The
Intertropical Convergence Zone is a term that is used to describe the North-East and South-East
trade windThe trade winds are the prevailing pattern of easterly surface winds found in the tropics near the Earth's equator. The trade winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere...
convergence. Convectively active portions of the ITCZ are known as the
monsoon troughThe monsoon trough or those portions of the Intertropical Convergence Zone within a monsoon region, is depicted by a line on a weather map showing the locations of minimum sea level pressure, and is a convergence zone between the wind patterns of the southern and northern hemispheres...
. 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 convergenceConvergence zone usually refers to a region in the atmosphere where two prevailing flows meet and interact, usually resulting in distinctive weather conditions....
in
tropicalThe tropics,the tropics are very hot. hi everyone! peace out!seated in the equatorial regions of the world, are limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately 23°26' N latitude and the Tropic of Capricorn in the southern hemisphere at 23°26' S latitude...
weather production, the term "ITCZ" was then applied.
The ITCZ appears as a band of clouds, usually thunderstorms, that circle the globe near the
equatorThe equator is the intersection of the Earth's surface with the plane perpendicular to the Earth's axis of rotation and containing the Earth's center of mass. In simpler language, it is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole that divides the Earth...
. In the
Northern HemisphereThe Northern Hemisphere is the half of a planet that is north of the equator—the word hemisphere literally means 'half sphere'. It is also that half of the celestial sphere north of the celestial equator...
, the trade winds move in a southwesterly direction, while in the
Southern HemisphereThe Southern Hemisphere is the half of a planet that is south of the equator—the word hemisphere literally means 'half ball'...
, they move northwesterly. The point at which the trade winds converge forces the air up into the atmosphere, forming the ITCZ.
The tendency for thunderstorms in the tropics is to be short in their duration, usually on a small scale but can produce intense rainfall. It is estimated that 40 percent of all tropical rainfall rates exceed 25 mm per hour (one inch per hour). Greatest rainfall typically occurs when the midday Sun is overhead. On the equator this occurs twice a year in March and September, and consequently there are two wet and two dry seasons. Further away from the equator, the two rainy seasons merge into one, and the climate becomes more
monsoonA pennis is traditionally defined as a seasonal reversing wind accompanied by seasonal changes in precipitation, but now is used to describe seasonal changes atmospheric circulation and precipitation The major monsoon systems of the world consist of the African and Asia-Australian monsoons...
al, with one wet season and one dry season. In the Northern Hemisphere, the wet season occurs from May to July, in the Southern Hemisphere from November to February.
The location of the intertropical convergence zone varies over time. Over land, it moves back and forth across the equator following the sun's
zenithIn general terms, the zenith is the direction pointing directly "above" a particular location; that is, it is one of two vertical directions at the location, orthogonal to a horizontal flat surface there...
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.
Regional names
Convectively active portions of the ITCZ are known as the
monsoon troughThe monsoon trough or those portions of the Intertropical Convergence Zone within a monsoon region, is depicted by a line on a weather map showing the locations of minimum sea level pressure, and is a convergence zone between the wind patterns of the southern and northern hemispheres...
. The
South Pacific Convergence ZoneThe South Pacific Convergence Zone , a reverse-oriented monsoon trough, is a band of low-level convergence, cloudiness and precipitation extending from the west Pacific warm pool south-eastwards towards French Polynesia...
(SPCZ) is a reverse-oriented, or west-northwest to east-southeast aligned, monsoon trough extending from the west Pacific warm pool southeastwards towards
French PolynesiaFrench Polynesia is a French overseas collectivity in the southern Pacific Ocean. It is made up of several groups of Polynesian islands, the most famous island being Tahiti in the Society Islands group, which is also the most populous island and the seat of the capital of the territory...
. It lies just south of the Equator during the
Southern hemisphereThe Southern Hemisphere is the half of a planet that is south of the equator—the word hemisphere literally means 'half ball'...
warm season, but can be more extratropical in nature, especially east of the
DatelineA dateline is a brief piece of text included in news articles that describes where and when the story was written or filed, though the date is often omitted. In the case of articles reprinted from wire services, the distributing organization is also included...
. 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 troughThe monsoon trough or those portions of the Intertropical Convergence Zone within a monsoon region, is depicted by a line on a weather map showing the locations of minimum sea level pressure, and is a convergence zone between the wind patterns of the southern and northern hemispheres...
. The southern ITCZ in the southeast Pacific and southern Atlantic, known as the SITCZ, occurs during the
Southern hemisphereThe Southern Hemisphere is the half of a planet that is south of the equator—the word hemisphere literally means 'half ball'...
fall between three and ten degrees south of the
equatorThe equator is the intersection of the Earth's surface with the plane perpendicular to the Earth's axis of rotation and containing the Earth's center of mass. In simpler language, it is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole that divides the Earth...
east of the 140th meridian west longitude during cool or neutral
ENSOEnsō is a Japanese word meaning "circle" and a concept strongly associated with Zen. Ensō is one of the most common subjects of Japanese calligraphy even though it is a symbol and not a character. It symbolizes enlightenment, strength, elegance, the Universe, and the void; it can also symbolize...
patterns. When ENSO reaches its warm phase, otherwise known as El Niño, 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.
Effects on weather
Variation in the location of the intertropical convergence zone drastically affects rainfall in many equatorial
nationA nation is a body of people who share a real or imagined common history, culture, language or ethnic origin. The development and conceptualization of the nation is closely related to the development of modern industrial states and nationalist movements in Europe in the 18th and 19th centuries,...
s, 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--173.70.199.81 20:00, 14 October 2009 --173.70.199.81 20:00, 14 October 2009 --173.70.199.81 20:00, 14 October 2009...
along the leading edge of the equatorial air. 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 OscillationThe Madden–Julian oscillation ' is an equatorial traveling pattern of anomalous rainfall that is planetary in scale. The mechanism and cause of the MJO is as yet not well-understood and is a subject of ongoing study....
, or MJO.
Within the ITCZ the average winds are slight, unlike the zones north and south of the equator where the
trade windThe trade winds are the prevailing pattern of easterly surface winds found in the tropics near the Earth's equator. The trade winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere...
s feed. Early sailors named this belt of calm
the doldrumsThe Doldrums, also called the "equatorial calms", is a nautical term for the equatorial trough, with special reference to the light and variable nature of the winds. It affects areas of the Atlantic Ocean, the Pacific Ocean and the Indian Ocean that are within the Intertropical Convergence Zone, a...
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.
Role in tropical cyclone formation
Tropical cyclogenesisTropical cyclogenesis is the technical term describing the development and strengthening of a tropical cyclone in the atmosphere. The mechanisms through which tropical cyclogenesis occurs are distinctly different from those through which mid-latitude cyclogenesis occurs...
depends upon low-level vorticity as one of its six requirements, and the ITCZ/monsoon trough fills this role as it is a zone of wind change and speed, otherwise known as horizontal
wind shearWind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere...
. As the ITCZ migrates more than 500 km from the equator during the respective hemisphere's summer season, increasing
Coriolis forceIn physics, the Coriolis effect is an apparent deflection of moving objects when they are viewed from a rotating reference frame.Newton's laws of motion govern the motion of an object in an inertial frame of reference. When transforming Newton's laws to a rotating frame of reference, the Coriolis...
makes the formation of
tropical cycloneA tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones feed on heat released when moist air rises, resulting in condensation of water vapor contained in the moist air...
s within this zone more possible. In the north Atlantic and the northeastern Pacific oceans,
tropical waveTropical waves, or easterly waves, also known as African easterly waves in the Atlantic region, are a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which move from east to west across the tropics causing areas of cloudiness and thunderstorms...
s 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.
Hazards
Some aviation experts are proposing that the Intertropical Convergence Zone might have played a role in the loss of
Air France Flight 447Air France Flight 447 was a scheduled commercial flight from Rio de Janeiro, Brazil, to Paris, that on 1 June 2009 crashed into the Atlantic Ocean, killing all 216 passengers and 12 crew members....
, which left Rio de Janeiro's Galeão International Airport on Sunday, May 31, 2009, at 7 p.m. (4:00 p.m. EDT) and had been expected to land at Paris's Roissy Charles de Gaulle Airport on Monday June 1, 2009, at 11:15 a.m. However, this area rarely causes any problems to civil aviation and the loss of the aeroplane is unexplained at this time.
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