The weather and climate of the tropics Part 1 – Setting the scene J. F. P. Galvin out within the tropical zone. What we know systems throughout the globe, providing – Weather September62, 9 No. 2007, Vol. of the weather (and, to some extent, the much of their energy. Met Office, Exeter climate) of the tropics remains limited and has typically focused on severe weather How can we define the In this series, I will describe various aspects events, such as tropical revolving storms tropics? of tropical weather and climate as it is (e.g. Emanuel, 2005), or data from a limited understood today. The series will cover range of observing stations. However, many The most commonly used definition of the these topics: the sub-tropical jet streams; factors of the day-to-day weather are impor- tropics is the zone within which the Sun is synoptic-scale weather systems; forecasting tant in the tropics, not least for aviation and directly overhead at some time during the significant cloud and associated weather; public safety. year, i.e. the zone between the tropics of dry environments; the development and For instance, the primary purposes of Cancer and Capricorn (23.45°N and 23.45°S, maintenance of monsoon systems; tropical forecasting for aircraft operations in the respectively). However, a more meteoro- revolving storms; and mesoscale convective tropics are safety and maximization of effi- logical definition, rather than the elevation complexes. Each part will include a descrip- ciency for the benefit of passengers and air- of the Sun at midday, is probably more tion of the effects of tropical weather. craft operators. The most accurate and useful to both the weather forecaster and This series is not intended to be a detailed appropriate forecasts will achieve this goal, the climatologist. mathematical description of tropical using a mixture of numerical weather pre- A simple method is to divide the globe weather. A number of very good texts are diction products, observed data and good into tropical and extra-tropical zones. This is available for those who wish to go into forecasting knowledge. It is the effects of a method often employed for the verifica- greater depth; in particular, I recommend the weather, in other words its outcomes, tion of numerical forecasts (WMO, 1982; Hastenrath (1991) and Riehl (1979). which must be considered. Fuller, 2004). One such method divides the Much of what we know about the The research carried out as part of the globe into two equal halves – tropics and weather has been focused on mid-latitude World Climate Research Programme since extra-tropics – with somewhat arbitrary weather systems: first, because most early the early 1970s is very important in allowing dividing lines at 30°N and 30°S. Con- researchers came from Western Europe and us to understand many of the processes and veniently, this latitude range includes eastern North America; and second, associated weather of the tropical zone almost all of the climatic zones that can be because of the risks and consequences of (Gates and Newson, 2006). Knowledge con- regarded as tropical: humid equatorial, weather systems prevalent in these zones. tinues to grow through more recent savannah, semi-desert scrub and hot desert However, although there are simple non- research programmes, such as TOGA, which (Figure 1). scientific descriptions of weather events investigates the important links between However, the zones of predominantly from the tropics going back hundreds of the tropical ocean and the global atmos- westerly winds make incursions equator- years, it is only since the late 1960s that phere (Fleming, 1986). It is clear that the ward of these lines of latitude, particularly in much scientific research has been carried tropics have an important effect on weather winter. In order to keep within a zone of Figure 1. Climatic zones of the tropics: Af, Am – tropical rainforest; Aw – savannah; BSh – tropical steppe; BWh – tropical and sub-tropical desert; Cs – dry-summer sub-tropical; H – highlands; Cf – warm temperate with no dry season; Cs – warm temperate with dry summer; Cw – warm temperate with dry winter; stippled– modification due to altitude. The effect of high ground has a profound influence on climate in the tropics, particularly above about 2km. (Using the system devised by Wladimir Koppen.) 