfrey J. S. ([email protected]) CSIRO Marine Research, Hobart, Tasmania 7001, Australia

xiv

CONTRIBUTORS

Gordon Arnold L. ([email protected]) Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA

Johnson G. C. ([email protected]) NOAA/Pacific Marine Environmental Laboratory, Seattle, WA 98115-0070, USA

Gould John ([email protected]) Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK

Joyce T. M. ([email protected]) MS 21, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

Grassl Hartmut ([email protected]) Max-Planck-Institüt für Meteorologie, Bundesstr. 55, D-20146 Hamburg, Germany

Katsaros Kristina B. ([email protected]) NOAA/Atlantic Oceanographic and Meteorological Laboratory, 4301 Rickenbacker Causeway, Miami, FL 33149, USA

Haidvogel D. B. ([email protected]) Department of Marine & Coastal Sciences, Rutgers University, New Brunswick, NJ 08903-0231, USA Hanawa Kimio ([email protected]) Department of Geophysics, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan Hogg Nelson G. ([email protected]) MS 21, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Hughes Chris W. ([email protected]) Proudman Oceanographic Laboratory, Bidston Observatory, Bidston Hill, Prenton, CH43 7RA, UK Hurrell Jim ([email protected]) Climate and Global Dynamics, National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307-3000, USA

Key R. ([email protected]) Princeton University, Guyot Hall, Princeton, NJ 08544-1003, USA King B. A. ([email protected]) Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK Large William G. ([email protected]) National Center for Atmospheric Research, Boulder, CO 80307, USA Lazier John ([email protected]) Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth NS, Canada B2Y 4A2 Legler David M. ([email protected]) COAPS, Florida State University, Tallahassee, FL 32306-2840, USA, presently at US CLIVAR Office, 400 Virginia Avenue, Suite 750, Washington, DC 20024, USA

Imawaki Shiro ([email protected]) Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan

Lindstrom Eric J. ([email protected]) NASA Headquartes Code YS, Washington, DC 20546, USA

Jenkins W. J. ([email protected]) Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK

Liu W. Timothy ([email protected]) Jet Propulsion Laboratory 300-323, 4800 Oak Grove Drive, Pasadena, CA 91109-8099, USA

CONTRIBUTORS Liu Zhengyu ([email protected]) Center for Climatic Research and Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison, 1225 W. Dayton Street, Madison, WI 53706-1695, USA Lupton J. ([email protected]) NOAA/PMEL, Hatfield Marine Science Center, Newport, OR 97365, USA McDougall Trevor J. ([email protected]) CSIRO Marine Research, Hobart, Tasmania 7001, Australia McPhaden M. J. ([email protected]) NOAA/Pacific Marine Environmental Laboratory, Seattle, WA 98115-0070, USA Niiler Peter ([email protected]) Physical Oceanography Research Division, Scripps Institution of Oceanography, La Jolla, CA 92093-0213, USA

Price James F. ([email protected]) MS21, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Reverdin G. ([email protected]) GRGS, 14 Avenue Edouard Belin, 31055 Toulouse Cedex, France Rhines Peter ([email protected]) University of Washington, Seattle, WA 98195, USA Rintoul Stephen R. ([email protected]) CSIRO Marine Research, Hobart, Tasmania 7001, Australia, and Antarctic CRC, Hobart, Tasmania 7001, Australia Roether W. ([email protected]) Abteilung Tracer-Ozeanographie, Institut für Umweltphysik, FB1, Universität Bremen, Postfach 33 04 40, 28334 Bremen, Germany Saunders Peter M. ([email protected]) Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK

Nurser A. J. George ([email protected]) Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK

Schlosser P. ([email protected]) Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964-8000, USA

Olbers Dirk ([email protected]) Alfred-Wegener-Institut für Polar- und Meeresforschung, Postfach 12 0161, D27515 Bremerhaven, Germany

Semtner A. J. ([email protected]) Naval Postgraduate School, Monterey, CA 93943-5000, USA

Owens Breck ([email protected]) MS21, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

