The Weather and Climate of the Tropics Part 1

Home , Middle latitudes

Weather – September 2007, Vol. 62, No. 9 245 warm temperate ropics, particularly above d sub-tropical desert; A simple method is to divide the globe However, the zones of predominantly systems throughout the globe, providing much of their energy. How can we define the tropics? The most commonly used definition of the tropics is the zone within which the Sun is directly overhead at some time during the year, i.e. the zone between the Cancer and Capricorn (23.45°N tropics and 23.45°S, of respectively). However, a more logical meteoro- definition, rather than the elevation of the Sun at useful midday, to both is the probably weather forecaster more and the climatologist. into tropical and extra-tropical zones. This is a method often employed for the verification of numerical forecasts Fuller, (WMO, 2004). One such 1982; method divides the globe into two equal halves – tropics and extra-tropics – with somewhat arbitrary dividing lines at 30°Nveniently, and this 30°S. latitude almost Con- all of range the climatic zones that can be includes regarded as tropical: humid savannah, semi-desert scrub and hot equatorial, desert (Figure 1). westerly winds make incursions in particularly latitude, of lines these of ward equator- winter. In order to keep within a zone of The The weather and climate climate of the tropics Part 1 – Setting the scene For instance, the primary purposes of The research carried out as part of the out within the tropical zone. What we know of the weather (and, to some climate) of extent, the tropics the remains limited and has typically focused on events, severe such weather as tropical (e.g. revolving Emanuel, 2005), or data storms from a limited range of observing stations. However, many impor- are weather day-to-day the of factors tant in the tropics, not least for aviation and public safety. forecasting for aircraft operations tropics in are safety the and maximization of effi- ciency for the benefit of passengers and aircraft operators. The most appropriate accurate forecasts will achieve this and goal, using a mixture of numerical weather pre- diction products, observed data and good forecasting knowledge. It is the effects the of weather, in other words its outcomes, which must be considered. World Climate Research Programme since the early 1970s is very important in allowing and processes the of many understand to us associated weather of the tropical (Gates and Newson, 2006). Knowledge con- zone tinues to grow research programmes, such as TOGA, which through more investigates recent the important links the between tropical ocean and the global phere atmos- (Fleming, 1986). It is clear tropics have an important effect on weather that the Much of what we know about the This series is not intended to be a detailed Figure 1. Climatic zones of the tropics: Af, Am – tropical rainforest; Aw – savannah; BSh – tropical steppe; BWh – tropical an Cs – dry-summer sub-tropical; H – highlands; Cf – warm temperate with no dry season; Cs – warm temperate with dry summer; Cw – with dry winter; stippled– modification due to altitude. The effect of high ground has a profound influence on climate in the t about 2km. (Using the system devised by Wladimir Koppen.) weather has been focused on mid-latitude weather systems: first, because most early researchers came from Western Europe and eastern North because America; of the risks and and consequences of weather systems prevalent second, in these zones. However, although there are simple scientific non- descriptions of weather from events the tropics going back years, hundreds it of is only since much the scientific late research 1960s has that been carried mathematical description weather. A number of of very good texts are available tropical for those who greater wish depth; to in go particular, I into recommend Hastenrath (1991) and Riehl (1979). In this series, I will describe various aspects of tropical weather and climate understood as today. The it series is will these topics: cover the sub-tropical jet synoptic-scale weather systems; streams; forecasting significant cloud and associated weather; dry environments; the development maintenance and of monsoon systems; tropical revolving storms; and mesoscale convective complexes. Each part will include a description of the effects of tropical weather. J. F. P. Galvin Met Office, Exeter 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 – 62, September9 Weather 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 The weather and climate of the tropics Weather – September 2007, Vol. 62, No. 9 247 (no ¶ As can be seen in Figure 1, the pre- Over