intertropical convergence zone

intertropical convergence zone (ITCZ), belt of converging trade winds and rising air that encircles Earth’s lower atmosphere near the Equator. The rising air produces high cloudiness, frequent thunderstorms, and heavy rainfall; the doldrums, oceanic regions of calm surface air, also occur within the zone. The ITCZ shifts north and south seasonally with the Sun. Over the Indian Ocean, it undergoes especially large seasonal shifts of 40°–45° of latitude. Other large seasonal shifts in the position of the ITCZ occur over the eastern Pacific Ocean, South America, and Africa. The ITCZ plays a substantial role in slowing the mixing of air between Earth’s Northern and Southern hemispheres (see also chemical equator).

Aloft, air moving away from the ITCZ is an important factor in atmospheric circulation. As air descends in the subtropical high-pressure belts near latitudes 30° N and 30° S (the horse latitudes), it causes the trade winds to blow westward and equatorward at Earth’s surface. These merge and rise in the ITCZ near the Equator and blow eastward and poleward at altitudes of 2 to 17 km (1 to 11 miles). Part of this flow descends in the subtropical high-pressure belts, and the remainder merges at high altitudes with the midlatitude westerly winds farther poleward. Precipitation generated within the ITCZ is a significant climatic control in several of Earth’s tropical climate zones, particularly the wet equatorial climate, tropical wet-dry climate, the tropical monsoon and trade-wind littoral climate, and the tropical and subtropical steppe climate (see also Köppen climate classification).