Siedler Gerold ([email protected]) Institut für Meereskunde, Universität Kiel, 24105 Kiel, Germany, and Instituto Canario de Ciencias Marinas, 35200 Telde, Spain

Philander S. G. H. ([email protected]) Department of Geosciences, Princeton University, Princeton, NJ 08540, USA

Smethie Jr W. M. ([email protected]) Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964-8000, USA

Pickart Robert ([email protected]) MS21, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

Smith Neville ([email protected]) Bureau of Meteorology Research Centre, Box 1289K, Melbourne, Victoria 3001, Australia

xv

CONTRIBUTORS

xvi

Stammer Detlef ([email protected]) Physical Oceanography Research Division, Scripps Institution of Oceanography, La Jolla, CA 92093-0230, USA Suginohara Nobuo ([email protected]) Center for Climate System Research, University of Tokyo, Tokyo 153-8904, Japan Talley Lynne D. ([email protected]) Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0230, USA Thompson B. J. ([email protected]) College of Marine Studies, University of Delaware, Lewes, DE 19958-1298, USA Toole John M. ([email protected]) Department of Physical Oceanography, MS 21, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

Wijffels Susan E. ([email protected]) CSIRO Marine Research, GPO Box 1538, Hobart, Tasmania 7001, Australia Willebrand J. ([email protected]) Institut für Meereskunde, Universität Kiel, 24105 Kiel, Germany Wong Annie ([email protected]) Joint Institute for the Study of the Atmosphere and Ocean (JISAO), University of Washington, Seattle, WA 98195, USA Wood Richard A. ([email protected]) Oceans Application Branch, Hadley Centre for Climate Prediction & Research, Met Office, Bracknell RG12 2SY, UK Wunsch Carl ([email protected]) Massachusetts Institute of Technology, Department of Earth, Atmosphere & Planetary Sciences, Cambridge, MA 02139-4307, USA

Wallace Douglas W. R. ([email protected]) Institut für Meereskunde, Universität Kiel, 24105 Kiel, Germany

Yashayaev Igor ([email protected]) Ocean Sciences Division, Bedford Institute of Oceanography, PO Box 1006, Dartmouth, NS B2Y 4A2, Canada

Webb D. J. ([email protected]) Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK

Zenk Walter ([email protected]) Institut für Meereskunde, Universität Kiel, 24105 Kiel, Germany

Foreword I am proud to join with my colleagues in celebrating the World Ocean Circulation Experiment (WOCE), one of the largest and most successful ocean endeavours ever undertaken by the international community. In 1988 I was Chairman of the WOCE Scientific Steering Group at the time of the International WOCE Conference in Paris. Scientific discussions throughout the 1980s had resulted in first a WOCE Science Plan and finally an Implementation Plan. At the International Conference, chaired by Gerold Siedler, countries made commitments to implementation. Now, some 13 years later, it gives me great pleasure to introduce this book. It encapsulates the enormous strides that have been made during the 1990s in understanding ocean circulation and its role in climate. This progress would not have been possible without the advances in technology (satellite and in-situ measurement, global high-accuracy navigation, supercomputing, the Internet) that we now take for granted. But WOCE provided the stimulus and the framework for addressing the oceans’ role in climate on a global scale. The obvious success of WOCE provides an object lesson in selfless international collaboration. Almost all WOCE data are already publicly available and will provide a unique resource for researchers to use for many years to come. WOCE also provides a sound scientific and technological foundation on which to build future research and operational ocean activities. Finally, this foreword should pay tribute both to the small group of far-sighted individuals who embarked on this project and to the many others who over the years have guided WOCE to the success it has now achieved.

