An Introduction to the Meteorology and Climate of the Tropics

Jim Galvin has had a long interest in tropical meteorology. He is a Senior Operational Meteorologist in the Met Office World Area Forecast Centre in Exeter, UK and also produces the Shipping Forecast. In 2014, he became Editor of the RMetS journal Weather. He enjoys photography and singing in a choir.

About the Author xi

Preface xii

Acknowledgements xiii

1 Setting the Scene 1

1.1 Introduction 1

1.2 What do we mean by ‘the tropics’? 2

1.3 The geography of the tropics 5

1.4 The tropical troposphere 5

1.5 Climate and population in the tropics 7

1.6 Question 8

2 The Energy Balance and the Dynamics of Weather in the Tropics 9

2.1 The tropical ‘heat engine’ 9

2.2 Absorption, reflection and apparent solar elevation 9

2.3 Emission from the surface 11

2.4 The radiation balance and the tropical zone 12

2.5 The dynamics of weather systems in the tropics 14

2.6 Questions 15

3 Winds, Temperature and Weather in the Tropical Zone 16

3.1 Winds 16

3.2 Temperature 20

3.3 The weather patterns and climates of the tropics 21

3.4 Clouds and fog in the tropics 23

3.5 Questions 28

4 The Subtropical Jet Streams 29

4.1 The formation of jet streams at the margins of the tropics 29

4.2 Weather associated with the subtropical jet stream 31

4.3 Folds and bifurcations in the flow 33

4.4 Clear‐air turbulence 34

4.5 Questions 36

5 Synoptic‐scale Weather Systems 38

5.1 Introduction 38

5.2 Convection in the tropics 38

5.3 The inter‐tropical convergence zone 42

5.4 The depth of convective clouds 47

5.5 Layer clouds and shallow convection 48

5.6 The effects of heavy rainfall in the tropics 48

5.7 Atmospheric teleconnections 49

5.8 Questions 49

6 Climate, Flora and Fauna 51

6.1 The relationship of climate to plants and animals 51

6.2 Tropical rainforest 51

6.3 Seasonal tropical forest 57

6.4 The savannas 58

6.5 Tropical deserts and scrublands 60

6.6 Mountain climates 61

6.7 Tropical oceans and coasts 65

6.8 Climatic variability 67

6.9 Questions 68

7 Dry Environments 69

7.1 Background 69

7.2 Wind and weather in the deserts 70

7.3 Fog and low cloud 78

7.4 Severe weather in the dry tropics 79

7.5 The effects of desert weather 82

7.6 Settlement and the over‐use of scarce water supplies 84

7.7 Questions 85

8 Monsoons 86

8.1 Introduction 86

8.2 The summer monsoon over southern Asia 86

8.3 The summer monsoon over East Asia 89

8.4 Variations of rainfall in the Asian summer monsoon 93

8.5 The Asian winter monsoon 94

8.6 The West African summer monsoon 96

8.7 The West African winter monsoon 103

8.8 Rainfall and the monsoons in East Africa 104

8.9 The South American–Caribbean ‘monsoons’ 106

8.10 The Australian summer monsoon 107

8.11 Variable broad‐scale factors affecting the monsoons 111

8.12 Questions 112

9 Tropical Revolving Storms 114

9.1 Broad‐scale convection and the development of tropical storms 114

9.2 Tropical storm development and decline 114

9.3 The effects of tropical revolving storms 120

9.4 Storm tracks in the Pacific Ocean 124

9.5 The formation and tracks of hurricanes in the North Atlantic–Caribbean 125

9.6 Tropical cyclones in the Indian Ocean 126

9.7 Tropical revolving storms in the south‐west Pacific 127

9.8 Variability in the development of tropical storms 128

9.9 Extra‐tropical transition 132

9.10 Conclusion 132

9.11 Questions 132

10 Mesoscale Weather Systems 134

10.1 Introduction 134

10.2 Mesoscale convective complexes 134

10.3 Sea‐ and land‐breeze convergence zones 135

10.4 Easterly waves and squall lines 139

10.5 Mesoscale convective systems in northern India 143

