Dynamics of the Tropical Atmosphere and Oceans
Advancing Weather and Climate Science Series

This book presents a unique and comprehensive view of the fundamental dynamical and thermodynamic principles underlying the large circulations of the coupled ocean-atmosphere system

Dynamics of The Tropical Atmosphere and Oceans provides a detailed description of macroscale tropical circulation systems such as the monsoon, the Hadley and Walker Circulations, El Niño, and the tropical ocean warm pool. These macroscale circulations interact with a myriad of higher frequency systems, ranging from convective cloud systems to migrating equatorial waves that attend the low-frequency background flow. Towards understanding and predicting these circulation systems.

A comprehensive overview of the dynamics and thermodynamics of large-scale tropical atmosphere and oceans is presented using both a ?reductionist? and ?holistic? perspectives of the coupled tropical system. The reductionist perspective provides a detailed description of the individual elements of the ocean and atmospheric circulations. The physical nature of each component of the tropical circulation such as the Hadley and Walker circulations, the monsoon, the incursion of extratropical phenomena into the tropics, precipitation distributions, equatorial waves and disturbances described in detail. The holistic perspective provides a physical description of how the collection of the individual components produces the observed tropical weather and climate. How the collective tropical processes determine the tropical circulation and their role in global weather and climate is provided in a series of overlapping theoretical and modelling constructs.

The structure of the book follows a graduated framework. Following a detailed description of tropical phenomenology, the reader is introduced to dynamical and thermodynamical constraints that guide the planetary climate and establish a critical role for the tropics. Equatorial wave theory is developed for simple and complex background flows, including the critical role played by moist processes. The manner in which the tropics and the extratropics interact is then described, followed by a discussion of the physics behind the subtropical and near-equatorial precipitation including arid regions. The El Niño phenomena and the monsoon circulations are discussed, including their covariance and predictability. Finally, the changing structure of the tropics is discussed in terms of the extent of the tropical ocean warm pool and its relationship to the intensity of global convection and climate change.

Dynamics of the Tropical Atmosphere and Oceans is aimed at advanced undergraduate and early career graduate students. It also serves as an excellent general reference book for scientists interested in tropical circulations and their relationship with the broader climate system.