D. James Baker Under Secretary for Oceans and Atmosphere Administrator, National Oceanic and Atmospheric Administration US Department of Commerce Washington, DC, USA October 2000

Preface As a moderator and initiator of climate variability and change, and as an economic resource, the global ocean is vital to life on earth. Quantifying the ocean’s role in climate has become a matter of urgency as society attempts to gauge how growing greenhouse gas concentrations will affect climate. The intergovernmental agreements of the Kyoto Protocol of 1997 indicate that policymakers are aware of the need to act on the basis of climate projections reported by the Intergovernmental Panel on Climate Change. Determining how to improve, extend and use these projections is an enormous challenge. The World Ocean Circulation Experiment (WOCE) is at the heart of this challenge. It predates the World Climate Research Programme, of which it is now a part. WOCE was the brainchild of a small group of scientists who, in the early 1980s, saw the prospects for instituting the first truly global ocean study, using new satellite technology and global ocean models made possible by increasing computer power. WOCE also needed unstinting international collaboration to collect a previously unobtainable in-situ global data set for validating and improving ocean circulation models. The satellite remote sensing, navigation and computer advances in the 1990s, coupled with developing electronic communication and the World Wide Web, made such an enterprise possible. In the process, WOCE revolutionized oceanography. From 1990, ship-based physical and chemical measurements and satellite remote sensing together resulted in a data set of unprecedented scope and precision. New observational techniques were also developed, maturing to form the basis of an operational observing system (Argo) for the interior of the global ocean. WOCE has resulted in a much closer interaction of observers and modellers. It also gave us the first generation of global data-assimilating models, which are immensely more powerful than the techniques previously available. The new data set and techniques are a key ingredient in decadal climate prediction and the results of WOCE have reinforced the evidence that the ocean plays a key role in the earth’s climate. Ocean Circulation and Climate – Observing and Modelling the Global Ocean presents the state of knowledge of the ocean and its role in climate change at the end of the twentieth century. The idea for the book originated from the ‘Ocean Circulation and Climate’ conference held in Halifax, Canada, in 1998. The conference marked the end of the observational phase of WOCE and the beginning of the Analysis, Interpretation, Modelling and Synthesis (AIMS) phase that is tackling many aspects of the ocean’s role in climate. After the conference, we approached the plenary speakers and other distinguished scientists to contribute to this book. We were delighted that so many agreed to join us in this project. The book chapters draw mainly from recent WOCE results, and naturally the contributors identify a number of outstanding questions and uncertainties. However, this book is not a final summary of WOCE’s achievements; WOCE researchers continue to publish a steady stream of new results in the refereed literature. Many of these results are based on the exploitation and synthesis of the unique WOCE basin- and global-scale observations and on the latest generation of ocean and climate models. The Ocean and Climate (Section 1) discusses the importance of the ocean in the climate system, describes the main ocean processes and reviews the history of WOCE. In Observations and Models (Section 2) some fundamental aspects of ocean observations and models are discussed. New Ways in Observing the Ocean (Section 3) introduces the new observational methods and instruments and the challenge of data and information management. The Global Flow Field (Section 4) discusses the state of knowledge of the global oceanic circulation from the surface to the deep ocean and from the Arctic to the tropical and Antarctic regions, as well as interocean

PREFACE

xix

exchanges. The Formation and Transport of Water Masses (Section 5) covers air–sea fluxes leading to the formation of surface water properties, the transport of these water masses into the ocean interior and the impact of ocean stirring and mixing. Large-Scale Ocean Transports (Section 6) describes the air–sea exchange, the storage and horizontal transport of heat, fresh water and carbon. Finally, The Future (Section 7) discusses the prospects for ongoing ocean and climate research and monitoring. The book is structured to guide the reader through the wide range of WOCE science in a consistent way. Cross-references between contributions have been added, and the book has a comprehensive index and unified reference list. Individual chapters can, however, stand alone. Indeed, where subject matter is duplicated, differences in approach and interpretation are evident. All chapters have been reviewed anonymously by specialists in the field; the editors thank the referees most warmly for their critical advice and guidance. Our hope is that this book will be read not only by oceanographers, meteorologists and climate scientists, but also by the wider scientific community of researchers, science managers and graduate students. A book of this size and complexity requires the hard work of many people. Notable among these are Lynne Hardwick and Julie Husin at CSIRO Hobart, Australia; Roberta Boscolo and Jean Haynes in the WOCE International Project Office in Southampton; and Serena Bureau and Gioia Ghezzi of Academic Press. Above all, this book should be regarded as a tribute to the originators of WOCE and to all the researchers, students and technicians who have transformed that early vision into a reality of which they can all feel justly proud.

