The Climates of the Tropics, and How They Are Changing
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STATE OF THE TROPICS 39 Essay 1 The climates of the Tropics, and how they are changing Blair Trewin Bureau of Meterology Blair Trewin has been a climate scientist with the Australian Bureau of Meteorology since 1998. He is a member of the World Meteorological Organisation’s (WMO) Expert Team on Climate Change Detection and Indices, and was the scientific co-ordinator of WMO’s annual Statement on the Status of the Global Climate in 2010 and 2011. He was the 2012-13 President of the Australian Meteorological and Oceanographic Society and is the editor of the Australian Meteorological and Oceanographic Journal. The climates of the Tropics, and how they are changing Blair Trewin Many features combine to make up the diverse this is a relatively narrow definition; on most more common to speak of seasons in terms of the climates of the Tropics. It is a warm region; continents, the poleward boundary of the zone monsoon, “warm” and “cool” or “wet” and “dry” mean annual temperatures exceed 20°C almost thus defined is some distance within the Tropics seasons. Outside the immediate proximity of the throughout the Tropics, except at high elevations, of Cancer and Capricorn, even at sea level. Such Equator, the coolest month is usually in “winter” and exceed 25°C in many parts of the tropical a definition also excludes high-altitude areas (most often January in the Northern Hemisphere zone. The tropical zone encompasses some of the within the Tropics, as mean temperatures typically and July in the Southern Hemisphere), but the wettest locations on Earth, as well as some of the decrease at a rate of about 6°C per 1000 metres. hottest month is not necessarily in “summer”. In world’s driest deserts. It also includes some of the continental regions where there is a sharp, well- world’s communities most vulnerable to natural A small annual range of temperature is defined wet season – the African Sahel, much of disasters; population pressures drive settlement of characteristic of the Tropics. Near the Equator, the Indian subcontinent, and to a lesser extent flood- or drought-prone areas, and less developed the difference in mean temperature between northern Australia – the hottest weather occurs countries lack the resources to create resilience to the warmest and coldest months is rarely more in the weeks prior to the onset of the wet season, extremes of climate. Limitations in physical and than 4°C, and at some locations (such as Quito, sometimes with a smaller secondary peak at the human infrastructure also contribute to limited Ecuador) it is less than 1°C. Annual ranges of end of the wet season (see Figure E1.2). capacity to warn of, or respond to, major disasters, temperature become larger as one moves further although this is an area where great strides have from the Equator, but even at the limits of the The equatorial zone is also characterised by been made in many countries in recent decades. tropical zone, few locations have an annual range consistent warmth and very small day-to-day of more than 15°C. temperature fluctuations; for example, at Singapore, the highest temperature on record What makes up a tropical climate? This low annual range of temperature makes is 36°C, and the lowest 18.9°C. Many locations traditional mid-latitude definitions of seasons of within 15° latitude of the Equator, especially on The classical definition of a tropical climate is one limited value; while it is possible to define “winter” islands or near the coast, have never reached 40°C in which the mean temperature of the coolest and “summer” in any location that is not actually and have highest recorded temperatures less month exceeds 18°C (see Figure E1.1), although on the Equator, in many tropical locations, it is than 5°C above the average daytime maximum temperature of the warmest month. This is in part due to island and coastal temperatures being largely controlled by ocean temperature, although Figure E1.2 Mean monthly rainfall and temperature at Bamako, Mali. in equatorial regions, even continental interiors (such as the Amazon basin) rarely, if ever, reach Rainfall (mm) Temperature (deg C) 40°C. Extreme temperatures are higher further from the Equator, although they still fall short of 350 35 the world’s highest extremes, most of which have occurred slightly outside the Tropics (including 300 30 most of the instances of temperatures measured under standard conditions of 50°C or above). 