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Dynamics of the Tropical Atmosphere and Oceans. Edition No. 1. Advancing Weather and Climate Science

  • ID: 5226804
  • Book
  • April 2020
  • 536 Pages
  • John Wiley and Sons Ltd

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.

Note: Product cover images may vary from those shown

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

1.2.1 Zonal Variability 5

1.2.1.1 Sea–Surface Temperature 5

1.2.1.2 Temperature and Humidity 5

1.2.1.3 Precipitation 6

1.2.1.4 Wind Fields 7

1.2.2 Spatial Variability in the Tropics 8

1.2.2.1 Surface Temperature 8

1.2.2.2 Precipitation 9

1.2.2.3 Surface Pressure 11

1.2.2.4 Wind Fields 12

1.2.2.5 Moisture Flux 13

1.2.3 Variability Along the Equator 14

1.2.3.1 Temperature and Moisture 14

1.2.3.2 Wind Fields 15

1.3 Macro-Scale Circulations 16

1.3.1 Hadley’s Circulation 16

1.3.2 Walker’s Circulation 17

1.3.3 Monsoon Circulations 19

1.3.3.1 Asian–Australasian Monsoons 20

1.3.3.2 Monsoons of the Americas 22

1.3.3.3 African Monsoon 23

1.3.4 Large-Scale Characteristics of Tropical Oceans 23

1.4 A Myriad of Variability 24

1.4.1 High-Frequency Variability 24

1.4.1.1 Waves in the Easterlies 25

1.4.1.2 Tropical Cyclones and Monsoon Depressions 26

1.4.1.3 The “Great Cloud Bands” 29

1.4.1.4 Mesoscale Convective Systems 29

1.4.2 Subseasonal Variability 29

1.4.3 Interannual Variability 31

1.4.3.1 El Niño and the Southern Oscillation 31

1.4.3.2 Atlantic Oscillations 32

1.4.3.3 Stratospheric Oscillations 33

1.4.4 Overlapping of Variance Bands: Waves Within Waves 33

Notes 34

2 Hydrological and Heat Exchange Processes 37

2.1 Water on Earth 38

2.1.1 An Inventory 38

2.1.2 Global Disposition of Rainfall 39

2.2 Thermodynamics of Water and Earth’s Climate 39

2.2.1 Implications of Clausius–Clapeyron 40

2.2.2 Role of Water in the Evolution of Earth’s Climate 41

2.2.3 Estimate of the Planetary Radiative Surface Temperature 42

2.3 Water and the Tropical System 43

2.3.1 Atmosphere 43

2.3.1.1 Clausius–Clapeyron and the Vertical Profiles of Temperature and Humidity 43

2.3.1.2 Distribution of Water Vapor and Liquid/Ice Water 44

2.3.1.3 Moisture, Lapse Rates and Gradients of Atmospheric Buoyancy 45

2.3.2 Ocean 46

2.3.2.1 Ocean Surface Layer: Warm and Fresh 49

2.3.2.2 Abyssal Water: Cold and Saline 49

2.3.2.3 The Thermocline 50

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

2.4.1 Concept of Buoyancy 50

2.4.2 Zonal Variability of Buoyancy Induced by Radiative Forcing 51

2.4.3 Poleward Heat Transport 51

2.5 Integrated Column Heating 53

2.5.1 Components of Total Heating 53

2.5.2 Latitudinal Distribution of Latent Heat Flux and Condensational Heating 54

2.5.3 Latitudinal Distributions of Total Columnar Heating 55

2.5.4 Longitudinal Disposition of Total Columnar Heating 56

2.5.5 Annual Cycle of Total Columnar Heating 56

2.6 Buoyancy in the Tropical Ocean 57

2.6.1 Net Heating of the Upper Ocean 58

2.6.2 Fresh Water Flux into the Tropical Ocean 59

2.6.3 Distribution of Ocean FB Buoyancy Flux 59

2.6.4 Observations of Ocean–Atmosphere Fluxes in the Tropics 61

2.6.4.1 Western Pacific Ocean Circulation Experiment (WEPOCS) 61

2.6.4.2 Surface Fluxes in the Bay of Bengal During JASMINE 63