Gerold Siedler Kiel, Germany and Telde, Spain

John Church Hobart, Australia

John Gould Southampton, UK

Acknowledgment The success of WOCE was dependent upon the collaboration of hundreds of researchers worldwide and the availability of technical and financial support to provide staff, equipment, ship resources, major facilities, satellites and supercomputers. On behalf of the WOCE scientific community, we as editors gratefully acknowledge the contributions from the many funding agencies that provided these resources and supported WOCE’s international planning and data management activities. GS acknowledges the support of the Deutsche Forschungsgemeinschaft and the German Federal Ministry of Research and Technology. JC acknowledges the support of the Australian Antarctic Cooperative Research Centre and CSIRO. This article is a contribution to the CSIRO Climate Change Program. JG acknowledges the UK Natural Environment Research Council for supporting the WOCE International Project Office contribution to this book.

SECTION

1 THE OCEAN AND CLIMATE Chapter 1.1

CLIMATE AND OCEANS Hartmut Grassl 3 Chapter 1.2

OCEAN PROCESSES AND CLIMATE PHENOMENA Allyn Clarke, John Church and John Gould 11 Chapter 1.3

THE ORIGINS, DEVELOPMENT AND CONDUCT OF WOCE B. J. Thompson, J. Crease and John Gould 31

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1.1 Climate and Oceans Hartmut Grassl

1.1.1 WOCE and The World Climate Research Programme The earth is mainly an ocean planet with 71% of the surface covered by water, including up to 6% coverage by sea ice. In total, perhaps as much as 85% of the earth’s surface is covered by either liquid water (oceans, lakes, rivers, wet vegetation) or solid water (snow, land-ice, sea-ice). Therefore, absorption of solar energy, which drives the earth’s climate system, and evapotranspiration of water at the surface, are dominated by the oceans. In addition, the global mean values of other energy fluxes, such as sensible and latent heat flux and net long-wave radiation flux, at the surface are predominantly a result of ocean–atmosphere interaction. The heat capacity of only three metres of the ocean corresponds to the heat capacity of the entire atmospheric column above. Thus, even if the horizontal ocean currents were much smaller, climate variability would still be to a large extent an ocean-related phenomenon. Both the atmosphere and the oceans exhibit a complicated circulation pattern and their interaction determines much of the climate variability on time scales from several hours (e.g. sea breezes) to seasons, years, decades, centuries and millennia. Understanding this variability is the essence of the World Climate Research Programme (WCRP), whose single and demanding goal has been formulated as follows: To understand and predict, to the extent possible, climate variability and climate change, including human influence on climate. Only if climate variability is at least partially understood will it be possible to detect and predict OCEAN CIRCULATION AND CLIMATE ISBN 0-12-641351-7

climate change arising from external forcing, be it by earth orbital parameter changes, solar irradiance variations, volcanic eruptions (the latter counted as external forcing despite belonging to the earth system) and/or human activities. Given the importance of the oceans to climate, it is not surprising to find the first two projects of WCRP, the World Ocean Circulation Experiment (WOCE) and the Tropical Ocean-Global Atmosphere (TOGA) study, were ocean related. Early discussions and the formulation of science plans for WOCE and TOGA were initiated by the Committee on Climate Changes and the Oceans (CCCO; Thompson et al., Chapter 1.3), jointly sponsored by the Scientific Committee for Oceanic Research (SCOR) of the International Council for Scientific Unions (ICSU), and by the Intergovernmental Oceanographic Commission (IOC) of UNESCO. The first decision on WOCE was made in 1978, before WCRP was initiated by WMO and ICSU in 1980. TOGA, which had been stimulated by the 1982–83 El Niño event, the most intense of the twentieth century until that date, was the first formally implemented WCRP project lasting from 1985 to 1994. In 1982, WOCE, focused on building models necessary for predicting climate change, became a central element of WCRP and effectively complemented the atmospheric projects within the broader scope of WCRP. Implementation of the field programme of WOCE commenced formally in 1990, at about the same time IOC became a sponsor of WCRP. Now, in 2001, we can look back to two very successful projects of WCRP. First, TOGA has made possible physically based predictions of Copyright © 2001 Academic Press All rights of reproduction in any form reserved

