Ocean circulation

287 Pages·1999·53.57 MB·English

by  Angela Colling

OCEAN C IR CU L ATlO N
This Page Intentionally Left Blank
Authors
Evelyn Brown (Waves, Tides, etc.; Ocean Chemistry.)
Angela Coiling (Ocean Circulation; Seawater (2nd edn); Waves, Tides, etc.
(2nd edn); Case Studies)
Dave Park (Waves, Tides, etc.)
John Phillips (Case Studies)
Dave Rothery (Ocean Basins)
John Wright (Ocean Basins; Seawater; Waves, Tides, etc. (2nd edn);
Ocean Chemistry; Case Studies)
Other members
Mark Brandon
Rachael James
Course Manager
Jennie Neve
Designers
Jane Sheppard
Liz Yeomans
Graphic Artist
Sue Dobson
Cartographer
Ray Munns
Editor
Gerry Bearman

This Volume forms part of an Open University course. For general
availability of all the Volumes in the Oceanography Series, please contact
your regular supplier, or in case of difficulty the appropriate Butterworth-
Heinemann office.
Further information on Open University courses may be obtained from:
The Admissions Office, The Open University, P.O. Box 48, Walton Hall,
Milton Keynes MK7 6AA, UK or from the Open University website:
h ttp://w w w. open. ac. uk
Cover illustration: Satellite photograph showing distribution of
phytoplankton pigments in the North Atlantic off the US coast in the region
of the Gulf Stream and the Labrador Current. (NASA, and O. Brown and
R. Evans, University of Miami.)
PREPARED BY ANGELA COLLING FOR THE COURSE TEAM
Butterworth-Heinemann
Linacre House, Jordan Hill, Oxford OX2 8DP
A division of Reed Educational and Professional Publishing Ltd
-~A member of the Reed Elsevier plc group
BOSTON JOHANNESBURG
MELBOURNE NEW DELHI OXFORD
Copyright (cid:14)9 1989 The Open University
First edition 1989
Second edition 2001. Reprinted 2004 (with corrections)
All rights reserved; no part of this publication may be reproduced, stored in a
retrieval system, or transmitted in any form or by any means, electronic,
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Road, London W1P 0LE This book may not be lent, resold, hired out or otherwise
disposed of by way of trade in any form of binding or cover other than that in
which it is published, without the prior consent of the Publishers.
British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library
ISBN 0 7506 5278 0
Library of Congress Cataloguing in Publication Data
A catalogue record for this book is available from the Library of Congress
Jointly published by the Open University, Walton Hall, Milton Keynes
MK7 6AA and Butterworth-Heinemann