10.6 Depressions in north‐west India, north Pakistan and Afghanistan 144

10.7 Cross‐equatorial flows 144

10.8 Mesoscale convective systems in the Gulf of Guinea 145

10.9 Local convection 145

10.10 Extra‐tropical interaction with moist tropical air masses 148

10.11 Conclusion 148

10.12 Questions 149

11 Forecasting Clouds and Weather 150

11.1 Background 150

11.2 Distribution of significant cloud 150

11.3 The effect of high ground as an elevated heat source 155

11.4 Tropical upper‐tropospheric troughs 157

11.5 Effects of severe convection on aviation 159

11.6 Questions 162

12 The Variability of Weather and Climate Change in the Tropics 164

12.1 Introduction 164

12.2 El Niño–La Niña 164

12.3 The Madden–Julian Oscillation 166

12.4 The quasi‐biennial oscillation 168

12.5 A discussion of anthropogenic climate change 171

12.6 How is climate likely to change in the tropical zone? 172

12.7 Modelling climate change 175

12.8 Conclusion 176

12.9 Questions 176

13 Tropical Agriculture 177

13.1 Agricultural productivity and tropical environments 177

13.2 Agriculture in the humid tropics and the effects of forest clearance 178

13.3 Agriculture in the savannas 180

13.4 Dry‐land agriculture 180

13.5 Weather and locust swarms 182

13.6 The effects of agriculture in the tropics 183

13.7 Agriculture and climate change 185

13.8 Question 186

14 The Importance of the Tropical Ozone Layer 187

14.1 Background 187

14.2 The role of the tropics in replenishing the extra‐tropical stratospheric ozone 191

14.3 The effect of ‘global warming’ on stratospheric ozone destruction in the tropics 193

14.4 The effects of exposure to short‐wave radiation 194

14.5 Current state of the stratospheric ozone layer 195

14.6 Question 195

15 Remote Sensing of Tropical Weather 196

15.1 Background 196

15.2 Satellite remote sensing 196

15.3 Precipitation 197

15.4 Wind profilers 199

15.5 Thunderstorm observation 199

15.6 Monitoring surface cover, fires and volcanic eruptions 199

15.7 Question 202

16 Tropical Weather and Health 203

16.1 Introduction 203

16.2 The effects of tropical sunshine and warmth 203

16.3 Lifted dust and its effects on health 203

16.4 Industrial and smoke pollution 204

16.5 Parasitic and infectious diseases 204

16.6 Response of the meteorological community 206

16.7 Questions 206

17 Conclusions and a Look to the Future 207

17.1 A summary 207

17.2 Forecasting the weather 208

17.3 Questions 208

Appendix 1 Observations from the Tropics 209

Appendix 2 Named Winds of the Tropics 213

Appendix 3 An Introduction to Cloud Types, Cloud Species and Precipitation 234

A3.1 Introduction 234

A3.2 The high clouds 235

A3.3 The medium‐level clouds 235

A3.4 The low clouds 236

A3.5 Cloud species and varieties 239

Appendix 4 An Introduction to Meteorological Diagrams, Stability, Instability and Aviation Weather Charts 246

A4.1 Temperature–pressure graphs 246

A4.2 Stability and instability 248

A4.3 Aviation‐significant weather charts 248

Appendix 5 Snow in the Desert: A Case Study 250

A5.1 Introduction 250

A5.2 Development of a depression over the desert 250

A5.3 The weather features associated with the upper low 253

A5.4 Summary 256

Appendix 6 A Climatic Summary for Tropical Countries and States 257

Appendix 7 Two Easterly Waves in West Africa in Summer 2009: A Case Study 265

A7.1 Introduction 265

A7.2 The wave of 28 August to 5 September 2009 266

A7.3 The wave of 2–11 September 2009 and the development of Hurricane Fred 270

A7.4 A model for the development of squall lines in the Sahel 272

A7.5 Conclusion 277

References 280

Glossary 293

Index 303