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climate variability on time scales of seasons to a year for those areas affected by the El Niño-Southern Oscillation (ENSO) events. Second, the field phase of WOCE has produced, for the first time, a global three-dimensional view of ocean structure as well as of the trace substance distribution of the global ocean (excluding the Arctic Basin and – due to operational constraints – parts of the Southern Ocean). At the same time there has been a significant improvement in the quality of ocean models and the WOCE data have become the basis for testing the ocean modules of climate (coupled ocean–atmosphere–land surface) models. This book is devoted to evaluation of ocean circulation as determined from the WOCE (and other) data, to the development of ocean models and to the understanding of the ocean’s circulation as a component of the climate system. Therefore I will concentrate on the role of the ocean as a component of the climate system.

1.1.2 The scientific approach to the complex climate system Complex systems are characterized by their ability to develop transient organized structures when reacting to internal or external forcing as a result of their internal non-linear dynamics. Examples are galaxies, stars, planets, the atmosphere, the ocean, ecosystems, our body and a single cell. The preferred scientific approach to complex systems is via experiments under controlled conditions for simpler subsystems, leading to models that approach reality and thus can be used for practical applications. In the geosciences, however, where deliberate experiments are impossible, the approach is via long-term, nearly global observations that may lead to a degree of understanding that allows numerical prediction for restricted time scales. The most successful application of this method is deterministic weather forecasting. In principle, weather forecasting can only be successful up to about 2 weeks and, at present, mostly does not incorporate more from the ocean than surface temperature as a starting field for deterministic predictions up to 1 week ahead. The climate system shows some statistical stability in its long-term behaviour despite reacting with deterministic instability to changed initial fields. Thus, climate predictions are probabilistic predictions that go beyond the predictability barrier for deterministic weather forecasts.

Since there were, until recently, no routine observations of large parts of the oceans’ interior, the barrier to climate variability predictions on monthly to seasonal time scales could not be surmounted. An exception was the application of very simple empirical rules to certain regions where persistent sea surface temperature anomalies allowed some extrapolation in time. Long-term observations on nearly global scale are needed. The highest priority for seasonal forecasts was for upper ocean observations in areas with especially strong seasonal to interannual variability. This, together with developing coupled ocean–atmosphere models, was exactly the strategy pursued by TOGA scientists. They were able to implement the TOGA observing system, including the Tropical Atmosphere/Ocean (TAO) Array, a set of up to 64 moored buoys across the tropical Pacific, measuring surface meteorological variables and upper ocean structure and expendable bathythermograph measurements from Voluntary Observing Ships (VOS). This observing system brought the breakthrough to physically based climate anomaly predictions for ENSO-affected areas by using coupled ocean– atmosphere models assimilating near-real-time observations. The success is due to the intrinsic time scales of ocean–atmosphere interaction of up to a year in the Pacific caused by the travel time of equatorial ocean Kelvin and Rossby waves across the Pacific basin. To understand and predict the full spectrum of climate variability, long-term global ocean observations are required (for a fuller discussion see Wunsch, Chapter 2.1 and Smith, Chapter 7.4). However, prior to WOCE no such system was considered feasible. Thus a major challenge for WOCE was to demonstrate that feasibility. Given the complexity of the oceans and the limited resources available, this required a range of different techniques (observations from research ships and merchant vessels, surface drifters, subsurface floats, moored instruments and, of course, satellites). All of these observing systems required some degree of development. WOCE’s main task was to observe for the first time (largely on a basin-by-basin manner) the three-dimensional structure of the global ocean as the basis for ocean model improvement needed for more reliable climate models. This required a commitment by the research community to the