continental areas, the periphery of Over the warmest ocean areas, the tropics Although a relatively narrow zone more than about 1500km wide, except in South-East Asia), it is the humid equatorial In tropics. the with associate many that zone this zone, rainfall can be relied upon round, year as the ITCZ Maxima are generally limited to the mid 30s is never far away. weather weather fluctuation within about 5–10°latitude of the mean position of the However, ITCZ. since most of deserts the have world’s their hot equatorial flank 20° within of the Equator, marked by transition to savannah vegetation (e.g. the Sahel of West Africa), seasonal variations can close be to these latitudes. large dominant climates of the tropics semi-desert scrub and ones: the hot deserts, are dry expansive savannahs. These lie towards the periphery of the tropical zone, in where regions anticyclonic subsidence predomi- nates and rainfall is ephemeral. either The hot deserts are seasonal noted for a or high diurnal range of temperature. A maximum of more than 40°C in places afternoon by may mid fall as much as 30 degC morning. by This range is solely effects of a dry due atmosphere with little or no to the cloud and such a low vapour pressure that diurnal temperatures can vary greatly. the humid zone is dominated by monsoon wind regimes. These bring weather in summer and wet predominantly dry humid is there Characteristically, winter. in weather a seasonal reversal of wind at low levels. In the northern summer, southeasterly winds cross the Equator and recurve to become south-westerlies (north-easterlies become north-westerlies in the Southern sphere), Hemi- bringing moist oceanic air across the Equator into areas that are under influence of the dry continental easterly winds during the winter. West Asia Africa, and southern northern Australia all experience these monsoon reversals. Although still seasonal, the situation is more complex the over Amazon basin and Caribbean, moist where westerlies cannot become lished, estab- due to the mountain barriers. Thus the motion Andes-Sierra of con- Madre vection is dependent on changes more in the atmospheric circulation complex and the influence of the Caribbean Sea, which warms and cools more than the neighbour- ing Atlantic Ocean. are characterized by the development tropical of revolving storms, associated with strong lower-tropospheric winds and (perhaps more importantly) heavy rainfall. ¶ The ITCZ is sometimes seen to split into two branches, each of which may be several hundred kilometres across. This is notable in the Indian Ocean and the western Pacific. . –1 in high summer with a –1 or more over east Asia and the –1 At medium levels, winds of moderate western Pacific. However, its speed is rarely more than 50 ms The weather patterns and climates of the tropics The tropical gradual region changes in experiences weather patterns between even small, generally are variations and only seasons. The main changes are between dry and wet seasons, marked (i) by the north- ward and southward movement inter-tropical of convergence the zone (ITCZ) the in central (equatorial) portion and (ii) by the winter incursion of cooler air at altitude near the poleward extremes. Even with this movement, there is almost no seasonal tropical jet stream (STJ) along the poleward edge of the tropical air mass, close to 30°N and 30°S. The STJ is present throughout the year in the Southern Hemisphere, but has large speed changes between winter and summer in the Northern Hemisphere. In the northern winter, its 110 ms speed may reach mean closer to 25 ms strength often form wave trains, known as easterly waves, which form in response to temperature differences and convective development. These have a strong association with severe weather, notably summer mesoscale weather systems over India, and the squall lines of Africa and Pacific parts of the Ocean (Atkinson, 2001). Over 1971; the Atlantic, easterly waves are Leroux, intimately associated with the development of tropical revolving storms (Emanuel 2005). The low value of the Coriolis force close to the Equator presents a difficulty assessment of wind speed and direction, so in the streamline analysis, rather than conven- tional pressure analysis, is generally used in the tropics, as described in Box 1. and –1* = 2 knots. –1 The easterly winds at high levels diverge Summer monsoon circulations have Wind speeds in this series are quoted in metres Figure 3. Mean sea-level pressure in the tropics (a) January, (b) July. Over high ground, in particular the Himalayas, mean sea-level pressure has no meaning, since the surface wind flow is influenced by pressure differences at higher levels. (Adapted from Godbole and Shukla, 1981.) north and south away from the upper high that forms close to the Equator. North and south of 15° latitude, they become westerlies (Figure 2). The reach upper jet stream strength westerlies to form may the sub- there is a local minimum over eastern Africa (Hastenrath, 1991). A similar, but weaker, jet stream forms over New Guinea and Indone- sia in response to the warming of Australia during the southern summer. westerlies at low levels, but upper-tropospheric easterlies strengthen as middle- and the troposphere warms and form deepens an upper high over the continents, to on the equatorward rim of which steep temperature and height gradient. The there is a greatest strengthening occurs as the equatorial high migrates away from the Equator and deepens. Close to the Equator, winds these may reach jet stream strength shallow in layer a near 15 km altitude. This stream is jet found only about 1000 m the below tropical tropopause. During the North- ern-Hemisphere summer, these stretch all the way from South-East winds Asia to Africa’s Gold Coast, although activity the and main highest speeds are across generally the southern tip of speeds India, occasionally where reach 60 ms 30°N and 25°S. Northern North-easterlies Hemisphere in converge the near Equator the with south-easterlies in the South- ern Hemisphere providing the forcing necessary for additional deep convection within the tropics, forming a belt of convective cloud: the inter-tropical convergence zone (ITCZ). However, significant reversals in the low-level wind flow occur over and around the tropical continents during the change from winter to summer. per second. 1 ms * Celsius‡ over land by the high water-vapour However, within each climatic zone there This brings a surprisingly cool and usually content of the air and minima are similarly are important variations, due to orography, dry climate north to only a few degrees restricted by cloudiness or the overnight latitude and longitude. Some of the world’s south of the Equator, although this climatic formation of dew, mist or fog. Over the sea, hot deserts receive most of their (meagre) zone, comparatively equable for the popula- temperature changes little day by day. Thus, rainfall in summer, others in winter – here tion, is narrow, extending only a few tens of throughout the tropics, the diurnal range of the variation is mainly by latitude and kilometres inland. Populations are compara- temperature is relatively small. Neverthe- altitude, with equatorward regions having a tively high in these zones, largely due to the less, this climate is uncomfortable for most summer rainfall peak. Examination of the excellent fishing available, due to the humans, who find it difficult to lose excess mean annual rainfall in the tropics reveals upwelling of plankton-rich deep waters. body heat in these conditions. Indeed, this that there are significant differences on a Heating and cooling have a significant energy-sapping weather is used as a test by broad scale within climatic zones, as shown effect on air pressure throughout the élite British army and Royal Marines regi- in Figure 4. In general, the desert areas tropics, the pressure falling in response to The weather The and weather climate of the tropics –1 ments, which carry out part of their training generally see less than 200 mm yr , diurnal heating and rising in response to in Brunei. although Australia’s dry interior is defined nocturnal cooling. As pressure rarely Where seasonality is the main effect on by annual rainfall less than 600 mm. The changes due to synoptic-scale weather rainfall, savannahs predominate. In these monsoon zones see between about 1000 systems in the tropics, the diurnal changes –1 areas of extensive grassland, annual evapo- and 2500 mm yr , occurring in summer. The are significant, as shown in Table 1. ration exceeds precipitation and trees grow semi-deserts and oceanic areas under the only in stunted groves. These areas are influence of the sub-tropical anticyclones Clouds and fog in the tropics home to relatively large populations in have a total rainfall between 200 and about –1 some parts of the world and agriculture is 1000 mm yr . Areas under the influence of Most Weather readers will be familiar with critically dependent on the summer rains upper-tropospheric troughs (see Parts 2 and the clouds observed in the middle latitudes –1 (both locally and to re-charge river flows), so 3) see about 1000 mm yr and the ITCZ and their typical range of heights (Pouncy, –1 any reduction or failure of seasonal rainfall experiences totals above 1000 mm yr with 2003). The clouds of the tropics are the same –1 often causes notable famines, especially in some areas seeing more than 3000 mm yr . types as those seen in the rest of the world, Weather – 62, September9 Weather No. 2007, Vol. recent years. Some areas that have some Orographic effects add further local detail. although convective clouds are predomi- seasonality, but a predominantly maritime High ground has two main effects: it nant, whilst altostratus and nimbostratus climate, such as the northern Caribbean, lowers the mean temperature (although are relatively rare. However, the increase in have an intermediate climate with extensive nights may be less cool, if the mountains are the depth of the troposphere means that forest, as well as grassland. Elsewhere, modest and the observation is not in a the range of height at which each species is steppe surrounds the arid deserts. Here few valley) and increases the rainfall, increasing seen is greater than in the middle latitudes. plants can grow, but there is sufficient rain- the likelihood of precipitation by ‘forced’ The World Meteorological Organization fall or run-off to support agriculture and convection or convergence. For instance, (WMO) recommends the range of cloud moderate-sized populations. Included in copious rainfall is generated by the Ethiopi- height given in Table 2 (WMO, 1956 ). † this climatic zone are the highlands of much an Highlands and India’s Western Ghats. All stratocumulus clouds in the tropics of Arabia. These highlands have a relatively equable are at a temperature above 0°C and it is climate within the almost universally hot altocumulus and altostratus (as well as occa- ‡ The ‘rule’ explaining the limitation of tempera- tropical zone and their climate is markedly sional nimbostratus) that occupy the level of ture assumes a boundary layer near saturation. different from that of their surroundings. This is comparatively rare, even in the humid Along the western margins of Africa and zone. The humidity mixing ratio (h.m.r.) locally † This assumes that the cloud height is not –1 the Americas, there are cool currents as a reaches 20 g kg in the humid zone, limiting relative to the observer. Clearly there are layer temperatures. However, the May pre-monsoon result of the upwelling of cool deep water clouds at heights less than 2000 m above high season, for instance, brings humid air across under the influence of the Trade Winds. The ground in the tropics that are at or below 0°C. India with an h.m.r. closer to 16 g kg–1, allowing effect is most marked along the south-west Typically, this would be the case for an observa- temperatures to rise to around 40°C under clear coast of South America, as well as the north- tion made at an altitude above 2500–4000 m in skies. In either case, it feels very close! western and south-western coasts of Africa. the tropics.

Figure 4. Mean annual rainfall (m) in the tropics (from Hastenrath, 1991, Figure 6.1:7). Local orographic effects are not evident at this scale, although a rainfall maximum in excess of 2000 mm yr–1 is shown over the southern Himalaya.

248 The weather and climate of the tropics Weather – September 2007, Vol. 62, No. 9 249 From From cumulus clouds, precipitation is Stratus and fog are also characteristic of Fog and stratus are also features of cooler Early-morning fog is also relatively com- Layer clouds are usually the product of Altocumulus castellanus (or floccus) Any unstable cloud whose top becomes generally slight. Nevertheless, this rainfall form can snowfall) mountains, high over (or, the majority of annual precipitation. parts of the tropics. Stratus with hill fog is frequently seen in the early morning over tropical woodland, in particular the tropical rainforest, following overnight Here, cooling. transpiration from the trees in a very moist environment assists its formation. parts of the tropical oceans quently seen along the east-facing coasts of and are fre- southern Africa and South America. In the former case, much of the meagre precipitation available for plants Namib to Desert is deposited from wet fogs. grow in the mon on the coasts air moist bring frequently breezes sea where of the Arabian Gulf, inland during the afternoon, humidifying the air and raising its summer fog point. cases Extreme (observed Bahrain), in when sea-surface temperatures in Doha and 35°C,reach may with Gulf fog the bring may visibility less than 100 m around dawn with air temperatures and dew 30°C points (Andy Dexter, Richard Young personal above communications)! These fogs moist, feel almost suffocating, making it difficult very for humans and animals to lose excess heat. As such, they can be dangerous to life in a very different way from