The tropical atmosphere differs significantly from that in middle latitudes. Temperature gradients are generally weak and weather systems are produced mainly by airstream convergence triggering convection in the moist surface layer. Strong longitudinal differences in climate exist as a result of the zones of subsidence (ascent) on the eastern (western) margins of the subtropical high-pressure cells. In the eastern oceans, there is typically a strong trade wind inversion at about 1 km with dry subsiding air above, giving fine weather. Downstream, this stable lid is raised gradually by the penetration of convective clouds as the trades flow westward. Cloud masses are frequently organized into amorphous 'clusters' on a subsynoptic scale; some of these have linear squall lines, which are an important source of precipitation in West Africa. The trade wind systems of the two hemispheres converge, but not in a spatially or temporally continuous manner. This intertropical convergence zone also shifts poleward over the land sectors in summer, associated with the monsoon regimes of South Asia, West Africa and northern Australia. There is a further South Pacific convergence zone in the southern summer.
Wave disturbances in the tropical easterlies vary regionally in character. The 'classical' easterly wave has maximum cloud buildup and precipitation behind (east of) the trough line. This distribution follows from the conservation of potential vorticity by the air. About 10 per cent of wave disturbances later intensify to become tropical storms or cyclones. This development requires a warm sea surface and low-level convergence to maintain the sensible and latent heat supply and upper-level divergence to maintain ascent. Cumulonimbus 'hot towers' nevertheless account for a small fraction of the spiral cloud bands. Tropical cyclones are most numerous in the western oceans of the northern hemisphere in the summer to autumn seasons.
The monsoon seasonal wind reversal of South Asia is the product of global and regional influences. The orographic barrier of the Himalayas and Tibetan Plateau plays an important role. In winter, the subtropical westerly jet stream is anchored south of the mountains. Subsidence occurs over northern India, giving northeasterly surface (trade) winds. Occasional depressions from the Mediterranean penetrate to northwestern India-Pakistan. The circulation reversal in summer is triggered by the development of an upper-level anticyclone over the elevated Tibetan Plateau with upper-level easterly flow over India. This change is accompanied by the northward extension of low-level southwesterlies in the Indian Ocean, which appear first in southern India and along the Burma coast and then extend northwestward. The summer 'monsoon' over East Asia also progresses from southeast to northwest, but the Mai-yu rains are mainly a result of depressions moving northeastward and thunderstorms. Rainfall is concentrated in spells associated with 'monsoon depressions', which travel westward steered by the upper easterlies. Monsoon rains fluctuate in intensity, giving rise to 'active' and 'break' periods in response to southward and northward displacements of the monsoon trough, respectively. There is also considerable year-to-year variability.
The West African monsoon has many similarities to that of India, but its northward advance is unhindered by a mountain barrier to the north. Four zonal climatic belts, related to the location of overlying easterly jet streams and east-west-moving disturbances, are identified. The Sahel zone is reached by the monsoon trough, but overlaying subsiding air greatly limits rainfall.
The climate of equatorial Africa is influenced strongly by low-level westerlies from the South Atlantic high (year-round) and easterlies in winter from the South Indian Ocean anticyclone. These flows converge along the Zaire air boundary (ZAB) with easterlies aloft. In summer, the ZAB is displaced southwards and northeasterlies over the eastern Pacific meet the westerlies along the ITCZ, oriented north-south from 0° to I2°S. The characteristics of African disturbances are complex and barely known. Deep easterly flow affects most of Africa south of I0°S (winter) or 15 to I8°S (summer), although the southern westerlies affect South Africa in winter.
In Amazonia, where there are broad tropical easterlies but no well-defined ITCZ, the subtropical highs of the North and South Atlantic both influence the region. Precipitation is associated with convective activity triggering low-level convergence, with meso- to synoptic-scale disturbances forming in situ, and with instability lines generated by coastal winds that move inland.
The equatorial Pacific Ocean sector plays a major role in climate anomalies throughout much of the tropics. At irregular, three- to five-year intervals, the tropical easterly winds over the eastern-central Pacific weaken, upwelling ceases off South America and the usual convection over Indonesia shifts eastward towards the central Pacific. Such warm ENSO events, which replace the normal La Niña mode, have global repercussions since teleconnection links extend to some extratropical areas, particularly East Asia and North America.
Variability in tropical climates also occurs through diurnal effects, such as land-sea breezes, local topographic and coastal effects on airflow, and the penetration of extratropical weather systems and airflow into lower latitudes.
Short-range tropical weather prediction is commonly limited by sparse observations and the poorly understood disturbances involved. Seasonal predictions show some success for the evolution of the ENSO regime, Atlantic hurricane activity and West African rainfall.
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