1.1 Climate and Oceans implementation of an internationally agreed plan, internationally agreed standards and the international management and sharing of data. Significant technical developments achieved during WOCE have now opened the door for an ongoing global ocean observing system. Two of the most significant technical developments are:

¥ accurate ocean surface topography measure-

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ments by satellite sensors, first of all by TOPEX/ POSEIDON, but also by ERS-1 and -2 altimeters (Fu, Chapter 3.3); development of profiling autonomous Lagrangian floats determining upper ocean structure and mean current on a prescribed pressure surface about every 2 weeks and with typical survival times of about 4 years (Davis and Zenk, Chapter 3.2).

Developing the new technologies and demonstrating the feasibility of a global observing system are only the first steps to building a truly global observing system. Applications like ocean weather forecasting, global seasonal climate variability predictions, better guidance for fisheries, etc., require the operational implementation of a global array of such floats and the continuation of altimeter measurements beyond the endorsed experimental phase. Sustained observations of the ocean interior and its surface have not only been recognized as prerequisites for progress in prediction of climate variability on seasonal time scales, but also for the understanding of decadal to century time-scale climate variability, a major challenge for climate science. Now that a near-real-time ocean observing system has been shown to be feasible and costeffective, both CLIVAR (Climate Variability and Predictability study of WCRP) and GODAE (Global Ocean Data Assimilation Experiment) are implementing within the Integrated Global Observing Strategy (IGOS) a pilot project to demonstrate the value of a global float array in combination with satellite altimetry. Besides operational ocean observations for climate research and predictions, we still lack certain types of observations in the atmosphere, namely, the threedimensional distribution of liquid water and ice, vertical profiles of minor constituents like ozone, and wind profiles in the lower troposphere. Since cloud–radiation interaction is a source of major uncertainty in the response of the climate system

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to an external forcing (by the sun or by human activities), I wrote, in 1996, on behalf of the Joint Scientific Committee for WCRP, to the major space agencies, asking for the development of an active sensor combination for the measurement of cloud water and ice. At the same time, I also pointed to the need for a better geoid determination in order to be able to exploit fully ongoing altimeter measurements for oceanography and climate research. To implement operational observing systems, it is necessary to start first with a research network and to demonstrate the benefit of the network for society as a whole. Only then will the resources for an ongoing operational commitment be provided. Scientists within WCRP are working to repeat these steps for as yet unobserved parts of the climate system, such as the deep ocean, sea-ice thickness, and the isotopic composition of precipitation, river water and snow. However, operational implementation will only follow if it is demonstrated to be a prerequisite for further progress and if cost-effective observational strategies are available or can be developed.

1.1.3 Ocean–atmosphere interaction and climate The ocean and atmosphere transport on average roughly the same amount of heat from low to high latitudes. However, this is achieved in remarkably different manners. The atmosphere does it mainly by transient eddies in the middle and high latitudes; the ocean does it mainly by boundary currents, large gyres (wind-driven to a large extent) and the vertical overturning of the ocean. Although the sun delivers more energy to the southern hemisphere since we are at present nearest to the sun in January, the northern hemisphere nevertheless contains the thermal equator at all seasons over the Atlantic and the Pacific. In the Atlantic, this is due to the shape of South America and Africa and to the Atlantic transporting – as WOCE studies have made clearer – about 1015 W of heat across the equator into the northern hemisphere (Bryden and Imawaki, Chapter 6.1). While the ocean is influenced by the atmosphere through fluxes of momentum, fresh water, incoming solar irradiance (Fig. 1.1.1, see Plate 1.1.1, p. 44) and atmospheric thermal radiation, sea surface temperature is the main parameter influencing the heat