2.7 Translations to the Broader Scale 66

2.7.1 Large-Scale Columnar Heating Gradients 66

2.7.2 Upper-Ocean Heating 68

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

2.9 Coupled Global Ocean–Atmosphere Synergies 70

2.9.1 The Notion of Interactive Zones 70

2.9.2 A Stable Global Interactive System 71

2.9.2.1 The Tropical Circulation 71

2.9.2.2 State of the Stratosphere 72

2.9.2.3 The Return Atmospheric Flow Between the Tropics and the Poles 72

2.9.2.4 The Polar Ocean–Atmosphere Interface and the Formation of Deep Water 72

2.9.2.5 The Return Ocean Flow Between the Poles and the Equator 73

2.9.2.6 Maintenance of the Warm Pool 73

2.10 Synthesis 73

Notes 73

3 Fundamental Processes 77

3.1 Some Fundamentals of Low-Latitude Atmospheric Dynamics 79

3.1.1 Basic Equations 79

3.1.2 Scaling Atmospheric Motions in the Tropics 80

3.1.2.1 Is the Tropical Atmosphere Hydrostatic? 81

3.1.2.2 A Consequence of Making the Hydrostatic Assumption: Total Kinetic Energy and the “Traditional Approximation” 82

3.1.2.3 Scaling Thermodynamic Variability in the Tropics 83

3.1.3 Early Interpretations 84

3.1.4 Conundrums 86

3.1.4.1 Hypothesis I: Tropical Convection is Driven by Extratropical Forcing 86

3.1.4.2 Hypothesis II: Convection Occurs Because of Reduced Static Stability in Regions of Convection 86

3.1.5 Geostrophic Adjustment in the Low-Latitude Atmosphere 87

3.1.5.1 Rossby Radius of Deformation (R) 87

3.1.5.2 Inertial Motion 88

3.1.5.3 Rotational or Buoyancy Waves? 89

3.1.5.4 Heating and Tropical Circulations 90

3.1.6 Overview 90

3.2 Dynamics of the Low-Latitude Upper Ocean 91

3.2.1 Scales of Motion 91

3.2.2 Geostrophic Adjustment in the Low-Latitude Ocean 93

3.2.3 Sverdrup Wind-Driven Transport 95

3.2.4 Ekman Transports 96

3.2.4.1 Formulation 96

3.2.4.2 Why is the Total Integrated Ekman Transport Orthogonal to the Surface Wind? 97

3.2.5 Induced Geostrophic Currents 98

3.2.6 Low-Latitude Wind-Driven Currents 99

3.2.6.1 Global Wind-Stress Fields and Surface Current Climatology 99

3.2.6.2 Geostrophic Currents 100

3.2.6.3 Currents and Counter-Currents 101

3.2.6.4 Equatorial Undercurrents 101

3.2.7 Overview 103

Notes 104

4 Kinematics of Equatorial Waves 107

4.1 Phase and Group Velocities, and Energy Propagation 107

4.1.1 Wave Characteristics in a Quiescent Basic State 107

4.1.1.1 Golf Ball in a Pond 107

4.1.1.2 Analysis of the Perturbation in the Pond 108

4.1.2 Kinematic Relationships Between Waves and Their Background Basic State 109

4.1.2.1 General Wave Kinematics in a Variable Basic Flow 110

4.1.2.2 Dispersion of Energy Away from a Source 111

4.1.2.3 Dispersion in a Quiescent or Constant Basic State 111

4.1.2.4 Constant Basic State 111

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

5.1.1 Governing Equations 115

5.1.1.1 Use the Equatorial 𝛽-Plane Approximation 115

5.1.1.2 Define Total Depth of the Fluid and the Background Basic State 116

5.1.1.3 Integrate the System in the Vertical 116

5.1.1.4 Linearization of the System 116

5.1.2 Doppler and Non-Doppler Effects 117

5.1.3 Equatorial Wave Equation 117

5.2 Upper Ocean 118

5.2.1 Governing Equations 118

5.2.2 Ocean Wave Equation 119

5.3 A Stratified Atmospheric Model 119

5.3.1 Separation of Variables 120