Preface xvii

Acknowledgments xix

Abbreviations xxiii

1 Climatology of the Tropical Atmosphere and Upper Ocean 1

1.1 The Growth of Tropical Meteorology 1

1.2 Seasonal Characteristics 4

1.3 Macro-Scale Circulations 16

1.4 A Myriad of Variability 24

Notes 34

2 Hydrological and Heat Exchange Processes 37

2.1 Water on Earth 38

2.2 Thermodynamics of Water and Earth’s Climate 39

2.3 Water and the Tropical System 43

2.4 Buoyancy, Differential Buoyancy, and the Generation of Horizontal Body Forces 50

2.5 Integrated Column Heating 53

2.6 Buoyancy in the Tropical Ocean 57

2.8 Convection–SST Relationships and the Vertical Scale of Tropical Motions 68

2.9 Coupled Global Ocean–Atmosphere Synergies 70

2.10 Synthesis 73

Notes 73

3 Fundamental Processes 77

3.1 Some Fundamentals of Low-Latitude Atmospheric Dynamics 79

3.2 Dynamics of the Low-Latitude Upper Ocean 91

Notes 104

4 Kinematics of Equatorial Waves 107

4.1 Phase and Group Velocities, and Energy Propagation 107

4.2 Dispersive and Non-dispersive Waves 111

4.3 Overview 112

Notes 113

5 Fundamental Prototypes of Tropical Systems 115

5.1 The Laplace Shallow Fluid System 115

5.2 Upper Ocean 118

5.3 A Stratified Atmospheric Model 119

5.4 Forced and Free Solutions and the Choice of H 121

5.5 Some Remarks 123

Notes 123

6 Equatorial Waves in Simple Flows 125

6.1 Atmospheric Modes in a Constant Basic State: Constant U 125

6.2 Atmospheric Waves in Latitudinal Shear Flow: Ū = Ū(y) 144

6.3 Physics of Equatorial Trapping 146

6.4 Large-Scale Low-Latitude Ocean Modes 151

6.5 Overview 157

Notes 158

7 Waves in Longitudinally and Vertically Varying Flows 159

7.1 Horizontal and Vertical Coupling of Equatorial Modes 160

7.2 Coupled Free and Forced Solutions of the Vertical Structure Equation 163

7.3 Wave Characteristics in a Zonally Varying Basic State Ū = Ū(x) 169

7.4 Numerical Substantiation of the Analytic Ray-Tracing Results 176

7.5 Zonally Varying Basic State and the “Longwave Approximation” 181

7.6 Vertical Trapping, Accumulation, and Lateral Emanation 182

7.7 Quasi-Biennial Oscillation (QBO) 183

Notes 184

8 Moist Processes and Large-Scale Tropical Dynamics 185

8.1 Convection and Large-Scale Budgets 186

8.2 Emerging Perspective on Tropical Convection 188

8.3 Comparison of Observed Waves and Waves from Theory 190

8.4 Dry and Moist Modes in the Tropics 191

8.5 Processes 193

8.6 Synthesis 201

Notes 203

9 Extratropical Influence on the Tropics 205

9.1 Lateral Wave Propagation in a Zonally Symmetric Basic State 205

9.2 Equatorial Wave Propagation in a Zonally Varying Basic State 208

9.3 Equatorward Wave Propagation in a Three-Dimensional Basic State 214

9.4 Overview 221

Notes 221

10 Tropical Influence on the Extratropics: A Zonally Averaged Perspective 223

10.1 Axisymmetric Meridional Circulation Models 223

10.2 Zonally Averaged Perspective of Meridional Circulations 225

10.3 Perspective 230

Notes 230

11 A Tropical–Extratropical Synergy 231

11.1 Mean and Transient Potential Vorticity on the 370 K Isentrope 231

11.2 Impermeability 234

11.3 Shallow Fluid Experiments 237

11.4 Recursively Breaking Rossby Waves 240

11.5 Conclusions 241

Notes 243

12 Arid and Desert Climates 245

12.1 Dynamics of Deserts 245

12.2 Radiative and Surface Fluxes 248

12.3 Diurnal Cycle of Divergence 250

12.4 Tropospheric Energy Balance 251

12.5 Nocturnal Stabilization of the Boundary Layer 251

12.6 Desert–Monsoon Relationships 255

Notes 256

13 Near-Equatorial Precipitation 257

13.1 Near-Equatorial Distributions of Precipitation 258

13.2 Dynamic Instabilities Associated with a Cross-Equatorial Pressure Gradient 262

13.3 Transient States of the Intertropical Convergence Zone 280

13.4 The Great Cloud Bands 290

13.5 Some Conclusions 298

Notes 299

14 Large-Scale, Low-Frequency Coupled Ocean–Atmosphere Systems 301

14.1 The Walker Circulation 302

14.2 The Southern Oscillation, El Niño and La Niña 305

14.3 Indian Ocean Interannual Oscillations 332

Notes 342

15 Intraseasonal Variability in the Tropical Atmosphere 345

15.1 Introduction 345

15.2 Structure of the Austral Summer ISV 345

15.3 Variability of Austral Summer ISVs 348

15.4 Mechanisms 351

15.5 Conclusions 358

Notes 358

16 Dynamics of the Large-Scale Monsoon 361

16.1 Overview 361

16.2 Theories of the Monsoon and Its Variability 364

16.3 Macroscale Structure of the Summer Monsoon 374

16.4 Macroscale Structure of the Winter Monsoon 388

16.5 Subseasonal Summer Monsoon Variability 391

16.6 Higher-Frequency Monsoon Variability 400

16.7 Some Comments 405

Notes 405

17 The Coupled Monsoon System 407

17.1 Coupled Characteristics of the Indian Ocean Region 407

17.2 Processes Determining the Indian Ocean SST 411

17.3 Do Ocean Heat Fluxes Regulate the Annual Cycle of the Monsoon? 415

17.4 Variability Within the Coupled Monsoon System 416

17.5 An Holistic View of the Monsoon System 421

Notes 428

18 The Changing Tropics 429

18.1 Tropical Warm Pool 429

18.2 Circulation Changes 438

18.3 Summary and Conclusions 442

Notes 444

19 Some Concluding Remarks 445

Notes 447

Appendix A Thermal Wind Relationship 449

Appendix B Stokes’ Theorem 451

Appendix C Dry and Moist Thermodynamical Stability 453

Appendix D Derivation of the Wave Equation (5.11) 455

Appendix E Conservation of Potential Vorticity of Shallow Water System 457

Appendix F Solutions to the Vertical Structure Equation for a Constant Lapse Rate Atmosphere 459

Appendix G Nonlinear Numerical Model 461

Appendix H Derivation of the Potential Vorticity Equation on an Extratropical β-Plane 463

Appendix I Derivation of the Barotropic Potential Vorticity Equation (13.25) with Friction and Heating 465

Appendix J Steady State Model of the Tropics 467

Appendix K Intermediate Ocean Model 469

References 471

Index 493

Peter J. Webster, PhD, is Professor Emeritus in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology.