Intertropical Convergence Zone

Last updated
The ITCZ is visible as a band of clouds encircling Earth near the Equator. IntertropicalConvergenceZone-EO.jpg
The ITCZ is visible as a band of clouds encircling Earth near the Equator.

The Intertropical Convergence Zone (ITCZ), known by sailors as the doldrums or the calms because of its monotonous, windless weather, is the area where the northeast and southeast trade winds converge. It encircles Earth near the thermal equator, though its specific position varies seasonally. When it lies near the geographic 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.

Contents

Meteorology

The ITCZ was originally identified from the 1920s to the 1940s as the Intertropical Front (ITF), but after the recognition in the 1940s and the 1950s of the significance of wind field convergence in tropical weather production, the term Intertropical Convergence Zone (ITCZ) was then applied. [1]

The ITCZ appears as a band of clouds, usually thunderstorms, that encircle the globe near the Equator. In the Northern Hemisphere, the trade winds move in a southwestward 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 Earth's rotation. 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. [2] 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 ITCZ gradually varies with the seasons, roughly corresponding with the location of the thermal equator. As the heat capacity of the oceans is greater than air over land, migration is more prominent over land. 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. [3] Sometimes, a double ITCZ forms, with one located north and another south of the Equator, one of which is usually stronger than the other. When this occurs, a narrow ridge of high pressure forms between the two convergence zones.

ITCZ over oceans vs. land

Seasonal variability of the Intertropical Convergence Zone (ITCZ), Congo air boundary (CAB), tropical rainbelt, and surface winds over Africa (adapted from Dezfuli 2017 with modification). This schematic shows that the ITCZ and the region of maximum rainfall can be decoupled over the continents. ITCZ Africa.png
Seasonal variability of the Intertropical Convergence Zone (ITCZ), Congo air boundary (CAB), tropical rainbelt, and surface winds over Africa (adapted from Dezfuli 2017 with modification). This schematic shows that the ITCZ and the region of maximum rainfall can be decoupled over the continents.

The ITCZ is commonly defined as an equatorial zone where the trade winds converge. Rainfall seasonality is traditionally attributed to the north–south migration of the ITCZ, which follows the sun. Although this is largely valid over the equatorial oceans, the ITCZ and the region of maximum rainfall can be decoupled over the continents. [4] [5] The equatorial precipitation over land is not simply a response to just the surface convergence. Rather, it is modulated by a number of regional features such as local atmospheric jets and waves, proximity to the oceans, terrain-induced convective systems, moisture recycling, and spatiotemporal variability of land cover and albedo. [4]

South Pacific convergence zone

Vertical air velocity at 500 hPa, July average. Ascent (negative values) is concentrated close to the solar equator; descent (positive values) is more diffuse Omega-500-july-era40-1979.png
Vertical air velocity at 500 hPa, July average. Ascent (negative values) is concentrated close to the solar equator; descent (positive values) is more diffuse

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 land mass during the summer than any other portion of the monsoon trough. [6] The southern ITCZ in the southeast Pacific and southern Atlantic, known as the SITCZ, occurs during the Southern Hemisphere fall between and 10° south of the equator east of the 140th meridian west longitude during cool or neutral El Niño–Southern Oscillation (ENSO) 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. [7]

Effects on weather

The ITCZ moves further away from the equator during the Northern summer than the Southern one due to the North-heavy arrangement of the continents. ITCZ january-july.png
The ITCZ moves further 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. [8] There appears to be a 15 to 25-day cycle in thunderstorm activity along the ITCZ, which is roughly half the wavelength of the Madden–Julian oscillation (MJO). [9]

Within the ITCZ the average winds are slight, unlike the zones north and south of the equator where the trade winds feed. As trans-equator sea voyages became more common, sailors in the eighteenth century named this belt of calm the doldrums because of the calm, stagnant, or inactive winds.