5.3.2 Basic Equations 120

5.3.3 Coupled Horizontal and Vertical Structure Equations 120

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 Ū 125

6.1.1 Governing Equations for a Motionless Basic State (Ū =0) 125

6.1.2 Governing Equations for a Constant Basic State (Ū =Constant) 125

6.1.3 General Dispersion Relationship for a Constant Basic State (Ū =Constant) 126

6.1.4 Eigenfrequencies for a Constant Basic State 127

6.1.4.1 Dispersion Diagrams 127

6.1.4.2 The Ubiquitous Nature of MRG and K Waves 128

6.1.4.3 Eigenfrequency Dependence on H 129

6.1.5 Eigensolutions 129

6.1.5.1 Equatorial Rossby Waves (ER) 129

6.1.5.2 Inertia-Gravity Waves (WIG, IG) 137

6.1.5.3 Mixed Rossby-Gravity Wave (MRG) 139

6.1.5.4 The Kelvin Wave 141

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

6.2.1 Regions of Shear in the Tropics 144

6.2.2 Impacts of Latitudinal Shear 145

6.2.2.1 Rossby Waves in Shear Flow 146

6.2.2.2 Mixed Rossby-Gravity Wave in Shear Flow 146

6.3 Physics of Equatorial Trapping 146

6.3.1 Simple Potential Vorticity Arguments 147

6.3.2 Induced Relative Vorticity in Simple Basic States 148

6.3.2.1 Motionless Basic State Ū = 0 148

6.3.2.2 Constant Non-Zero Basic State (Ū =constant) 149

6.3.2.3 Shear Flow: Ū = Ū(y) 150

6.4 Large-Scale Low-Latitude Ocean Modes 151

6.4.1 Simple Model of the Upper Ocean – Geopotential Surfaces 151

6.4.2 Rotational Ocean Waves 152

6.4.3 Impact of Boundaries on Near-Equatorial Ocean Modes 153

6.4.4 The Longwave Approximation 156

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

7.1.1 Coupled Group Speeds 160

7.1.2 Coupled Dispersion Relationships and Group Speeds 160

7.1.2.1 Equatorial Rossby (ER) Waves 160

7.1.2.2 Mixed Rossby-Gravity (MRG) Wave 162

7.1.2.3 Kelvin Wave 163

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

7.2.1 Free Solutions 164

7.2.1.1 Isothermal Atmosphere 164

7.2.1.2 Constant Lapse Rate Atmosphere 166

7.2.1.3 Construction of Realistic Temperature Profiles 166

7.2.2 Forced Motions in an Isothermal Atmosphere 166

7.2.2.1 Case 1: External Solutions 168

7.2.2.2 Case 2: Oscillatory Solutions 168

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

7.3.1 Rays in the Longitudinal Plane 170

7.3.1.1 Rossby Wave Characteristics 170

7.3.1.2 MRG Wave Characteristics 172

7.3.2 Impact of Longitudinal Displacement of Wave Sources in a Zonally Varying Flow 173

7.3.2.1 Accumulating Modes 174

7.3.2.2 Anomalous Non-accumulating Propagating Mode 174

7.4 Numerical Substantiation of the Analytic Ray-Tracing Results 176

7.4.1 Equatorial Accumulation 176

7.4.2 Equatorial Emanation Regions to Higher Latitudes 177

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

8.3.1 Stratospheric Modes 190

8.3.2 Transient Tropospheric Modes 190

8.3.3 Stationary Modes in the Tropics 191

8.4 Dry and Moist Modes in the Tropics 191

8.5 Processes 193

8.5.1 Convective Dissipation 194

8.5.2 Stability and Convection 196

8.5.3 Surface Flux Feedbacks 197

8.5.4 Convective Instability of the Second Kind: CISK 198

8.5.5 Spatial Variation of the Basic State 199

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

9.2.1 Numerical Experiments in a Zonally Varying Basis State 210

9.2.1.1 Weak Equatorial Easterlies (Basic State A) 210

9.2.1.2 Weak Westerly Zone (Basic State B) 210

9.2.1.3 Strong Westerly Zone (Basic State C) 212

9.2.1.4 Latitudinal Distributions of PKE 212

9.2.2 Synthesis 212

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

9.3.1 Structure of the Mean Fields in the Upper and Lower Troposphere 214

9.3.2 Transient Behavior 216

9.3.3 One-Point Correlation Fields in the Horizontal Plane 217

9.3.4 One-Point Correlation Fields in the Vertical Plane 218

9.3.5 Impacts of Extratropical Wave Incursion into the Tropics 219

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

10.2.1 An Atmospheric Zonally Averaged Model 225

10.2.2 Observed Eddy Momentum and Heat Fluxes 226

10.2.3 Eddies and the Mean Circulation 227

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

11.3.1 Potential Vorticity Substance in a Shallow Fluid 237

11.3.2 Simulations with “Equatorial” and “Monsoon” Heating 238

11.3.3 Role of PVS Fluxes in Determining the Zonally Averaged Tropical Circulation 239

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

12.5.1 Development of a Nocturnal Desert Jet Stream 251

12.5.2 Dynamics of the Low-Level Nocturnal Jet 251

12.5.3 A Subsiding Lateral Exhaust 254

12.6 Desert–Monsoon Relationships 255

Notes 256

13 Near-Equatorial Precipitation 257

13.1 Near-Equatorial Distributions of Precipitation 258

13.1.1 Relationships Between Convection, MSLP, and SST 258

13.1.2 Theories of the Location of Near-Equatorial Convection 260

13.1.2.1 Collocation of Convection and Maximum SST 260

13.1.2.2 Locus of Near-Equatorial Disturbances 261

13.1.2.3 Zonally Symmetric Arguments 261

13.1.2.4 Dynamic–Thermodynamic Optimization and Feedbacks 261

13.1.2.5 Coupled Ocean–Atmosphere Explanations 262

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

13.2.1 Distributions of Absolute Vorticity 263

13.2.2 Geophysical Context for Inertial Instability 265

13.2.3 Concept of “Perpetual” Instability 266

13.2.4 Near-Equatorial Inertial Instability 268

13.2.5 Processes Determining the Distribution of Absolute Vorticity 270

13.2.6 Vertical and Latitudinal Structures 272

13.2.7 Is the Existence of a CEPG a Sufficient Condition for Inertial Instability? 273

13.2.8 Dynamic Estimate of the Latitude of the Mean ITCZ in Regions of Strong CEPG 275

13.2.9 Low-Level Near-Equatorial Westerlies in Regions of Strong CEPG 276

13.2.10 A Potential Vorticity View of Ameliorating Secondary Circulations 278

13.3 Transient States of the Intertropical Convergence Zone 280

13.3.1 Character of the Transients 281

13.3.2 Transient Composites 284

13.3.3 Diagnostics of ITCZ Transients 288

13.3.4 Origin of “Easterly Waves” 289

13.4 The Great Cloud Bands 290

13.4.1 Climatology of the GCBs 291

13.4.2 Variability within the GCBs 291

13.4.3 Theories of the Formation, Location, and Orientation of the GCBs 292

13.4.3.1 Anchoring 292

13.4.3.2 Orientation of the GCBs 292

13.4.3.3 Continental and Orographic Forcing 295

13.4.4 High-Frequency Variance in the GCBs 295

13.5 Some Conclusions 298

Notes 299

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

14.1 The Walker Circulation 302

14.1.1 Early Depictions 302

14.1.2 Nature of Zonal Circulations 303

14.1.3 Simple Model Simulations of Zonal Circulations 303