Edited, typeset, illustrated and designed by The Open University
Printed in Singapore by Kyodo under the supervision of MRM
Graphics Ltd., UK
s330v3i2.3
ABOUT THIS VOLUME
ABOUT THIS SERIES
1.1 THE RADIATION BALANCE OF THE EARTH-OCEAN-ATMOSPHERES YSTEM 14
1.2 SUMMARY OF CHAPTER 1 16
2.1 THE GLOBAL WIND SYSTEM 18
2.2 POLEWARD TRANSPORT OF HEAT BY THE ATMOSPHERE 22
2.2.1 Atmospheric circulation in mid-latitudes 22
2.2.2 Vertical convection in the atmosphere 26
2.3 ATMOSPHERE-OCEAN INTERACTION 29
2.3.1 Easterly waves and tropical cyclones 29
2.3.2 A brief look ahead 34
2.4 SUMMARY OF CHAPTER 2 35
3.1 THE ACTION OF WIND ON SURFACE WATERS 39
3.1.1 Frictional coupling with the ocean 40
3.1.2 Ekman motion 42
3.2 INERTIA CURRENTS 44
3.3 GEOSTROPHIC CURRENTS 46
3.3.1 Pressure gradients in the ocean 46
3.3.2 Barotropic and baroclinic conditions 49
3.3.3 Determination of geostrophic current velocities 53
3.3.4 Pressure, density and dynamic topography 60
3.4 DIVERGENCES AND CONVERGENCES 64
3.5 THE ENERGY OF THE OCEAN: SCALES OF MOTION 69
3.5.1 Kinetic energy spectra 69
3.5. 2 Eddies 71
3.6 SUMMARY OF CHAPTER 3 75
4.1 THE GULF STREAM 79
4.1.1 Early observations and theories 79
4.2 THE SUBTROPICAL GYRES 85
4.2.1 Vorticity 85
4.2.2 Why is there a Gulf Stream? 90
4.2.3 The equations of motion 98
4.2.4 Investigating the ocean through computer modelling 102
4.3 MODERN OBSERVATIONSA ND STUDIES OF THE NORTH ATLANTIC GYRE 107
4.3.1 The Gulf Stream system 107
4.3.2 Geostrophic flow in the Gulf Stream 109
4.3.3 Insights from MODE 114
4.3.4 Measuring currents directly 117
4.3.5 Mapping the Gulf Stream using water characteristics 122
4.3.6 Gulf Stream 'rings' 124
4.3.7 Other methods of current measurement 129
4.3.8 Modelling the circulation of the North Atlantic 131
4.4 COASTAL UPWELLING IN EASTERN BOUNDARY CURRENTS 133
4.5 THE NORTH ATLANTIC OSCILLATION 137
4.6 SUMMARY OF CHAPTER 4 140
5.1 EQUATORIAL CURRENT SYSTEMS 143
5.1.1 The Equatorial Undercurrent 146
5.1.2 Upwelling in low latitudes 153
5.2 MONSOONAL CIRCULATION 156
5.2.1 Monsoon winds over the Indian Ocean 156
5.2.2 The current system of the Indian Ocean 157
5.3 THE ROLE OF LONG WAVES IN OCEAN CIRCULATION 162
5.3.1 Oceanic wave guides and Kelvin waves 164
5.3.2 Rossby waves 167
5.4 EL NINO-SOUTHERN OSCILLATION 170
5.5 CIRCULATION IN HIGH LATITUDES 176
5.5.1 The Arctic Sea 177
5.5.2 The Southern Ocean 181
5.6 SUMMARY OF CHAPTER 5 186
6.1 THE OCEANIC HEAT BUDGET 191
6.1.1 Solar radiation 191
6.1.2 The heat-budget equation 192
6.2 CONSERVATION OF SALT 202
6.2.1 Practical application of the principles of conservation and continuity 203

6.3 OCEAN WATER MASSES 206
6.3.1 Upper and intermediate water masses 208
6.3.2 Deep and bottom water masses 213
6.4 OCEANIC MIXING AND TEMPERATURE-SALINITY DIAGRAMS 223
6.4.1 Mixing in the ocean 223
6.4.2 Temperature-salinity diagrams 225
6.5 NON-CONSERVATIVE AND ARTIFICIAL TRACERS 234
6.6 GLOBAL FLUXES OF HEAT AND FRESHWATER 240
6.6.1 The global thermohaline conveyor 240
6.6.2 The World Ocean Circulation Experiment 243
6.6.3 Oceanography in the 21st century: predicting climatic change 248
6.7 SUMMARYO F CHAPTER 6 250
This is one of a Series of Volumes on Oceanography. It is designed so that it
can be read on its own, like any other textbook, or studied as part of $330
Oceanography, a third level course for Open University students. The
science of oceanography as a whole is multidisciplinary. However, different
aspects fall naturally within the scope of one or other of the major
'traditional' disciplines. Thus, you will get the most out of this Volume if
you have some previous experience of studying physics. Other Volumes in
this Series lie more within the fields of geology, chemistry and biology (and
their associated sub-branches).
Chapter 1 establishes the essential causes of the circulation patterns of air
and water in the atmosphere and the oceans. These are: first, the need for
heat to be redistributed over the surface of the Earth; and secondly, that the
Earth is both rotating and spherical. Chapter 2 follows on from this by
describing the global wind system, and showing how the atmospheric
circulation transports heat from low latitudes to high latitudes. It
emphasizes that the atmosphere and oceans are continuously interacting,
effectively forming one system, and illustrates this idea through the
spectacular example of tropical cyclones.
Chapter 3 explains the fundamental principles relating to wind-driven
surface currents. The theoretical basis is established with the minimum use
of mathematical equations, which are explained and applied in context to
enable you to understand their relevance. The Chapter ends with a
discussion of the energy of the ocean, and emphasizes the importance of
mesoscale eddies which, although they are believed to contain most of the
energy of the ocean circulation, were only discovered in the 1970s.
Chapter 4 uses the example of the North Atlantic subtropical gyre,
particularly the Gulf Stream, to explore how ideas about ocean circulation
have evolved from the sixteenth century up to the present day. Some of the
theoretical models of the ocean, devised by oceanographers to help them
understand why the subtropical gyres have the form they do, are discussed
with a minimum of mathematics. The equations of motion, which are the
basis for all dynamic models, are introduced in a qualitative way, and some
recent computer models are briefly discussed, so as to bring out the
fundamental purpose of modelling. The last Section introduces the idea of
'teleconnections', with discussion of the North Atlantic Oscillation - a