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fluxes from the ocean to the atmosphere. The results of these ocean influences are reduced seasonal variation in maritime areas compared with continental regions, weaker meridional gradients over ocean areas and strong longitudinal dependence of yearly average temperature, especially in the northern North Atlantic (Fig. 1.1.2, see Plate 1.1.2, p. 44). Here the deviation from the zonal mean temperature exceeds 10°C off Norway. A major reason for these high temperatures (besides transport of heat from the subtropical Atlantic Ocean) is the high salinity in the northern latitude Atlantic leading to intermittent deep convection events either near to the sea-ice edge (Greenland Sea) or in the open ocean regions such as the Labrador Sea. Long-term mean ocean–atmosphere interaction is responsible for the mean differences between marine and continental climates. Internal variability of the atmosphere–ocean–cryosphere climate system on all time scales, supplemented by external forcing (volcanic eruptions and solar cycles), produces the observed climate variability. On very long time scales, the forcing by the changed orbit of the earth around the sun (Milankovich cycles) becomes dominant. We have good statistical descriptions of climate variability, at least for the last few decades, for many places and large continental areas. However, we lack even the basic understanding of the causes for some of the multiyear to decadal time scale ocean-focused phenomena like the Antarctic Circumpolar Wave in the Southern Ocean and the North Atlantic Oscillation. Only the ENSO and the QuasiBiennial Oscillation (QBO) stand out as being partly understood. Since ENSO is the cause of a large part of interannual variability in at least the tropics, the next breakthrough concerning the understanding of climate variability may well be related to mid-latitude variability. As well as ocean–atmosphere interaction, it is likely also to involve land ice–atmosphere, vegetation–atmosphere and sea ice–atmosphere interaction since all these interactions lead to interannual and decadal time scale variability. For example, understanding the variability of monsoons will need cooperation between two WCRP projects – the Climate Variability and Predictability (CLIVAR) study and the Global Energy and Water Cycle Experiment (GEWEX) – in coordinated enhanced observing periods.

1.1.4 Rapid changes related to the oceans The tendency of the climate system to react strongly and rapidly to minor changes is now well established, both from direct observations and palaeoclimate reconstructions. An example (not related to the ocean) is the Antarctic ozone hole. Less than a billionth of all air molecules, the chlorofluorocarbons (CFCs) and halons, have caused through their chlorine- and bromine-containing decay products, the complete disappearance of the ozone in areas with polar stratospheric clouds (12–20 km height) in the Antarctic during early spring since the mid-1980s. (Ozone acts as a UV-B filter and is the third most important greenhouse gas after water vapour and CO2.) This has led through mixing with mid-latitude air to the weakening of the strong latitudinal gradient of UV-B radiation. Now the daily UV-B dose reaching the surface during a sunny day in late spring and early summer is sometimes as high in New Zealand and on the Antarctic Peninsula as it is in Darwin, tropical North Australia (Seckmeyer et al., 1995). Palaeo-evidence on the instability of the global ocean conveyer belt stimulated by increased freshwater input from melting ice sheets into the Atlantic is now abundant (Keigwin et al., 1994). Also model studies (e.g. Rahmstorf and Willebrand, 1995) have suggested that this may happen for comparatively slight changes in the freshwater budget of the Atlantic, north of about 30°S. It is suggested that addition of 0.1 Sverdrup (1 Sverdrup: 106 m3 s91) of fresh water could completely stop deep convection in the northern North Atlantic and remove the 4°C positive sea surface temperature anomaly (compared with the Eastern Pacific at a latitude of about 50°N). In addition, coupled ocean–atmosphere GCMs (General Circulation Models) run under steadily increasing greenhouse gas concentrations show a spin-down of the strength of the meridional overturning in the Atlantic (e.g. Manabe and Stouffer, 1993) and in the Southern Ocean (Hirst, 1998). It is therefore urgent that the monitoring of the flow over the sills from the Nordic Seas into the North Atlantic and of the oceanic overturning itself, as conducted during WOCE, be continued in order to validate the models’ ability to represent adequately realistic large-scale ocean circulation and its variability. Another potential rapid change related to the ocean would be the disappearance of multiyear sea

