Intertropical Convergence Zone (ITCZ)
The Intertropical Convergence Zone (ITCZ), is the area encircling the earth near the equator where winds originating in the northern and southern hemispheres come together.
When it lies near the equator, it is called the near-equatorial trough. Where the ITCZ is drawn into and merges with a monsoonal circulation, it is sometimes referred to as a monsoon trough, a usage more common in Australia and parts of Asia.
In the seamen’s speech the zone is referred as the doldrums because of its erratic weather patterns with stagnant calms and violent thunderstorms.
The ITCZ appears as a band of clouds, usually thunderstorms, that circle the globe near the equator. In the Northern Hemisphere, the trade winds move in a southwestern direction from the northeast, while in the Southern Hemisphere, they move northwestward from the southeast. When the ITCZ is positioned north or south of the equator, these directions change according to the Coriolis effect imparted by the rotation of the earth. For instance, when the ITCZ is situated north of the equator, the southeast trade wind changes to a southwest wind as it crosses the equator.
The ITCZ is formed by vertical motion largely appearing as convective activity of thunderstorms driven by solar heating, which effectively draw air in; these are the trade winds.
The ITCZ is effectively a tracer of the ascending branch of the Hadley cell, and is wet. The dry descending branch is the horse latitudes.
The location of the intertropical convergence zone varies over time. Over land, it moves back and forth across the equator following the sun’s zenith point.
Over the oceans, where the convergence zone is better defined, the seasonal cycle is more subtle, as the convection is constrained by the distribution of ocean temperatures. Sometimes, a double ITCZ forms, with one located north and another south of the equator. When this occurs, a narrow ridge of high pressure forms between the two convergence zones, one of which is usually stronger than the other.
In summer the ITCZ will be well north of the Equator, it cuts the West African coast at about Dakar so the SE trades cross the equator, change direction and become SW monsoons affecting West Africa.
In winter the ITCZ is only slightly north of the equator, at about 3 to 5 degrees latitude north, so the wind affecting West Africa then will be the NE Trade winds. There will still be a very small area that still gets SW monsoons, but it is negligible.
The ITCZ will reach Northern Australia at its furthest south and the winds changing direction in the Southern Hemisphere will become the NW Monsoon bringing extremely bad weather to Papua New Guinea and Darwin.
Exam Question Tips:
The inter-tropical convergence zone (ITCZ) particularly affects, western Africa between 10 and 20 deg N and the northern coasts of the Arabian sea in July.
Regarding the movement of the ITCZ in the region of West Africa: It reaches its maximum northerly position of 15 to 20 deg N in July. Its in the vicinity of Dakar (Dakar latitude is 14.6 deg N).
ITCZ is most likely to be encountered in January, between Dakar and Rio de Janeiro, in bands of latitude from 0-7 deg North.
The broken (dashed) line labelled “Z” and “Y” is the mean position of the ITCZ in July and January respectively: