Intertropical Convergence Zone

Intertropical Convergence Zone or ITCZ, the low-pressure zone near the equator where the trade winds of both hemispheres converge, and that is characterized by high humidity, intermittent heavy rainfall, and light, variable winds. The ITCZ is also known as the equatorial trough, the Intertropical Confluence, and the Intertropical Discontinuity, the last name reflecting the fact that it does not form a continuous girdle of convergence around the Earth. Rather, it comprises linear segments, several thousands of kilometres long, which are often clearly visible on satellite photographs as narrow, well-defined bands of cloud. In the eastern Pacific Ocean two ITCZs sometimes appear; the upwelling of cool water to the ocean surface can split the zone into northern and southern parts.

The ITCZ is a product of the convergence of the trade winds near the equator and of the convection currents caused by the intense solar heating of the equatorial zone. The two combine to force air to rise, leaving behind a surface low-pressure area, the ITCZ. The strong upward currents form the rising limbs of the Hadley cells—the thermally driven atmospheric circulation systems that exist between the equator and latitude 30° in each hemisphere. As it rises, the air cools and condenses until it reaches the tropopause—the boundary between the troposphere and stratosphere—when it begins to move poleward. It reaches around latitude 30° where, as a result of further cooling due to radiation, it begins to sink. To complete the Hadley cell most of the cooled air begins to move back towards the equator, being deflected towards the west as it does so by the effect of the Earth’s rotation to become the north-easterly and south-easterly trade winds of the Northern and Southern hemispheres respectively. The trade winds pick up both latent heat and moisture as they cross the warm seas of the tropics. As it cools again after being reincorporated into the rising limbs of the Hadley cells, this moist air produces the towering cumulonimbus clouds and afternoon thunderstorms characteristic of the ITCZ. Another characteristic of the zone are the gentle, intermittent winds that occur at ground level, and which led sailors to give the ITCZ the name it is best known by, the doldrums.

The ITCZ generally coincides with the thermal equator, the zone around the globe of most intense surface heating, which is usually indicated by a line connecting points of highest mean annual temperature for their longitude. Unlike the geographical equator, however, the thermal equator is neither a geometric circle around the Earth, nor is it static; instead it forms a meandering feature that shifts with the seasons. The apparent movement of the Sun relative to the geographical equator, caused by the Earth’s elliptical orbit, draws the thermal equator with it, so that it is found in the summer hemisphere—that is the hemisphere in which the Sun appears to be. The Sun is in the Northern hemisphere between the March and September equinoxes, and in the Southern hemisphere between the September equinox and the equinox of the following March. The equinoxes are the two days each year (March 20 or 21 and September 22 or 23) when the Sun is directly over the geographical equator. The fact that the land heats up far more than the sea, which varies relatively little in temperature over the year, means that the thermal equator extends poleward more over the land than the sea.

The ITCZ also follows the apparent movement of the Sun, lagging by about one month, and is furthest from the geographical equator over land in the summer hemisphere. It is to be found at its extreme position in the Northern hemisphere in July and August, when it reaches 3° north over India and south-eastern Asia; and in the Southern hemisphere in January and February, when its location ranges from about 18° south over southern Africa and Australia to about 7° north over the eastern Pacific. In March and September it is over the equator. The zone of wind convergence does not coincide exactly with the low-pressure zone, being several degrees of latitude equatorward of it.

Because the ITCZ is associated with considerable cloud formation it plays an important role in rainfall generation and timing in the tropics. At the poleward limit of the ITCZ there is only one rainy season, elsewhere there are two rainy seasons, which occur just after the equinoxes, apart from the Amazon and Congo basins, and parts of south-eastern Asia, where there is no dry season.

The influence of the ITCZ on rainfall is exemplified by the south-west monsoonal rains of the Indian subcontinent. The movement of the thermal equator and ITCZ north over India and Pakistan during the Northern hemisphere summer draws in towards the low-pressure zone warm, moist air from over the Indian Ocean. This air moves first northward, and then because of the effects of the Earth’s rotation is diverted north-eastward. It begins to rise and cool, a result of convection and the meeting of the trade winds, eventually shedding its moisture as rain. Rainfall is increased over the subcontinent because the Himalaya ranges act as a barrier. Other major areas of monsoon associated with the movement of the ITCZ are eastern and south-eastern Asia, the western coast of Africa, and the northern coast of Australia. The movement of the ITCZ is also important for bringing rain to southern Africa and the Sahel region of western Africa. These areas, which have only one rainfall season, are particularly vulnerable to the changes that can occur in the movement of the ITCZ. In years when the ITCZ does not migrate as far south as usual, droughts can occur.

Recent research has indicated links between the movement of the ITCZ in some years, and associated rainfall amounts, particularly in southern Africa and Australia, and the appearance of the El Niño phenomenon. El Niño is an oceanic and atmospheric phenomenon in the Pacific Ocean, which occurs about every three to seven years, and during which unusually warm ocean conditions appear along the western coast of Ecuador and Peru, causing widespread climatic disturbances of varying severity. The climate disturbances caused by El Niño occur when ocean currents become sufficiently warm and persistent to cause a reversal in the normal weather conditions of the eastern and western Pacific. Normally, the waters of the western tropical Pacific are warm, with temperatures more than 10° C (18° F) higher than the eastern waters off Peru and Ecuador. This is because the easterly trade winds push Sun-warmed surface waters westward, allowing cold water to rise to the surface in the eastern Pacific. Not only is the water cold, but air pressure is high (in contrast to the ITCZ over the western Pacific) creating the typically arid conditions of west-coast South America.

During El Niño, however, the easterly trade winds collapse or even reverse. As the slight weakening of the winds causes a modest change in sea surface temperatures, the change in wind and pressure increases. The warm water of the western Pacific flows back eastward, and sea-surface temperatures increase significantly off the western coast of South America. As this happens, the wet-weather conditions associated with the ITCZ move to the east, and the arid conditions common in the east appear in the west. This brings heavy rains to South America and has caused drought in southern Africa and northern Australia. The El Niño event of 1982-1983 was the most severe of the 20th century.