Tuesday, June 9, 2009

Intertropical Convergence Zone

Last week's crash of Air France 447, over the central Atlantic, has brought the Intertropical Convergence Zone (ITCZ) into the news. Though the cause for this tragedy remains uncertain, it appears that the perpetual stormy weather of this equatorial band was not a major factor.

The ITCZ is a zone of cloudiness and thunderstorms that circles the globe, over or near the equator. The Earth's rotation produces low-level, northeast trade winds in the Northern Hemisphere and low-level, southeast trade winds in the Southern Hemisphere; these converge in the ITCZ and, in combination with solar heating, force the surface air to rise, producing extensive bands of showers and thunderstorms (the latter produce heavy rain but, devoid of upper level jet stream energy, are rarely intense). As one might expect, storms build through the day (in response to solar radiation) and dissipate overnight. Beneath the rising air, calm conditions develop at the surface (as in the eye of a hurricane) and sailors have long experienced the becalming effects of the ITCZ doldrums.

Though centered over the equator through the course of a year, the zone shifts north and south with the seasons, following the most direct rays of the sun; due to the retention of heat by rocks and soil, this shift is more pronounced over the equatorial Continents than over the oceans. Moving toward the Tropic of Cancer during the Northern Hemisphere summer, the ITCZ is the focal point of Atlantic hurricane formation, triggered primarily by upper level lows as they move westward from the coast of Africa.