Intertropical Convergence Zone

The low-level circulation over the Atlantic and Pacific oceans is dominated by the easterly flow around the equatorward flank of the subtropical high-pressure belts near 30N and 30S. Superimposed on this planetary-scale easterly flow are weak synoptic-scale disturbances that cause the wind to fluctuate between northeasterly and southeasterly. These disturbances are called easterly waves because they move from east to west, in contrast to middle-latitude disturbances that usually move from west to east. Also endemic to the tropical oceans is a class of extremely intense, circular vortices called tropical cyclones, which account for the strongest sustained surface winds observed anywhere in the earth's atmosphere. Most of these storms develop during the warm season, over a few well-defined areas.

Within the lowest kilometer above the sea surface the easterly flow is particularly steady and has a distinct equatorward component. Hence, the prevailing winds in the tropical North Atlantic and North Pacific are from the northeast, and those in the tropical South Atlantic and South Pacific are from the southeast. In the early days of sailing ships these wind regimes came to be known as the northeast and southeast trades, respectively. Until recently, it was widely believed that the tradewind belts in the northern and southern hemispheres were separated by a region of calm winds along the equator called the "doldrums". More recent evidence indicates that, with the exception of the extreme Western Pacific, the transition between the northeast and southeast trades usually takes place within a very narrow belt located several degrees north of the equator. This is the Intertropical Convergence Zone (ITCZ), where the northeast and southeast trades flow together, and are characterized by strong upward motion and heavy rainfall. The ITCZ is most clearly defined over the eastern portions of the Pacific and Atlantic oceans.

At the longitudes of the major continents, the low-level tropical wind field exhibits a strong seasonal dependence, with a tendency toward onshore (sea to land) flow during summer and offshore flow during winter. The seasonal reversal is particularly pronounced over southeast Asia and adjacent regions of the Indian Ocean where the prevailing winds blow from the southwest during summer and northeast during winter. These seasonal wind regimes are known as monsoons (from the Arabic word mausin - a season). Over most of India, the summer (southwest) monsoon is characterized by heavy rainfall while the winter (northeast) monsoon is extremely dry.

From Wallace, J. M. and P. V. Hobbs, 1977: Atmospheric Science: An Introductory Survey. Academic Press, 467 pp.