The Inter-Tropical Convergence Zone (ITCZ) is a low-pressure belt located near and parallel to the Earth's equator, within a few degrees of latitude. Because it is a low-pressure belt and located in the vicinity of the equator, near-surface winds on both sides of this belt tend to converge into the belt. A band of clouds and accompanying precipitation is associated with the ITCZ, which can therefore be easily identified from satellite pictures.

The wind on the north side of the ITCZ comes from the northeast (blowing toward the southwest), and therefore, by meteorological convention, is called the "northeasterly." The obverse wind on the south side of the ITCZ is the "southeasterly." The northeasterly and southeasterly are also historically called "trade winds," because explorers, navigators, and merchant fleets once used these winds to sail to many tropical regions for trade and adventure (Henson, 2007). Thus, the ITCZ is the belt where the trade winds meet.

The ITCZ migrates slightly, oscillating across the equator with the change of seasons. It is primarily located north of the equator, although during winter in the Northern Hemisphere it sometimes moves to the south of the equator. The ITCZ can often be well organized, in the sense that the low pressure, clouds, and precipitation associated with the ITCZ form a solid band structure and make almost an entire circle around the globe. However, this well-organized structure can break down from time to time. When it breaks down, the ITCZ becomes separated into several blocks or a few segments.

The ITCZ is an important atmospheric phenomenon in tropical meteorology (Ahrens, 2005). In meteorology, a low-pressure center always draws air flows around it coming in, causes convection, and therefore generates a weather system. The ITCZ serves precisely this role. As the trade winds converge into the low-pressure belt, air starts to ascend to higher altitudes. Because the ITCZ is typically formed over the tropical oceans, the air in the zone is very moist. The ascending of this moist air in the ITCZ typically leads to cloud formation as the air condenses at higher, colder altitudes. As the air continues to rise and condense, water will eventually precipitate back to the surface. In the ITCZ, this ascending stream of tropical air forms the ascending branch of the Hadley cell. This air-circulation cell ascends in the tropics as a result of convection in the ITCZ, moves outward toward the poles, descends in the subtropical regions, and returns inward toward the equator at lower altitudes.

Hadley circulation thus typically transports warm, moist, tropical air to higher latitudes at upper levels. The air gradually cools and dries out as it travels poleward. In the subtropics, about 30œ latitude north and south of the equator, the air begins to descend. This effect is believed to cause many of the large deserts in the subtropics, such as the Sahara Desert, because the descending air has lost most of its moisture to precipitation before it descends. Thus, the ITCZ plays an important role in global atmospheric circulation and global climate.

As a driving engine of global atmospheric circulation, the ITCZ plays a great enough role in Earth's climate that any change in the position and intensity of the ITCZ may have significant effects upon the climate. The ITCZ itself comprises the planet's deepest convective cloud systems with the most frequent thunderstorm activities and produces the greatest rainfall on the Earth. The Hadley circulation, driven in party by the ITCZ, has a profound impact on a global scale. For example, the intensification of the ITCZ may cause more evaporation of water vapor from tropical oceans and enhance the transport of water vapor to high latitudes.

Many scientists believe that the ITCZ may intensify and produce even more precipitation with the increase of global surface temperature. The position, movement, and intensification of the ITCZ are also influenced by various climate regimes, such as the El Nino-Southern Oscillation and La Nina events. Scientists have also shown that when the ITCZ breaks down, some of its segments can form into tropical disturbances, the initial stage of tropical cyclones, hurricanes, and typhoons. However, it is still unclear whether global warming can cause more frequent breakdown of the ITCZ, thereby producing more tropical storms, although scientists do believe that hurricane intensity will increase as the climate warms.

References

1. Ahrens, Donald C. Essentials of Meteorology: An Invitation to the Atmosphere. 4th ed. New York: Brooks/Cole, 2005.

2. Henson, Robert. The Rough Guide to Weather. 2d ed. New York: Rough Guides, 2007.

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