Intertropical Convergence Zone

Intertropical Convergence Zone

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The Intertropical Convergence Zone (ITCZ), known by sailors as The Doldrums, is the area encircling the earth near the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

 where winds originating in the northern and southern hemispheres 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
Convergence zone
Convergence zone usually refers to a region in the atmosphere where two prevailing flows meet and interact, usually resulting in distinctive weather conditions....

 in tropical
Tropics
The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately  N and the Tropic of Capricorn in the southern hemisphere at  S; these latitudes correspond to the axial tilt of the Earth...

 weather production, the term "ITCZ" was then applied. When it lies near the equator, it is called the near-equatorial trough
Near-equatorial trough
Since 1954, studies concluded that the near-equatorial trough was associated with a maximum in oceanic sea surface temperature. It is sometimes used as a synonym for the terms near equatorial tradewind convergence, Intertropical convergence zone, and monsoon trough. Surges of higher pressure from...

. Where the ITCZ is drawn into and merges with a monsoonal circulation
Monsoon
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea...

, it is sometimes referred to as a monsoon trough
Monsoon trough
The monsoon trough is that portion of the Intertropical Convergence Zone which extends into or through a monsoon circulation, as depicted by a line on a weather map showing the locations of minimum sea level pressure, and as such, is a convergence zone between the wind patterns of the southern and...

, 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
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

. In the Northern Hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its 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 southwestern direction from the northeast, while in the Southern Hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...

, 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
Coriolis effect
In physics, the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right...

 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
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

 activity of thunderstorm
Thunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...

s 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
Hadley cell
The Hadley cell, named after George Hadley, is a circulation pattern that dominates the tropical atmosphere, with rising motion near the equator, poleward flow 10–15 kilometers above the surface, descending motion in the subtropics, and equatorward flow near the surface...

, and is wet. The dry descending branch is the horse latitudes
Horse latitudes
Horse Latitudes or Subtropical High are subtropical latitudes between 30 and 35 degrees both north and south. This region, under a ridge of high pressure called the subtropical high, is an area which receives little precipitation and has variable winds mixed with calm.The consistently warm, dry...

.

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
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...

 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.

South Pacific Convergence Zone


The South Pacific Convergence Zone
South Pacific convergence zone
The 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, trough extending from the west Pacific warm pool southeastwards towards French Polynesia
French Polynesia
French Polynesia is an overseas country of the French Republic . 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 hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...

 warm season, but can be more extratropical in nature, especially east of the International Date Line
International Date Line
The International Date Line is a generally north-south imaginary line on the surface of the Earth, passing through the middle of the Pacific Ocean, that designates the place where each calendar day begins...

. It is considered the largest and most important piece of the ITCZ, and has the least dependence upon heating from a nearby land mass
Land mass
Land mass refers to the total area of a country or geographical region . The Earth's total land mass is which is about 29.2% of its total surface. Water covers approximately 70.8% of the Earth's surface, mostly in the form of oceans....

 during the summer than any other portion of the monsoon trough
Monsoon trough
The monsoon trough is that portion of the Intertropical Convergence Zone which extends into or through a monsoon circulation, as depicted by a line on a weather map showing the locations of minimum sea level pressure, and as such, is a convergence zone between the wind patterns of the southern and...

. The southern ITCZ in the southeast Pacific and southern Atlantic, known as the SITCZ, occurs during the Southern hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...

 fall between
3rd parallel south
The 3rd parallel south is a circle of latitude that is 3 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Southeast Asia, Australasia, the Pacific Ocean and South America....

 and 10° south
10th parallel south
The 10th parallel south is a circle of latitude that is 10 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Australasia, the Pacific Ocean and South America....

 of the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

 east of the 140th meridian west
140th meridian west
The meridian 140° west of Greenwich is a line of longitude that extends from the North Pole across the Arctic Ocean, North America, the Pacific Ocean, the Southern Ocean, and Antarctica to the South Pole....

 longitude during cool or neutral ENSO
Enso
Ensō 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 the Absolute enlightenment, strength, elegance, the Universe, and the void; it can...

 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 nation
Nation
A nation may refer to a community of people who share a common language, culture, ethnicity, descent, and/or history. In this definition, a nation has no physical borders. However, it can also refer to people who share a common territory and government irrespective of their ethnic make-up...

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
Weather front
A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front...

 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 Oscillation
Madden-Julian oscillation
The Madden–Julian oscillation ' is the largest element of the intraseasonal variability in the tropical atmosphere. It is a large-scale coupling between atmospheric circulation and tropical deep convection...

, or MJO.

Within the ITCZ the average winds are slight, unlike the zones north and south of the equator where the trade wind
Trade wind
The trade winds are the prevailing pattern of easterly surface winds found in the tropics, within the lower portion of the Earth's atmosphere, in the lower section of the troposphere near the Earth's equator...

s 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 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
Tropical cyclogenesis
Tropical cyclogenesis is the term that describes 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 fills this role as it is a zone of wind change and speed, otherwise known as horizontal wind shear
Wind shear
Wind 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 force
Coriolis effect
In physics, the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right...

 makes the formation of tropical cyclone
Tropical cyclone
A 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 strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...

s within this zone more possible. In the north Atlantic and the northeastern Pacific oceans, tropical wave
Tropical wave
Tropical waves, easterly waves, or tropical 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...

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


Thunderstorms along the Intertropical Convergence Zone played a role in the loss of Air France Flight 447
Air France Flight 447
Air France Flight 447 was a scheduled airline flight from Rio de Janeiro-Galeão to Paris-Roissy involving an Airbus A330-200 aircraft that crashed into the Atlantic Ocean on 1 June 2009, killing all 216 passengers and 12 aircrew. The investigation is still ongoing, and the cause of the...

, which left Rio de Janeiro-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. The aircraft crashed with no survivors while flying through a large ITCZ thunderstorm, although the exact manner in which the aircraft was lost is not yet concluded. Ice forming rapidly on airspeed sensors is the leading theory at this time. Most aircraft flying these routes are able to avoid the larger convective cells without incident.

See also


List of countries located on the Equator

External links

  • The ITCZ in Africa via the University of South Carolina
    University of South Carolina
    The University of South Carolina is a public, co-educational research university located in Columbia, South Carolina, United States, with 7 surrounding satellite campuses. Its historic campus covers over in downtown Columbia not far from the South Carolina State House...

  • A Shifting Band of Rain from March 2011 Scientific American
    Scientific American
    Scientific American is a popular science magazine. It is notable for its long history of presenting science monthly to an educated but not necessarily scientific public, through its careful attention to the clarity of its text as well as the quality of its specially commissioned color graphics...