Role in tropical cyclone formation

Hurricanes Celia and Darby in the eastern Pacific and the precursor to Hurricane Alex in the Intertropical Convergence Zone. ITCZ jun 25 2010.jpg
Hurricanes Celia and Darby in the eastern Pacific and the precursor to Hurricane Alex in the Intertropical Convergence Zone.

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 to tropical - subtropical latitudes and even beyond (Shandong province of the People's Republic of China) during the respective hemisphere's summer season, increasing Coriolis force makes the formation of tropical cyclones within this zone more possible. Surges of higher pressure from high latitudes can enhance tropical disturbances along its axis. [10] 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 clusters of thunderstorms can develop under weak vertical wind shear.

Hazards

Thunderstorms along the Intertropical Convergence Zone played a role in the loss of Air France Flight 447, which left Rio de Janeiro–Galeão International Airport on Sunday, May 31, 2009, at about 7:00 p.m. local time (6:00 p.m. EDT or 10:00 p.m. UTC) and had been expected to land at Charles de Gaulle Airport near Paris on Monday, June 1, 2009, at 11:15 a.m. (5:15 a.m. EDT or 9:15 a.m. UTC). [11] The aircraft crashed with no survivors while flying through a series of large ITCZ thunderstorms, and ice forming rapidly on airspeed sensors was the precipitating cause for a cascade of human errors which ultimately doomed the flight. Most aircraft flying these routes are able to avoid the larger convective cells without incident.[ citation needed ]

In the Age of Sail, to find oneself becalmed in this region in a hot and muggy climate could mean death when wind was the only effective way to propel ships across the ocean. Calm periods within the doldrums could strand ships for days or weeks.[ citation needed ] 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.

In literature

The doldrums are notably described in Samuel Taylor Coleridge's poem The Rime of the Ancient Mariner (1798).

See also

Notes

  1. Barry, Roger Graham; Chorley, Richard J. (1992). Atmosphere, weather, and climate . London: Routledge. ISBN   978-0-415-07760-6. OCLC   249331900. Atmosphere, weather, and climate.
  2. "Inter-Tropical Convergence Zone". JetStream - Online School for Weather. NOAA. 2007-10-24. Retrieved 2009-06-04.
  3. "Inter Tropical Convergence Zone (ITCZ) - SKYbrary Aviation Safety". www.skybrary.aero. Retrieved 2018-04-12.
  4. 1 2 3 Dezfuli, Amin (2017-03-29). "Climate of Western and Central Equatorial Africa". Oxford Research Encyclopedia of Climate Science. doi:10.1093/acrefore/9780190228620.013.511.
  5. Nicholson, Sharon E. (February 2018). "The ITCZ and the Seasonal Cycle over Equatorial Africa". Bulletin of the American Meteorological Society. 99 (2): 337–348. doi:10.1175/bams-d-16-0287.1. ISSN   0003-0007.
  6. E. Linacre and B. Geerts. Movement of the South Pacific convergence zone Retrieved on 2006-11-26.
  7. Semyon A. Grodsky; James A. Carton (2003-02-15). "The Intertropical Convergence Zone in the South Atlantic and the Equatorial Cold Tongue" (PDF). University of Maryland, College Park. Retrieved 2009-06-05.
  8. Djurić, D: Weather Analysis. Prentice Hall, 1994. ISBN   0-13-501149-3.
  9. Patrick A. Harr. Tropical Cyclone Formation/Structure/Motion Studies. Office of Naval Research Retrieved on 2006-11-26. Archived November 29, 2007, at the Wayback Machine
  10. C.-P. Chang, J.E. Erickson, and K.M. Lau. Northeasterly Cold Surges and Near-Equatorial Disturbances over the Winter MONEX Area during December 1974. Part I: Synoptic Aspects. Retrieved on 2007-04-26.
  11. "Q & A Turbulences" 1.June.2009 The Guardian

Related Research Articles

Horse latitudes subtropical latitudes 30–35 degrees north and south

Horse latitudes, subtropical ridges or subtropical highs are the subtropical latitudes between 30 and 35 degrees both north and south where Earth's atmosphere is dominated by the subtropical high, an area of high pressure, which suppresses precipitation and cloud formation, and has variable winds mixed with calm winds.

