The trade winds

Circulations in the vertical and horizontal planes

The vertical change of pressure with height depends on the temperature structure. High-(low-) pressure systems intensify with altitude in a warm (cold) air column; thus warm lows and cold highs are shallow features. This 'thickness' relationship is illustrated by the upper-level subtropical anticyclones and polar vortex in both hemispheres. The intermediate mid-latitude west­erly winds thus have a large 'thermal wind' component. They become concentrated into upper tropospheric jet streams above sharp thermal gradients, such as fronts.

The upper flow displays a large-scale long­-wave pattern, especially in the northern hemi­sphere, related to the influence of mountain barriers and land-sea differences. The surface pressure field is dominated by semi-permanent subtropical highs, sub-polar lows and, in winter, shallow cold continental highs in Siberia and north-western Canada. The equatorial zone is predominantly low pressure. The associated global wind belts are the easterly trade winds and the mid-latitude westerlies. There are more variable polar easterlies and over land areas in summer a band of equatorial westerlies repre­senting the monsoon systems. This mean zonal (west-east) circulation is intermittently inter­rupted by 'blocking' highs; an idealized sequence is known as the index cycle.

The atmospheric general circulation, which transfers heat and momentum polewards, is predominantly in a vertical meridional plane in low latitudes (the Hadley cell), but there are also important east-west circulations (Walker cells) between the major regions of subsidence and convective activity. Heat and momentum exchanges in middle and high latitudes are accomplished by horizontal waves and eddies (cyclones/anticyclones). Substantial energy is also carried polewards by ocean current systems. Surface currents are mostly wind driven, but the slow deep ocean circulation (global conveyor belt) is due to thermohaline forcing.

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The trades (or tropical easterlies) are important because of the great extent of their activity; they blow over nearly half the globe. They originate at low latitudes on the margins of the subtropical high-pressure cells, and their constancy of direction and speed (about 7 m s-1) is remarkable. Trade winds, like the westerlies, are strongest during the winter half-year, which suggests they are both controlled by the same fundamental mechanism.

The two trade wind systems tend to converge in the Equatorial Trough (of low pressure). Over the oceans, particularly the central Pacific, the conver­gence of these air streams is often pronounced and in this sector the term Intertropical Convergence Zone (ITCZ) is applicable. Generally, however, the convergence is discontinuous in space and time. Equatorward of the main belts of the trades over the eastern Pacific and eastern Atlantic are regions of light, variable winds, known traditionally as the doldrums and much feared in past centuries by the crews of sailing ships. Their seasonal extent varies considerably: from July to September they spread westward into the central Pacific while in the Atlantic they extend to the coast of Brazil. A third major doldrum zone is located in the Indian Ocean and western Pacific. In March-April it stretches 16,000 km from East Africa to 180° longi­tude and it is again very extensive during October-December.

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