fogs in the middle latitudes. convection and are often relatively thick, as a result. Most of these clouds of convective origin are of genera altocumulus or stratocumulus. However, there are some excep- tions and altostratus sometimes form, usually in association with or nimbostratus cyclonic disturbances. These will cussed be later dis- in the series. Hours of precipitation may steady fall from these deep layer clouds. clouds are also characteristic of tropical air. Principally, they form in response to high- level cooling. This occurs in two ways. First, there is the dynamical cooling associated with advection, often troughs. ahead This is of usually evident upper above the 700-hPa level. Second, there is wave cooling of the atmosphere the during the long- evening, causing a decrease in stability at the cloud tops. Castellanus may development prolong the life through of the cumulonimbus night. In some cases, the stabilization de- is associated with instability at lower levels, which feeds into the unstable medium-level clouds. glaciated is reclassified as cumulonimbus. Thus the base of clouds, these originally cumulonimbus altocumulus castellanus, may be at 4000 to 5000 m or more. Most , 1995) et al. in the ITCZ), can produce large –1 20 g kg ∪ diurnal variation amounts of precipitable water and are the source of thunderstorms of the ITCZ. Thun- association in seen frequently are derstorms with the deep convection of However, they the are more common tropics. near the instability where ITCZ, the of edge poleward and convective available potential energy (CAPE) values (defined in Galvin are larger than nearer to the Equator, especially over the continents Despite (see the Figure presence 5). of tropics the of much hail, produce may cumulonimbus, which is simply too warm for ground, hail although to it reach does the occur where in wind places shear and ducive. vorticity Waterspouts are and con- tornadoes also may form, but only where there is sufficient vorticity. from air with a high humidity mixing ratio (r HPa hPa HPa hPa frequently well into the high-cloud étage. Precipitation from cumulonimbus clouds Cumulonimbus clouds always have tops * Nimbostratus usually has a base that extends down into the low-cloud étage and a top that is Correction Correction to the observed pressure necessary to allow for diurnal variation Average component of the 3-hourly barometric change due to the Table Table 1 Typical pressure changes due to the heating and cooling of the atmosphere in the tropics. 0000010002000300 –0.60400 –0.10500 +0.30600 +0.70700 +0.80800 –0.5 +0.60900 –0.1 +0.21000 +0.3 –0.41100 +0.7 0000-0300 –0.91200 +0.8 0100-0400 –1.31300 +0.6 0200-0500 –1.41400 +0.2 0300-0600 +1.1 –1.11500 0400-0700 –0.3 +0.9 –0.61600 0500-0800 +0.3 –0.8 +0.11700 0600-0900 –1.1 –0.5 +0.71800 –1.2 –1.2 +1.3 0700-1000 +1.0 1900 –1.0 –1.5 +1.5 0800-1100 +0.9 2000 –0.5 –1.5 +1.4 0900-1200 +0.3 2100 +0.1 +1.0 1000-1300 –1.0 –0.5 2200 +0.7 +0.5 1100-1400 –0.2 –1.1 2300 +1.1 1200-1500 +0.7 –0.1 –1.4 +1.3 1300-1600 +1.5 –0.6 –1.3 +1.2 1400-1700 +1.8 –0.9 –0.9 +0.9 1500-1800 +1.9 –0.9 –0.4 +0.3 1600-1900 +1.4 +0.6 1700-2000 +0.7 –0.2 +1.3 1800-2100 –0.6 –0.3 +1.7 1900-2200 –0.8 –1.0 +1.6 –0.8 –1.5 2000-2300 +1.2 –1.6 2100-2400 +0.5 –1.4 2200-0100 –0.2 2300-0200 –0.8 –1.0 –1.4 0.0 +0.8 –1.5 +1.1 –1.1 –0.6 +0.1 +0.7 +1.0 Local time 0°–10°N or S 10°–20°N or S Local time 0°–10°N or S 10°–20°N or S Table 2 The variation of range of cloud-base heights in tropical, middle-latitude and polar air masses. Étage High (Ci, Cs, Cc) Middle (Ac, As, Ns*) Low (St, Sc, Cu, Cb) 2000–4000 m 3000–8000 m 0–2000 m Polar regions 2000–7000 m 5000–13000 m Middle latitudes 0–2000 m 6000–18000 m 2000–8000 m Tropical regions 0–3000 m is usually heavy. These clouds, resulting extending into the middle levels and quently fre- into the high-cloud étage. This the is usual state in the tropics, where cumulonimbus clouds rarely have 12 tops km. In hot-desert below regions, where there is sufficient moisture, cumulus or, if there sufficient is moisture at high lonimbus clouds, may form around levels, the time cumu- of maximum temperature. transition to supercooled water. The tops of the deepest medium-level contain clouds a significant may amount of ice as temperature the of the cloud falls below about –20°C. This temperature altitude occurs between 8 at km and, an Clouds formed predominantly of ice are not locally, 9 km. found below 8 km in the tropics. thunderstorms that develop over the hot deserts of the tropics have such high bases. Precipitation from these high-based cumulonimbus clouds usually evaporates before reaching the ground and strong down- draughts are the result.

