Intertropical Convergence Zone

The Intertropical Convergence Zone (ITCZ), known by sailors as the doldrums, is the area encircling the earth near the equator where the northeast and southeast trade winds come together.

The ITCZ was originally identified from the 1920s to the 1940s as the "Intertropical Front" (ITF), but after the recognition in the 1940s and 1950s of the significance of wind field convergence in tropical weather production, the term "ITCZ" was then applied. 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 to 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.

Convergence zone

A convergence zone in meteorology is a region in the atmosphere where two prevailing flows meet and interact, usually resulting in distinctive weather conditions. This create a mass accumulation that eventually leads to a vertical movement and the formation of clouds and precipitation. Large scale convergence, called synoptic scale convergence, provides weather systems such as baroclinic troughs, low-pressure areas, and cyclones . Small scale convergence will give from isolated cumulus clouds to large areas of thunderstorms. The inverse of the convergence is the divergence.

Large scale

An example of a convergence zone is the Intertropical Convergence Zone (ITCZ), a low pressure area which girdles the Earth at the Equator. Another example is the South Pacific convergence zone that extends from the western Pacific Ocean toward French Polynesia.

Mesoscale

Convergence zones also occur at a smaller scale. Some examples are the Puget Sound Convergence Zone which occurs in the Puget Sound region in the U.S. state of Washington; Mohawk–Hudson convergence in the U.S. state of New York; the Brown Willy effect which can be generated when south-westerly winds blow over Bodmin Moor in Cornwall; and the Pembrokeshire Dangler which can form when northerly winds blow down the Irish Sea. They can also be associated with sea breeze fronts.