Anticyclone Weather phenomenon which is the opposite of a cyclone

An anticyclone is a weather phenomenon defined by the United States National Weather Service's glossary as "a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere". Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure. Mid-tropospheric systems, such as the subtropical ridge, deflect tropical cyclones around their periphery and cause a temperature inversion inhibiting free convection near their center, building up surface-based haze under their base. Anticyclones aloft can form within warm core lows such as tropical cyclones, due to descending cool air from the backside of upper troughs such as polar highs, or from large scale sinking such as the subtropical ridge. The evolution of an anticyclone depends upon variables such as its size, intensity, and extent of moist convection, as well as the Coriolis force.

Atmospheric circulation The large-scale movement of air, a process which distributes thermal energy about the Earths surface

Atmospheric circulation is the large-scale movement of air and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth.

Tropical climate climate in the tropical regiop

A tropical climate in the Köppen climate classification is a non-arid climate in which all twelve months have mean temperatures warmer than 18 °C (64 °F). Tropical climates are normally found from the equator to 25 north and south latitude. Tropical climates are typically frost-free, and changes in the solar angle are small since they occupy low latitudes. In tropical climates, the temperature remains relatively constant throughout the year. Sunlight is intense.

Low-pressure area region where the atmospheric pressure is lower than that of surrounding locations

A low-pressure area, low, low area or low is a region on the topographic map where the air pressure is lower than that of surrounding locations. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the atmosphere. The formation process of a low-pressure area is known as cyclogenesis. Within the field of meteorology, atmospheric divergence aloft occurs in two areas. The first area is on the east side of upper troughs, which form half of a Rossby wave within the Westerlies. A second area of wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength. Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity.

Trade winds permanent east-to-west prevailing winds that flow in the Earths equatorial region

The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere, strengthening during the winter and when the Arctic oscillation is in its warm phase. Trade winds have been used by captains of sailing ships to cross the world's oceans for centuries and enabled colonial expansion into the Americas and trade routes to become established across the Atlantic and Pacific oceans.

Tropical wave type of atmospheric trough

A tropical wave, in and around the Atlantic Ocean, is a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which moves from east to west across the tropics, causing areas of cloudiness and thunderstorms. Tropical waves form in the easterly flow along the equatorward side of the subtropical ridge or belt of high air pressure which lies north and south of the Intertropical Convergence Zone (ITCZ). Tropical waves are generally carried westward by the prevailing easterly winds along the tropics and subtropics near the equator. They can lead to the formation of tropical cyclones in the north Atlantic and northeastern Pacific basins. A tropical wave study is aided by Hovmöller diagrams, a graph of meteorological data.

The South Pacific Convergence Zone (SPCZ), a reverse-oriented monsoon trough, is a band of low-level convergence, cloudiness and precipitation extending from the Western Pacific Warm Pool at the maritime continent south-eastwards towards French Polynesia and as far as the Cook Islands. The SPCZ is a portion of the Intertropical Convergence Zone (ITCZ) which lies in a band extending east-west near the Equator 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. The SPCZ can affect the precipitation on Polynesian islands in the southwest Pacific Ocean, so it is important to understand how the SPCZ behaves with large-scale, global climate phenomenon, such as the ITCZ, El Niño–Southern Oscillation, and the Interdecadal Pacific oscillation (IPO), a portion of the Pacific decadal oscillation.

Equatorial Counter Current A shallow eastward flowing current found in the Atlantic, Indian, and Pacific Oceans

The Equatorial Counter Current is an eastward flowing, wind-driven current which extends to depths of 100-150m in the Atlantic, Indian, and Pacific Oceans. More often called the North Equatorial Countercurrent (NECC), this current flows west-to-east at about 3-10°N in the Atlantic, Indian Ocean and Pacific basins, between the North Equatorial Current (NEC) and the South Equatorial Current (SEC). The NECC is not to be confused with the Equatorial Undercurrent (EUC) that flows eastward along the equator at depths around 200m in the western Pacific rising to 100m in the eastern Pacific.