14.1.4 Role of an Interactive Ocean 304

14.2 The Southern Oscillation, El Niño and La Niña 305

14.2.1 Evolution 309

14.2.2 Annual Cycle of the Upper Pacific Ocean 309

14.2.3 Interannual Variability of the Annual Cycle 313

14.2.4 Large-Scale Signals of El Niño and La Niña 313

14.2.5 ENSO Theories 318

14.2.5.1 The “Bjerknes Hypothesis”: Positive Feedback Between the Ocean and the Atmosphere 319

14.2.5.2 ENSO Theories with Negative Feedbacks 320

14.2.6 Predictability of ENSO 326

14.2.6.1 Annual Cycle of Persistence and the Boreal Springtime “Persistence Barrier” 326

14.2.6.2 The Boreal Springtime “Predictability Barrier” 327

14.2.6.3 Real-Time Forecasts of ENSO Variability 328

14.2.6.4 Can Forecasts of ENSO Extrema and Their Impacts Be Improved? 331

14.3 Indian Ocean Interannual Oscillations 332

14.3.1 The 1961 Event 332

14.3.2 The 1997–1998 Event 333

14.3.3 Association of the IOD with ENSO 334

14.3.4 What Produced the 1997–1998 IOD Episode? 336

14.3.5 Composite Structure of the IOD 337

Notes 342

15 Intraseasonal Variability in the Tropical Atmosphere 345

15.1 Introduction 345

15.2 Structure of the Austral Summer ISV 345

15.2.1 Early Constructions 345

15.2.2 More Recent Analyses 346

15.3 Variability of Austral Summer ISVs 348

15.4 Mechanisms 351

15.4.1 A Local Instability Mechanism for the Initiation of an ISV Event 351

15.4.1.1 Destabilization Phase 353

15.4.1.2 A Convective Phase 355

15.4.1.3 Restoration Phase 355

15.4.2 The Indian Ocean as an ISV Generation Region 356

15.4.2.1 Feedbacks from Wave Dynamics 356

15.4.2.2 Extratropical Influence 357

15.4.2.3 Impact of Climatological State 358

15.5 Conclusions 358

Notes 358

16 Dynamics of the Large-Scale Monsoon 361

16.1 Overview 361

16.1.1 Slow Component (Months to Years) 362

16.1.2 Intermediate Component (Weeks to Months) 362

16.1.3 Faster Components (2–15 Days) 362

16.1.4 Connective Components (All Timescales) 364

16.2 Theories of the Monsoon and Its Variability 364

16.2.1 Early Descriptions 364

16.2.1.1 Sir Edmund Halley’s Tropical Wind Climatology 364

16.2.1.2 Halley’s Differential Buoyancy Hypothesis 365

16.2.1.3 Determining the Origin of Monsoon Flow 365

16.2.2 Attempts to Determine Remote Influences on the Monsoon 366

16.2.2.1 Walker’s Surmise 366

16.2.2.2 The Demise of the Walker Relationships: A Mid-century Conundrum 367

16.2.2.3 Relationships Revisited Using a Longer Data Series 367

16.2.3 Circulations Associated with Strong and Weak South Asian Monsoons 370

16.2.3.1 Identification of an Anomalous Monsoon 370

16.2.3.2 Impact of an Anomalous Monsoon on the Indian Ocean SST 372

16.2.3.3 Indian Ocean SST Anomalies and Monsoon Precipitation 372

16.2.3.4 Annual Cycle of Anomalous 850 and 200 hPa Winds 373

16.3 Macroscale Structure of the Summer Monsoon 374

16.3.1 Mean Seasonal PV Distributions 374

16.3.2 Annual Cycle of PV Fields 375

16.3.3 A Physical Basis for the Character of the Macroscale Monsoon 376

16.3.3.1 Anomalous Location of South Asian Monsoon Precipitation 376

16.3.3.2 Seasonal Distribution of Mean Upper Tropospheric Temperature and Specific Humidity 378

16.3.3.3 Mean Monthly Geopotential Sections 379

16.3.3.4 Comparison of Surface Heat Fluxes 380

