book - Introduction - to - Physical - Oceanography

book - Introduction - to - Physical - Oceanography

(Parte 1 de 10)

Introduction To Physical Oceanography

Robert H. Stewart

Department of Oceanography Texas A & M University

Copyright 2008 September 2008 Edition i i

Contents

Preface vii

1.1 Physics of the ocean1
1.2 Goals2
1.3 Organization3
1.4 The Big Picture3
1.5 Further Reading5

1 A Voyage of Discovery 1

2.1 Definitions8
2.2 Eras of Oceanographic Exploration8
2.3 Milestones in the Understanding of the Ocean12
2.4 Evolution of some Theoretical Ideas15
2.5 The Role of Observations in Oceanography16
2.6 Important Concepts20

2 The Historical Setting 7

3.1 Ocean and Seas2
3.2 Dimensions of the ocean23
3.3 Sea-Floor Features25
3.4 Measuring the Depth of the Ocean29
3.5 Sea Floor Charts and Data Sets3
3.6 Sound in the Ocean34
3.7 Important Concepts37

3 The Physical Setting 21

4.1 The Earth in Space39
4.2 Atmospheric Wind Systems41
4.3 The Planetary Boundary Layer43
4.4 Measurement of Wind43
4.5 Calculations of Wind46
4.6 Wind Stress48
4.7 Important Concepts49

4 Atmospheric Influences 39 i iv CONTENTS

5.1 The Oceanic Heat Budget51
5.2 Heat-Budget Terms53
5.3 Direct Calculation of Fluxes57
5.4 Indirect Calculation of Fluxes: Bulk Formulas58
5.5 Global Data Sets for Fluxes61
5.6 Geographic Distribution of Terms65
5.7 Meridional Heat Transport68
5.8 Variations in Solar Constant70
5.9 Important Concepts72

5 The Oceanic Heat Budget 51

6.1 Definition of Salinity73
6.2 Definition of Temperature7
6.3 Geographical Distribution7
6.4 The Oceanic Mixed Layer and Thermocline81
6.5 Density83
6.6 Measurement of Temperature8
6.7 Measurement of Conductivity or Salinity93
6.8 Measurement of Pressure95
6.9 Temperature and Salinity With Depth95
6.10 Light in the Ocean and Absorption of Light97
6.1 Important Concepts101

6 Temperature, Salinity, and Density 73

7.1 Dominant Forces for Ocean Dynamics103
7.2 Coordinate System104
7.3 Types of Flow in the ocean105
7.4 Conservation of Mass and Salt106
7.5 The Total Derivative (D/Dt)107
7.6 Momentum Equation108
7.7 Conservation of Mass: The Continuity Equation1
7.8 Solutions to the Equations of Motion113
7.9 Important Concepts114

7 The Equations of Motion 103

8.1 The Influence of Viscosity115
8.2 Turbulence116
8.3 Calculation of Reynolds Stress:119
8.4 Mixing in the Ocean123
8.5 Stability127

CONTENTS v

9.1 Inertial Motion133
9.2 Ekman Layer at the Sea Surface135
9.3 Ekman Mass Transport143
9.4 Application of Ekman Theory145
9.5 Langmuir Circulation147
9.6 Important Concepts147

9 Response of the Upper Ocean to Winds 133

10.1 Hydrostatic Equilibrium151
10.2 Geostrophic Equations153
10.3 Surface Geostrophic Currents From Altimetry155
10.4 Geostrophic Currents From Hydrography158
10.5 An Example Using Hydrographic Data164
10.6 Comments on Geostrophic Currents164
10.7 Currents From Hydrographic Sections171
10.8 Lagrangian Measurements of Currents172
10.9 Eulerian Measurements179
10.10Important Concepts180

10 Geostrophic Currents 151

1.1 Sverdrup’s Theory of the Oceanic Circulation183
1.2 Western Boundary Currents189
1.3 Munk’s Solution190
1.4 Observed Surface Circulation in the Atlantic192
1.5 Important Concepts197

1 Wind Driven Ocean Circulation 183

12.1 Definitions of Vorticity199
12.2 Conservation of Vorticity202
12.3 Influence of Vorticity204
12.4 Vorticity and Ekman Pumping205
12.5 Important Concepts210

12 Vorticity in the Ocean 199

13.1 Defining the Deep Circulation211
13.2 Importance of the Deep Circulation212
13.3 Theory for the Deep Circulation219
13.4 Observations of the Deep Circulation2
13.5 Antarctic Circumpolar Current229
13.6 Important Concepts232

13 Deep Circulation in the Ocean 211

14.1 Equatorial Processes236
14.2 El Nino240
14.4 Observing El Nino250
14.5 Forecasting El Nino251
14.6 Important Concepts254

vi CONTENTS

15.1 Introduction–Some Words of Caution255
15.2 Numerical Models in Oceanography257
15.3 Global Ocean Models258
15.4 Coastal Models262
15.5 Assimilation Models266
15.6 Coupled Ocean and Atmosphere Models269
15.7 Important Concepts272

15 Numerical Models 255

16.1 Linear Theory of Ocean Surface Waves273
16.2 Nonlinear waves278
16.3 Waves and the Concept of a Wave Spectrum278
16.4 Ocean-Wave Spectra284
16.5 Wave Forecasting288
16.6 Measurement of Waves289
16.7 Important Concepts292
17.1 Shoaling Waves and Coastal Processes293
17.2 Tsunamis297
17.3 Storm Surges299
17.4 Theory of Ocean Tides300
17.5 Tidal Prediction308
17.6 Important Concepts312

17 Coastal Processes and Tides 293 References 313

Preface

This book is written for upper-division undergraduates and new graduate students in meteorology, ocean engineering, and oceanography. Because these students have a diverse background, I have emphasized ideas and concepts more than mathematical derivations.

Unlike most books, I am distributing this book for free in digital format via the world-wide web. I am doing this for two reasons:

1. Textbooks are usually out of date by the time they are published, usually a year or two after the author finishes writing the book. Randol Larson, writing in Syllabus, states: “In my opinion, technology textbooks are a waste of natural resources. They’re out of date the moment they are published. Because of their short shelf life, students don’t even want to hold on to them”—(Larson, 2002). By publishing in electronic form, I can make revisions every year, keeping the book current.

2. Many students, especially in less-developed countries cannot afford the high cost of textbooks from the developed world. This then is a gift from the US National Aeronautics and Space Administration nasa to the students of the world.

Acknowledgements

I have taught from the book for several years, and I thank the many students in my classes and throughout the world who have pointed out poorly written sections, ambiguous text, conflicting notation, and other errors. I also thank Professor Fred Schlemmer at Texas A&M Galveston who, after using the book for his classes, has provided extensive comments about the material.

I also wish to thank many colleagues for providing figures, comments, and helpful information. I especially wish to thank Aanderaa Instruments, Bill Allison, Kevin Bartlett, James Berger, Gerben de Boer, Daniel Bourgault, Don Chambers, Greg Crawford, Thierry De Mees, Richard Eanes, Peter Etnoyer, Tal Ezer, Gregg Foti, Nevin S. Fuckar, Luiz Alexandre de Araujo Guerra, Hazel Jenkins, Jody Klymak, Judith Lean, Christian LeProvost, Brooks Martner, Nikolai Maximenko, Kevin McKone, Mike McPhaden, Thierry De Mees, Pim van Meurs, Gary Mitchum, Joe Murtagh, Peter Niiler, Nuno Nunes, Ismael Nunez-Riboni, Alex Orsi, Kym Perkin, Mark Powell, Richard Ray, Joachim Ribbe, Will Sager, David Sandwell, Sea-Bird Electronics, Achim Stoessel, David vii viii PREFACE

Stooksbury, Tom Whitworth, Carl Wunsch and many others.

Of course, I accept responsibility for all mistakes in the book. Please send me your comments and suggestions for improvement.

Figures in the book came from many sources. I particularly wish to thank

Link Ji for many global maps, and colleagues at the University of Texas Center for Space Research. Don Johnson redrew many figures and turned sketches into figures. Trey Morris tagged the words used in the index.

I especially thank nasa’s Jet Propulsion Laboratory and the Topex/Poseidon and Jason Projects for their support of the book through contracts 960887 and 1205046.

Cover photograph of the resort island of Kurumba in North Male Atoll in the Maldives was taken by Jagdish Agara (copyright Corbis). Cover design is by Don Johnson.

The book was produced in LATEX2ε using TeXShop 2.14 on an Intel iMac computer running OS-X 10.4.1. I especially wish to thank Gerben Wierda for his very useful i-Installer package that made it all possible, and Richard Koch, Dirk Olmes and many others for writing the TeXShop software package. Their software is a pleasure to use. All figures were drawn in Adobe Illustrator.

Chapter 1 A Voyage of Discovery

The role of the ocean on weather and climate is often discussed in the news. Who has not heard of El Nino and changing weather patterns, the Atlantic hurricane season and storm surges? Yet, what exactly is the role of the ocean? And, why do we care?

1.1 Why study the Physics of the ocean?

The answer depends on our interests, which devolve from our use of the ocean. Three broad themes are important:

1. We get food from the ocean. Hence we may be interested in processes which influence the sea just as farmers are interested in the weather and climate. The ocean not only has weather such as temperature changes and currents, but the oceanic weather fertilizes the sea. The atmospheric weather seldom fertilizes fields except for the small amount of nitrogen fixed by lightning.

2. We use the ocean. We build structures on the shore or just offshore. We use the ocean for transport. We obtain oil and gas below the ocean. And, we use the ocean for recreation, swimming, boating, fishing, surfing, and diving. Hence we are interested in processes that influence these activities, especially waves, winds, currents, and temperature.

3. The ocean influence the atmospheric weather and climate. The ocean influence the distribution of rainfall, droughts, floods, regional climate, and the development of storms, hurricanes, and typhoons. Hence we are interested in air-sea interactions, especially the fluxes of heat and water across the sea surface, the transport of heat by the ocean, and the influence of the ocean on climate and weather patterns.

These themes influence our selection of topics to study. The topics then determine what we measure, how the measurements are made, and the geographic areas of interest. Some processes are local, such as the breaking of waves on a beach, some are regional, such as the influence of the North Pacific on Alaskan

2 CHAPTER 1. A VOYAGE OF DISCOVERY weather, and some are global, such as the influence of the ocean on changing climate and global warming.

If indeed, these reasons for the study of the ocean are important, lets begin a voyage of discovery. Any voyage needs a destination. What is ours?

At the most basic level, I hope you, the students who are reading this text, will become aware of some of the major conceptual schemes (or theories) that form the foundation of physical oceanography, how they were arrived at, and why they are widely accepted, how oceanographers achieve order out of a random ocean, and the role of experiment in oceanography (to paraphrase Shamos, 1995: p. 89).

More particularly, I expect you will be able to describe physical processes influencing the ocean and coastal regions: the interaction of the ocean with the atmosphere, and the distribution of oceanic winds, currents, heat fluxes, and water masses. The text emphasizes ideas rather than mathematical techniques. I will try to answer such questions as:

1. What is the basis of our understanding of physics of the ocean?

(Parte 1 de 10)

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