245 predominantly easterly winds at most levels, Although use of the STJ as the northern Despite its relatively uniform depth for a narrower zone must be used. Where an and southern limits of the tropics means much of the year, some variation occurs automated system, such as the verification that the tropical zone extends north of 40°N with the changing of the seasons and these of numerical forecasts, is not required, there over Asia during the northern summer, it is variations are notably marked in the North- are better climatological or synoptic-mete- appropriate, since the air to the south of it ern Hemisphere summer. Between late May orological ways to define the tropics. retains tropical characteristics. To provide and late September, the tropopause is high- A more useful definition for the climatolo- consistency throughout this series of er over north Africa and south Asia than it is gist is based on the small annual variation of papers, some areas poleward of the mean close to the Equator. The intense warming climate typical in the tropics. Riehl (1979, latitude of the STJ will be discussed, since of these land masses causes the tropo- Ch. 2) proposed the definition as the area these areas spend part of the year within the sphere to expand and the tropopause occa- within which the diurnal temperature range meteorological tropics. This also allows sionally reaches a height of 18 km over exceeds the range of annual mean tem- inclusion of areas frequently affected by Tibet. Smaller expansions occur over The weather The and weather climate of the tropics perature. This has great value, since data can upper-tropospheric troughs in the STJ. Australia, Africa and South America during easily be sorted using this definition. How- the southern summer. The expansion is a ever, the weather forecaster usually needs The tropical troposphere key element in the development of the sum- something more closely related to the daily mer monsoon circulations (to be described weather, without reference to seasonal vari- The height of the tropopause varies little in in Part 6). ations. the tropics, but is related to the mean tem- Associated with the great depth of the Using current weather, it is possible to perature of the troposphere. Thus the tropi- tropical troposphere is a high total (1000– define the tropical zone on a daily basis. The cal zone effectively contains a single air 500 hPa) thickness. This generally has a temperature difference between the tropics mass. However, variations in mean tempera- minimum around 580 decametres along the and middle latitudes causes a jet stream to ture and dynamics cause some variation and northern and southern boundaries of the develop at the poleward limit of the tropics. the highest tropopause heights are tropical zone and may reach 590 deca- This sub-tropical jet stream (STJ) has a core generally found close to the Equator. The metres or more in places. close to 30°N and 30°S and has little high- height of the troposphere rarely extends Weather – September62, Weather 9 No. 2007, Vol. amplitude wave development along it above 17 km* and is most often between Winds in the tropical zone (Figure 2). The area between these jet 15 km and 16 km. Height gradients are usu- streams has a tropospheric depth character- ally small, but increase somewhat near the Within the tropics, winds are often relatively istic of the tropical zone. This depth allows us sub-tropical jet streams. Indeed, the associa- light, in particular at upper levels. Over the to define the periphery of the tropics, even tion of the STJ with a tropospheric depth of Atlantic and much of the Pacific, these are when the STJ weakens or is absent, as often about 15 km provides a definition of tropical westerlies throughout the year. Over the occurs in summer, as is usually the case in the air. western Pacific, Indian Ocean and Africa, Northern Hemisphere in summer. Further- there are high-level easterlies close to the more, the STJs coincide with the transition * Altitudes given in this series of articles Equator (Figures 2 and 3). At low levels, from westerly lower tropospheric winds on approximate the true heights. In the tropics, Trade-Wind flows predominate, originating their poleward side to the easterly Trade these are about 7% greater than the ICAO in the sub-tropical high-pressure systems Winds on their equatorward side. heights corresponding to pressure levels. (the areas of the ‘doldrums’) centred near January 200 mbar July 200 mbar Figure 2. Mean annual wind flow (streamlines) at 200hPa (c. 12km) in the tropics with vector-mean isotachs (ms–1) shown by dashes. The mean core of the sub-tropical jet streams is shown by arrows. The effect of using gridded vector-mean speed, as in this case, is to reduce the apparent speed of the winds, which are instantaneously stronger at all times and in all seasons. (The use of the 200-hPa also omits the strongest winds observed near the Equator, which develop between the 150-hPa and 100-hPa levels; these are indicated by arrows.) The easterly and westerly branches of the Walker circulation can be seen close to the Equator. (Adapted from Godbole and Shukla, 1981.) 246 weather fluctuation within about 5–10° latitude of the mean position of the ITCZ. However, since most of the world’s hot The and weather climate of the tropics deserts have their equatorial flank within 20° of the Equator, marked by transition to savannah vegetation (e.g.