continual climatic 'sea-saw', which is the most important factor affecting
winter conditions over the northern Atlantic Ocean and Nordic Seas.
Chapter 5 considers the major current systems outside the subtropical gyres,
in both high and low latitudes. The characteristics of the current systems of
the tropical regions are described and explained in terms of the direct and
indirect effects of the wind- the Trade Winds in the Atlantic and Pacific,
and the seasonally reversing monsoonal winds in the Indian Ocean. The
importance of the Equator as a 'wave guide', which channels disturbances
in the upper ocean, is briefly discussed, particularly in the context of the
climatic oscillation known as E1 Nifio. The Chapter ends with a description
of the Antarctic Circumpolar Wave, a large-scale wave-like disturbance
involving the atmosphere, ocean and ice-cover, which travels around the
Antarctic continent once every 8-10 years.
The last Chapter takes up the theme of the first, and revises many of the
topics introduced in earlier Chapters. It considers the transport of heat and
water through the ocean-atmosphere system and begins with a discussion of
processes occurring at the air-sea interface, where all oceanic water masses
acquire their characteristics. Changes in the rates of formation of the cold,
dense water masses which flow through the deep basins of the ocean are
closely related to climatic change, and this topic is discussed in relation to
climatic fluctuations in the North Atlantic. Finally, there is a discussion of
the importance of the World Ocean Circulation Experiment (initiated at the
end of the 1980s) and a brief look at future developments in oceanography.
You will find questions designed to help you to develop arguments and/or
test your own understanding as you read, with answers provided at the back
of this Volume. Important technical terms are printed in bold type where
they are first introduced or defined.
The Volumes in this Series are all presented in the same style and format,
and together provide a comprehensive introduction to marine science.
Ocean Basins deals with structure and formation of oceanic crust,
hydrothermal circulation, and factors affecting sea-level. Seawater
considers the seawater solution and leads naturally into Ocean Circulation,
which is the 'core' of the Series. Waves, Tides and Shallow-Water Processes
introduces the physical processes which control water movement and
sediment transport in the nearshore environment (beaches, estuaries, deltas,
shelves). Ocean Chemistry and Deep-Sea Sediments is concerned with
biogeochemical cycling of elements within the seawater solution and with
water-sediment interaction on the ocean floor. Case Studies in
Oceanography and Marine Affairs examines the effect of human
intervention in the marine environment and introduces the essentials of
Law of the Sea. The two case studies respectively review marine affairs in
the Arctic from an historical standpoint, and outline the causes and effects
of the tropical climatic phenomenon known as E1 Nifio.
Biological Oceanography: An Introduction (by C. M. Lalli and T. R. Parsons)
is a companion Volume to the Series, and is also in the same style and
format. It describes and explains interactions between marine plants and
animals in relation to the physical/chemical properties and dynamic
behaviour of the seawater in which they live.