The trades (or tropical easterlies) are important because of their great extent, affecting almost half the globe (see Figure 7.13). 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 airstreams 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 (see Plate 24). 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 to April it stretches 16,000 km from East Africa to 180° longitude and is again very extensive during October to December.
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