1.1 Climate and Oceans ice in the Arctic Basin, since it would alter air–sea fluxes and especially precipitation (snowfall) strongly in high altitudes. This would certainly have a hemispheric impact. Whether the recent decline of the multiyear sea-ice area in the Arctic Basin by 18% from 1978 to 1998 reported by Johannessen et al. (1999) is related to a natural recent intensification of the North Atlantic Oscillation or is an effect of anthropogenic climate change is not yet clear. In order to establish reliable data sets on sea-ice thickness (the multiyear sea-ice decline is related to shrinking sea-ice thickness), an extension of the small networks of upward-looking sonars established within WCRP’s Arctic Climate System Study (ACSYS), both in the Arctic and in the sea-ice area around Antarctica, is required. The free access to earlier classified data from submarines in combination with these networks would be a validation data set for the upcoming altimeter data sets, that could give estimates of sea-ice thickness. It was one of the roles of WOCE to develop models that would allow the assessment of the sensitivity of the climate system to these types of changes.

1.1.5 Cryosphere and the oceans Ice–ocean interactions are an important part of the climate system. Mountain glaciers, small ice caps and ice sheets influence global sea level. Deep convection in the Arctic and Southern Oceans is largely related to sea-ice formation on continental shelves or in coastal polynyas (slope convection) and open ocean deep convection is often caused by atmospheric forcing at the sea-ice edge. Sea ice is treated interactively in most coupled ocean–atmosphere models and improved parameterizations for dynamical sea-ice behaviour have been selected by the Sea Ice Modelling Intercomparison Project within ACSYS (Lemke et al., 1997). However, although meltwater runoff from land-ice and the melting of tabular icebergs in the Antarctic sea-ice zone influence the thermohaline circulation, as yet no adequate parameterizations exist for these processes. The positive freshwater balance of the Arctic Ocean creates a thin and cold, less saline layer on top of a much warmer and more saline intermediate layer of Atlantic origin. The heat content of this deeper layer is sufficient to melt the entire multiyear sea ice. Therefore, changes in the salinity

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of the top layer through changes in Arctic river runoff, snow-depth on sea ice and evaporation could have a strong impact on Arctic sea ice, and in turn on Arctic Ocean circulation and at least regional climate. However, the observation of solid precipitation is at present not adequate to detect changes. This is because earlier observations (for example, snow depth on sea ice measured regularly in spring at many stations where ocean profiling took place) have largely ceased following the collapse of the Soviet Union. As yet, remote sensing methods of sufficient accuracy to detect these precipitation changes have not been developed.

1.1.6 Anthropogenic climate change and the oceans The ocean as a key component of the climate system also plays a major role in anthropogenic climate change. First, ocean heat absorption delays the full global warming. Second, the oceans, particularly their regional pattern of heat transport and absorption, lead to significant changes in regional climate and thus rainfall and temperature change. Third, the oceans are a major sink for anthropogenic CO2. Fourth, ocean heat absorption leads to thermal expansion of the oceans and sea-level rise and as a result to coastal erosion and flooding. Fifth, as already mentioned, changes in the formation of deep water masses at high latitudes in the North Atlantic and the Southern Ocean could lead to abrupt changes in the global ocean thermohaline circulation and a major rearrangement of global climate. As indicated by Kattenberg et al. (1996), the warming response is dependent on the rate of increase of greenhouse gases because of uptake of heat by the oceans. In coupled models, a slow increase in greenhouse gas concentrations of 0.25% yr91 to doubled preindustrial levels results in 70% of the full equilibrium warming at the time of doubling. For a more rapid increase of 4% yr91, only about 40% of the equilibrium warming is achieved at the time of doubling. The present rate of increase of greenhouse gas concentrations is below 1% yr91, accounting for the combined effect of all greenhouse gases. Thus only about 60% of the warming we are committed to because of increases in past greenhouse gas concentrations should have been realized to date. In addition to slowing the rate of warming, changes in precipitation patterns

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will depend strongly on the warming pattern of the ocean. The ocean water contains about 50 times the carbon stored in the atmosphere. On average about 90 billion tons of carbon (GtC) are released from the ocean and about 92 GtC are absorbed by the ocean each year. Carbon models indicate a net uptake of 2