Population in the tropics On average, population density is low in the tropics: typically around 10 per km2. The weather The and weather climate of the tropics However, in India, Bangladesh, southern China and parts of South-East Asia, the population density is much higher, reaching 100 per km2 or more. Away from rivers, the hot-desert environment is not able to support large populations and tropical rain forest is a dark foreboding environment with trees that are difficult to clear, unless heavy machinery is available. Thus populations are found mainly along coasts or rivers, where transport has been available for centuries or millennia. It is near the mouths of the larger

Weather – 62, September9 Weather No. 2007, Vol. rivers that the largest cities are found. The savannahs are moderately populated, the land easy to clear for agriculture and road building. However, in some areas the population density is close to the ability of the land to support it. In Africa and parts of South America, this presents a problem, since populations have grown rapidly Figure 5. Radiosonde profile for Niamey, Niger, in Africa's Sahel at 1200 UTC on 30 July 2006. This during the late twentieth century and the ascent is absolutely unstable and has a convective cloud base near 850 hPa, in contrast to ascents rainfall is not sufficiently reliable in these –1 θ typical of the ITCZ. It shows a typically high level of CAPE (1273 J kg ) and w falling with height. areas for there to be confidence that populations can survive without a major risk of drought and famine. The degree of urbanization, by contrast, varies considerably from continent to conti- Box 1. The effect of latitude on geostrophic balance nent. In South America, it is generally above Over much of the globe, wind speed and direction are the result of a balance of forces, 50%, the proportion in Venezuela above the most significant of which are the pressure-gradient force and the Coriolis force, 90% and in Australasia 80%. But in much of which are in equilibrium where the flow is straight and parallel to the Equator (Persson, Asia and Africa it is below 45%, Ethiopia, 2000). However, this relationship breaks down close to the Equator, where the Coriolis Uganda, Malawi and Nepal having fewer force becomes very small, since the air is subject to very little rotation around the Earth’s than 15% of their populations living in axis. towns. This stresses the relative importance Nevertheless, winds remain in reasonable balance until they reach the latitudes of of agriculture in these countries, even those about 6° and even then momentum generally carries the wind in the direction it was that are rapidly industrializing, such as India moving when in near-geostrophic balance. Bigger problems for the analyst include the and China. However, there are significant distance between isobars, which become very large in the tropics, such that barometric variations from country to country and with- errors (and those of correction to sea level) make standard analysis, based on mean sea- in countries. level pressure (or geopotential height), almost impossible. As a result, most analysis in the tropics uses streamlines. If we define the tropics as the zone between the sub-tropical discontinuities, the El Niño and the Southern tropical atmosphere is barotropic, so horizontal temperature differences are small. Fur- thermore, the apparent progress of the sun from east to west across the sky causes a Oscillation pressure disturbance as the air is heated and cooled by incoming and outgoing radia- The El Niño phenomenon is generally tion. The wavelength of this disturbance is 12 hours and its amplitude is approximately accepted to have a profound underlying 2hPa at the surface (Table 1); the amplitude is somewhat larger than the usual change effect in the tropics, changing the broad- of pressure in the atmosphere. scale circulation patterns as pressure Over land areas, the use of mean sea-level pressure is also inappropriate since the changes across the Pacific to form the correction of observed pressure to sea level causes errors. Thus, where geostrophic Southern Oscillation. The Southern Oscilla- balance needs to be measured, the height of the 925 hPa or 850 hPa surface should be tion is a relative change of pressure in the used. eastern tropical Pacific basin (usually measured at Tahiti) compared with that in the 250 The weather and climate of the tropics Weather – September 2007, Vol. 62, No. 9 251 . : 53 : 69–72, 77–80. 