250 25 While tropical climates are characterised by 200 20 small variations in temperature, they include an immense variety of rainfall regimes. The Tropics 150 15 include the driest parts of the world, the Atacama Desert on the west coast of South America where 100 10 mean annual rainfall is less than 1 millimetre, as well as some of the wettest, locations in western 50 5 Colombia and in Hawaii which receive more than 10,000 millimetres a year. 0 0 JanFeb MarApr MayJun JulAug SepOct NovDec The classical model of tropical rainfall is one of Source: WMO (2012) an equatorial zone where rain occurs all year, Note: Blue bars are montly rainfall averages and the red line is mean temperature. transitioning to a “monsoon” or “savanna” 40 SECTION 2 THE ECOSYSTEM ESSAY 1 THE CLIMATES OF THE TROPICS STATE OF THE TROPICS 41 Figure E 1.1 Map of the major global climatic zones A B C Source: Chen & Chen 2013 Note: Major global climatic zones include tropical zones (blue, type A), according to the Köppen climatic classification. In this classification tropical zones are defined by the mean temperature of the coolest month, and by rainfall (A: Tropical, B: Dry, C Mild temperate, D: Snow, E: Polar). WHO/ UNICEF (2012). Figure E1.3 Mean monthly rainfall and temperature at four near-equatorial locations: (A) Singapore, Singapore; (B) Manaus, Brazil; (C) Nairobi, Kenya and (D) Lambarene, Gabon. Rainfall (mm) Temperature (deg C) Rainfall (mm) Temperature (deg C) 350 35 350 35 300 30 300 30 250 25 250 25 200 20 200 20 150 15 150 15 100 10 100 10 50 5 50 5 0 0 0 0 JanFeb MarApr MayJun JulAug SepOct NovDec JanFeb MarApr MayJun JulAug SepOct NovDec Source: WMO (2012) Note: Blue bars are montly rainfall averages and the red line is mean temperature. climate with a well-defined wet and dry season especially on the north side of the Equator. Some receive their heaviest rain when the monsoon (in summer and winter respectively), with the wet examples of rainfall regimes in near-equatorial retreats in autumn and the trade winds become season becoming shorter with increasing latitude locations are shown in Figure E1.3. re-established. Examples include the central coast before it disappears altogether as one moves into of Vietnam and the east coasts of southern India the arid zone near the Tropics. This structure exists Moving away from the Equator and into the 10- and Sri Lanka (all of which have their peak rainfall most clearly in west and central Africa north of the 20° latitude range, climates where there is a single, in October and November), and parts of coastal Equator, and in the Indonesian/Australian region. clear-cut wet season, most commonly in that Brazil whose rainfall peaks between April and June. hemisphere’s summer, become the norm. In some Figure E1.4 illustrates some climates of this type. Reality is a good deal more complex than that, but parts of the world, such as west and central Africa strong seasonal variations of rainfall are a common (both north and south) and much of northern Island climates can be more complex still, with feature of tropical climates. Even near the Equator, Australia, almost all the year’s rain falls within substantial changes in the seasonal regime over a wide range of rainfall regimes exist. Regions four or five months with the remainder of the year short distances depending on aspect. In the such as the equatorial parts of the Indonesian being almost completely dry (e.g. Figure E1.2). southwest Pacific, windward southeast sides of archipelago, parts of equatorial Africa, and Elsewhere, especially within reasonable proximity islands receive significantly more rainfall than western Colombia are wet all year, but some other of east coasts (e.g. southern coastal China, the leeward sides during the “dry” season, and thus equatorial regions have a distinct drier season – east coasts of Africa and South America south of have a more even spread of rainfall than leeward sometimes relatively but not totally dry (as with the Equator, large parts of India, and much of the sides or island interiors. On the larger islands the the mid-year period over much of the Amazon), Queensland coast in Australia), rain can occur heating of land and convergence of sea breezes but in some regions, the near-total seasonal in all months of the year but there is still a clear during the warmest months of the year causes the dryness of the savanna zone extends all the way distinction between the wetter and drier months. development of convective cloud and showers in to the Equator, as in western Africa. Highland A variation on this theme comes in some parts of the late afternoon. East Africa has a more complex pattern still, with the world where coasts are at right angles to the two rainy seasons (March-May and October- trade winds; many of these locations are relatively At the outer limits of the Tropics, in areas November), and dry conditions in between, rainshadowed during the summer monsoon and sufficiently far from east coasts, the wet 42 SECTION 2 THE ECOSYSTEM ESSAY 1 THE CLIMATES OF THE TROPICS STATE OF THE TROPICS 43 Figure E1.3 Mean monthly rainfall and temperature at four near-equatorial locations: (A) Singapore, Singapore; (B) Manaus, Brazil; (C) Nairobi, Kenya and (D) Lambarene, Gabon.