16.3.3.5 Comparison of Vertical Temperature Profiles over the Gangetic Plains and the HTP 381

16.3.3.6 Precipitation over the HTP and the Evolution of the Elevated Surface Cyclonic Vortex 382

16.3.3.7 A Heating Threshold for a Subtropical Meridional Circulation 382

16.3.4 West African Summer Monsoon 386

16.4 Macroscale Structure of the Winter Monsoon 388

16.4.1 Siberian Cold Anticyclone 389

16.4.2 Limitations on Central Pressure 389

16.5 Subseasonal Summer Monsoon Variability 391

16.5.1 Identification of Propagating Intraseasonal Signals 391

16.5.2 Impacts of Monsoon Intraseasonal Oscillations (MISOs) 392

16.5.3 Inter-event Variability 393

16.5.4 Composite Structure of the MISO 394

16.5.5 Theories Regarding the Genesis and Maintenance of the MISO 396

16.5.5.1 External Forcing of the MISO 396

16.5.5.2 MISO as an Internal Mode: Self-Induction and Self-Regulation 397

16.5.5.3 Extensions of the Internal Mode Theory 398

16.5.5.4 Northward Propagation of the Boreal Summer MISO 398

16.6 Higher-Frequency Monsoon Variability 400

16.6.1 Summer Monsoon 400

16.6.2 Winter Monsoon 401

16.7 Some Comments 405

Notes 405

17 The Coupled Monsoon System 407

17.1 Coupled Characteristics of the Indian Ocean Region 407

17.1.1 Monsoonal Moisture Fluxes 408

17.1.2 Annual Cycle in the Indian Ocean Region 408

17.1.3 The Surface Flux–SST Tendency Paradox in the Indian Ocean 408

17.2 Processes Determining the Indian Ocean SST 411

17.2.1 Ocean Heat Transport 411

17.2.2 Changes in Ocean Heat Storage 413

17.2.3 Spatial and Temporal Variability of Ocean Heat Flux 413

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

17.3.1 Balanced Interhemispheric Heat Fluxes 415

17.3.2 Cross-Equatorial Ocean Ekman Heat Transport 415

17.4 Variability Within the Coupled Monsoon System 416

17.4.1 Biennial Variability 416

17.4.1.1 Ocean-Atmosphere Feedbacks 417

17.4.1.2 Pacific Warm Pool Seasonal Cycle Instability 418

17.4.1.3 Indian Ocean Feedbacks I, The Meehl Theory 418

17.4.1.4 Indian Ocean Feedbacks II: A Dynamic Ocean 419

17.4.1.5 ENSO and Internal Dynamics 419

17.4.2 Intraseasonal Variability in the Indian Ocean 421

17.5 An Holistic View of the Monsoon System 421

17.5.1 Indian Ocean Sector 422

17.5.2 Speculations on the Interaction of the Indian and Pacific Ocean Sectors 424

Notes 428

18 The Changing Tropics 429

18.1 Tropical Warm Pool 429

18.1.1 Changes in the Ocean Warm Pool During Last Century 429

18.1.2 The Mid-Twentieth Century SST Plateau 430

18.1.3 Longer-Term Changes in the Tropical SST 431

18.1.4 Relationship Between SST and Convection in the OWP 432

18.1.4.1 Surface Energy Balance Regulation 433

18.1.4.2 Cloud-Radiation Feedbacks 433

18.1.4.3 Ocean Feedbacks 433

18.1.5 SST and Column Integrated Heating (CIH) 433

18.1.6 Why is the Area of Organized Convection Relatively Constant? 435

18.2 Circulation Changes 438

18.2.1 Definition of a Broad-Scale Monsoon System 438

18.2.2 Variability and Trends of the Northern Hemisphere Monsoon System 439

18.2.3 Why Has the Northern Hemisphere Monsoon System Intensified? 441

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

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Peter J. Webster Georgia Institute of Technology.
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