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. Consequences of the ITCZ’s movement are the “monsoon seasons” that affect areas in the tropics, as well as the formation of tropical storms during the local hemispheric summer (north or south of the equator). Navigators in the age of sail learned to utilize the “trade winds” that blow to the east along the edge of the ITCZ.
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. Weather phenomenon formed when trade winds converge in tropics and rise to form rain (nimbus) clouds. ITCZ is characterized by heavy rainfall and is responsible for most of the precipitation in Africa. 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 aries 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. Doldrums- An area of calm found at the equator. Also called the Intertropical Convergence Zone (ITCZ) •Tropical Easterlies – When warm air from the equator rises, it cools, and flows back toward the equator. It appears to flow to the west because of the Coriolis Effect. •Prevailing Westerlies – When air moves toward the poles, it flows from west to east. •Polar Easterlies – Air over the poles cools and sinks back down, it eventually returns to the equator. South Pacific Convergence Zone
The South Pacific Convergence Zone (SPCZ) is a reverse-oriented, or west-northwest to east-southeast aligned, trough extending from the west Pacific warm pool southeastwards towards French Polynesia. It lies just south of the Equator during the Southern hemisphere warm season, but can be more extratropical in nature, especially east of the International Date Line. It is considered the largest and most important piece of the ITCZ, and has the least dependence upon heating from a nearby landmass during the summer than any other portion of the monsoon trough. 3] The southern ITCZ in the southeast Pacific and southern Atlantic, known as the SITCZ, occurs during the Southern hemisphere fall between 3° and 10° south of the equator east of the 140th meridian west longitude during cool or neutral ENSO patterns. When ENSO reaches its warm phase, otherwise known as El Nino, the tongue of lowered sea surface temperatures due to upwelling off the South American continent disappears, which causes this convergence zone to vanish as well. [4] Effects on weather
The ITCZ moves farther away from the equator during the Northern summer than the Southern one due to the North-heavy arrangement of the continents. Variation in the location of the intertropical convergence zone drastically affects rainfall in many equatorial nations, resulting in the wet and dry seasons of the tropics rather than the cold and warm seasons of higher latitudes. Longer term changes in the intertropical convergence zone can result in severe droughts or flooding in nearby areas.
In some cases, the ITCZ may become narrow, especially when it moves away from the equator; the ITCZ can then be interpreted as a front along the leading edge of the equatorial air. [5] There appears to be a 15-25 day cycle in thunderstorm activity along the ITCZ, which is roughly half the wavelength of the Madden-Julian Oscillation, or MJO. [6] Within the ITCZ the average winds are slight, unlike the zones north and south of the equator where the trade winds feed. Early sailors named this belt of calm the doldrums because of the inactivity and stagnation they found themselves in after days of no wind.
To find oneself becalmed in this region in a hot and muggy climate could mean death in an era when wind was the only effective way to propel ships across the ocean. Even today the leisure and competitive sailors attempt to cross the zone as quickly as possible as the erratic weather and wind patterns may cause unexpected delays. Role in tropical cyclone formation Tropical cyclogenesis depends upon low-level vorticity as one of its six requirements, and the ITCZ fills this role as it is a zone of wind change and speed, otherwise known as horizontal wind shear.
As the ITCZ migrates more than 500 km from the equator during the respective hemisphere’s summer season, increasing Coriolis force makes the formation of tropical cyclones within this zone more possible. In the north Atlantic and the northeastern Pacific oceans, tropical waves move along the axis of the ITCZ causing an increase in thunderstorm activity, and under weak vertical wind shear, these clusters of thunderstorms can become tropical cyclones. characteristic for Intertropical convergence zone — Heavy precipitation, little wind convection cell in the atmosphere borders on the intertropical convergence zone — hadley
Does the Intertropical Convergence Zone have high pressure with rising air? Low pressure is associated with rising air. The ITCZ is an area of low pressure. Characteristics of an Intertropical Convergence Zone By Wanda Marie Thibodeaux, eHow Contributor Many of the Earth’s clouds form in the Intertropical Convergence Zone. Weather on the Earth is driven by multiple factors, including thermal energy from within the Earth’s core and from the sun. Certain areas of the Earth are known for specific weather patterns that occur as a result of these factors.
One area that scientists, geologists and meteorologists study frequently is the Intertropical Convergence Zone, which is a band near the equator where the southern and northern trade winds meet. Low Air Pressure 1. In the Intertropical Convergence Zone, the northern and southern trade winds come together. Because of the rotation of the Earth, the winds cannot really cross the equator without losing energy. Instead of continuing over the Earth horizontally, the winds thus move vertically toward the upper atmosphere.
The heating of the Earth’s ocean currents by the sun assists in this process, making the air warmer and letting it rise. The result is that the Intertropical Convergence Zone has low air pressure near the Earth’s surface. The lack of horizontal wind movement in the region caused sailors to nickname the Intertropical Convergence Zone, “the doldrums. ” The frequent rising of air in the Intertropical Convergence Zone means that moisture constantly is being brought high enough in the atmosphere to a point cool enough to allow the moisture to condense into clouds.
The Intertropical Convergence Zone therefore can see incredible amounts of precipitation and high humidity. Although some areas of the zone do have a dry season, others do not. Afternoon showers are a feature of the zone. Storm Rainfall in the Intertropical Convergence Zone typically is not gentle rainfall that lasts for long periods. Instead, the high amounts of energy from thermal and solar heating cause moisture to condense quickly into clouds in the hottest part of the day. Circular typhoons thus often form as the air currents move.
Some of the strongest winds on the Earth have been recorded in these storms. Thunderstorms with heavy lightening also are common. The Intertropical Convergence Zone is characterized by inconsistent location around the equator. As the Earth moves with the seasons, the area which receives the highest amount of heat energy from the sun varies. The thermal equator around which the Intertropical Convergence Zone forms thus moves, depending on the season. In some cases, this shift can result in the complete reversal of normal trade wind patterns, particularly in the Indian Ocean.
The characteristics of the Intertropical Convergence Zone have an enormous impact on weather all around the globe. Shifting of wind patterns in the Intertropical Convergence Zone can move thermal energy and moisture to different parts of the Earth than usual and can slow or even stop ocean currents. This affects all plant and animal life either directly or indirectly, since ecosystems are dependent largely on weather patterns and temperature. Seasonal Migration of the ITCZ in Africa The migration of the inter-tropical convergence zone (ITCZ) n Africa affects seasonal precipitation patterns across that continent. The blue shading on the map shows the areas of highest cloud reflectivity, which correspond to the average monthly position of the ITCZ. Use the links below to explore the seasonal progression of the ITCZ and how it affects the climate at selected stations ITCZ Disturbance Sometimes as a tropical wave passes a cluster of thunderstorms develops and separates from the ITCZ. If the cluster of thunderstorms persists for 24 hours it is called an ITCZ disturbance.
