Vallis - Climate and the Oceans

CLIMATE AND THE OCEANS

Princeton Primers in Climate

David Archer, The Global Carbon Cycle

Geoffrey K. Vallis, Climate and the Oceans

Shawn J. Marshall, The Cryosphere

Geoffrey K. Vallis

Copyright 2012 by Princeton University Press

Published by Princeton University Press
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In the United Kingdom: Princeton University Press
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All Rights Reserved
ISBN: 978-0-691-14467-2 (cloth)
ISBN: 978-0-691-15028-4 (pbk)

Library of Congress Cataloging-in-Publication Data

Vallis, Geoffrey K.
Climate and the oceans / Geoffrey K. Vallis.
p. cm. (Princeton primers in climate)
Includes bibliographical references and index.
1. OceanographyResearch. 2. Ocean circulation.
3. Ocean-atmosphere interaction. 4. Climatic changes.
I. Title.
GC57.V26 2011
551.5246dc22
2011014372

British Library Cataloging-in-Publication Data is available

This book has been composed in Minion Pro
Printed on acid-free paper.
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1

The truth and nothing but the truth, but not
the whole truth.

THIS IS A BOOK ON CLIMATE, WITH AN EMPHASIS ON THE role of the ocean. The emphasis is on large-scale processes and phenomena, and on the physical aspects of the ocean rather than its chemical or biological properties. It is not a textbook on physical oceanography, of which there are several good ones, nor is it a textbook on climate, of which there are some good ones. Rather, and as its size may indicate, the book is an introduction to, or a primer on, the ocean-climate system.

This book could be used to provide an introductory big picture for more advanced students or for scientists in other fields, or it could provide advanced reading for undergraduate students taking courses at a more elementary level. The book is somewhat more mechanistic than most books at this level: the emphasis is on how things work, and in particular how the ocean works and how it influences climate. I discuss observations to motivate the discussion, but the main emphasis is not on what things happen to be, but why and how they happen to be.

This is a fast book, although it is not, I hope, a loose book. It covers a lot of ground, quickly, and tries not to get bogged down in too much detail. Having said that, one of the most important questions to answer in the study of climate is to understand just what is a detail and what is essential. If one is studying the climate as a whole, then one might regard the presence of a small island in the northeast Atlantic as a detail, and it surely is (unless one is studying the climate of that island). One might also regard the precise way in which carbon dioxide molecules vibrate and rotate when electromagnetic radiation impinges upon them as a detail, yet it is precisely this motion that gives us the greenhouse effect, which makes our planet habitable, and which also gives us global warming, and which may affect our economy to the tune of trillions of dollars. Hardly a detail! But can we, and need we, understand all such matters to understand climate? Without answering that question, it is clear that in a short book such as this, choices have to be made. I will occasionally, but only occasionally, simply tell you how things are, without delving into the mechanisms if they are tangential to our narrative.

Given that we do move around sharp bends quite quickly, a certain amount of sophistication is assumed of the reader, or at least a willingness to think a little and puzzle things out and perhaps even look up one or two references. On the other hand, I havent assumed much background knowledgejust a bit of basic physics and mathematics and some general knowledge about the ocean and climate. In other words, Im writing the . Some readers may wish to skip ahead directly to chapters that particularly interest them, and this way of reading should be possible by referring back to the earlier chapters as needed.

I thank Peter Gent and Carl Wunsch for their perceptive reviews of an early draft of this book. They saved me from myself in a number of ways. Thanks also to Ryan Rykaczewski, Stephanie Downes, Mehmet Ilicak, Rym Msadek, Caroline Mller, Amanda ORourke, Ilissa Ocko, Thomas Spengler, and Antoine Venaille, for many comments and constructive criticisms, even if they did it for the beer. The chapter on ocean circulation owes much to a couple of vigorous conversations with Max Nikurashin. Finally, my thanks to Cathy Raphael for expertly creating many of the figures.

CLIMATE AND THE OCEANS

The climates delicate, the air most sweet.

William Shakespeare, A Winters Tale

TO APPRECIATE THE ROLE OF THE OCEAN IN CLIMATE, we need to have a basic understanding of how the climate system itself works, and that is the purpose of this chapter. Our emphasis here is the role of the atmospherewe dont pay too much attention to the oceans as well get more of that (lots more) in later chaptersand we assume for now that the climate is unchanging. So without further ado, lets begin.

Earth is a planet with a radius of about 6,000 km, moving around the sun once a year in an orbit that is almost circular, although not precisely so. Its farthest distance from the sun, or aphelion, is about 152 million km, and its closest distance, perihelion, is about 147 million km. This ellipticity, or eccentricity, is small, and for most of the rest of the book we will ignore it. (The eccentricity is not in fact constant and varies on timescales of about 100,000 years because of the influence of other planets on Earths orbit; these variations may play a role in the ebb and flow of ice ages, but that is a story for another day.) Earth itself rotates around its own axis about once per day, although Earths rotation axis is not parallel to the axis of rotation of Earth around the sun. Rather, it is at an angle of about 23, and this is called the obliquity of Earths axis of rotation. (Rather like the eccentricity, the obliquity also varies on long timescales because of the influence of the other planets, although the timescale for obliquity variations is a relatively short 41,000 years.) Unlike the ellipticity, the obliquity is important for todays climate because it is responsible for the seasons, as we will see later in this chapter.