58 Weather . Springer/Praxis, Climate Climate Dynamics of : 234–239. . Geneva. The Meteorology and Cli- 55 1991. The Climate and Weather of Weather 1998. Westerly wind bursts International cloud atlas Manual on the Global Data- 2003. A history of cloud codes 2000. Back to basics: Coriolis. Academic Press, London. Kluwer Kluwer Academic Publishers, 2001. 1979. 1956. 1982. Weather 282–284. WMO. Abridged atlas. Geneva. WMO. Processing System Hastenrath Hastenrath S. Riehl H. the Tropics. Verbickas S. in the tropical Pacific. the Tropics. Dordrecht. Leroux M. mate of Tropical Africa Chichester. Persson A. Part 3. The Coriolis force on the physical earth. Pouncy FJ. and symbols. Correspondence to: Jim Galvin, Met Office, FitzRoy Road, Exeter EX1 3PB. Email: [email protected] © Crown Copyright, 2007. DOI: 10.1002/wea.53 NASA ICSU and , June 2004, : 237–250. Oxford Universi- 55 , No. 9), so no more 2006. World Cli- 1981. Global Q. J. R. Meteorol. Soc. 53 Forecasters’ Guide to . University Press of The Tropical Ocean and . Goddard Space Flight Divine Wind: The History Weather NWP Gazette 1971. 2002. The response of the : 210–216. 1986. 2005. 55 2004. Recent forecast model . in 1998 (Vol. : 1–23. Atkinson GD. Tropical Meteorology the Pacific, Honolulu. Emanuel K. Godbole RV, Shukla J. analysis of mean sea-level pressure. Tech. Memo. 82097 Centre, Greenbelt. and Science of Hurricanes. ty Press, New York. Fedorov AV. coupled tropical ocean-atmosphere to westerly wind bursts. 128 Fleming RJ. Global Atmosphere programme. improvements. WMO International TOGA Project Office, Boulder. Fuller SR. 10. Galvin JFP, Bennett PH, Couchman PB. 1995. Two thunderstorms in summer 1994 at Birmingham. Gates WL, Newson RL. mate Research Programme: a history. WMO Bull A review of El Niño and its global con- References References sequences sequences appeared as a special Weather issue of will be said here. on 19 August 2006. The surface dew point temperature fell rapidly, 2.5 degC in 7 minutes, as the leading edge UTC A particularly threatening advancing squall line, as seen from Stratfield Mortimer, Berkshire, looking south-west at 1812 of the squall line passed overhead; this was followed by two hours of showery rain starting a few minutes after the photograph was taken. (© Stephen Burt.) La Niña is an amplification of the west- Relatively strong westerlies are associated are westerlies strong Relatively The consequence of the changes is to ward flow of the Trade Winds and associated increased warmth of the western Pacific. It is measured as an increase from the nor- mal pressure difference Tahiti–Darwin. Its consequences are an increase in west Pacif- ic tropical storm activity and copious additional rainfall across South-East north-eastern Australia. Asia and with the development of the El-Niño anom- alies in the eastern Pacific (Verbickas, 1998; Fedorov, 2002) and their formation mecha- nism seems to be directly linked with El-Niño the phenomenon. In view of the mass transport associated with such a change in wind velocity, accompanied by amount the of water vapour that can be carried vast by such tropical winds, it is easy to see how El Niño has global consequences. increase precipitation in the eastern Pacific (and over the Atlantic and western Indian Ocean basins), while decreasing it South-East across Asia and Australia (as perhaps, east Africa). well as, west (measured at Darwin). warming El of Niño the is eastern a equatorial Ocean Pacific and causes mean sea-level pressure to fall over the eastern Pacific, whereas over the western Pacific, pressure rises.