Convergence zone meteorological phenomenon

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 causes a mass accumulation that eventually leads to a vertical movement and to the formation of clouds and precipitation. Large-scale convergence, called synoptic-scale convergence, is associated with weather systems such as baroclinic troughs, low-pressure areas, and cyclones. The large-scale convergence zone formed over the equator, Hadley Cell, has condensed and intensified as a result of the global increase in temperature. Small-scale convergence will give phenomena from isolated cumulus clouds to large areas of thunderstorms.

Geography of South America

The geography of South America contains many diverse regions and climates. Geographically, South America is generally considered a continent forming the southern portion of the landmass of the Americas, south and east of the Panama–Colombia border by most authorities, or south and east of the Panama Canal by some. South and North America are sometimes considered a single continent or supercontinent, while constituent regions are infrequently considered subcontinents.

Hadley cell A global scale tropical atmospheric circulation feature

The Hadley cell, named after George Hadley, is a global scale tropical atmospheric circulation that features air rising near the Equator, flowing poleward at a height of 10 to 15 kilometers above the earth's surface, descending in the subtropics, and then returning equatorward near the surface. This circulation creates the trade winds, tropical rain-belts and hurricanes, subtropical deserts and the jet streams. Hadley cells are the low-altitude overtuning circulation that have air sinking at roughly zero to 30 degree latitude.

Monsoon trough

The monsoon trough is a portion of the Intertropical Convergence Zone in the Western Pacific, 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 northern hemispheres.

Tropical cyclogenesis

Tropical cyclogenesis is 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 temperate cyclogenesis occurs. Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment.

Tropical rainforest climate type of tropical climate in which there is no dry season

A tropical rainforest climate is a tropical climate usually found within 10 to 15 degrees latitude of the equator, and has at least 60 mm of rainfall every month of the year. Regions with this climate are typically designated Af by the Köppen climate classification. A tropical rainforest climate is typically hot, very humid and wet.

Tropical monsoon climate climate subtype in the Köppen climate classification system

An area of tropical monsoon climate is a type of climate that corresponds to the Köppen climate classification category "Am". Tropical monsoon climates have monthly mean temperatures above 18 °C (64 °F) in every month of the year and a dry season. Tropical monsoon climates is the intermediate climate between the wet Af and the drier Aw.

Climate of Africa

The climate of Africa is a range of climates such as the equatorial climate, the tropical wet and dry climate, the tropical monsoon climate, the semi-desert climate (semi-arid), the desert climate, and the subtropical highland climate. Temperate climates are rare across the continent except at very high elevations and along the fringes. In fact, the climate of Africa is more variable by rainfall amount than by temperatures, which are consistently high. African deserts are the sunniest and the driest parts of the continent, owing to the prevailing presence of the subtropical ridge with subsiding, hot, dry air masses. Africa holds many heat-related records: the continent has the hottest extended region year-round, the areas with the hottest summer climate, the highest sunshine duration, and more.

The Atlantic Equatorial Mode or Atlantic Niño is a quasiperiodic interannual climate pattern of the equatorial Atlantic Ocean. It is the dominant mode of year-to-year variability that results in alternating warming and cooling episodes of sea surface temperatures accompanied by changes in atmospheric circulation. The term Atlantic Niño comes from its close similarity with the El Niño-Southern Oscillation (ENSO) that dominates the tropical Pacific basin. The Atlantic Niño is not the same as the Atlantic Meridional (Interhemispheric) Mode that consists of a north-south dipole and operates more on decadal timescales. The equatorial warming and cooling events associated with the Atlantic Niño are known to be strongly related to atmospheric climate anomalies, especially in African countries bordering the Gulf of Guinea. Therefore, understanding of the Atlantic Niño has important implications for climate prediction in those regions. Although the Atlantic Niño is an intrinsic mode to the equatorial Atlantic, there may be a tenuous causal relationship between ENSO and the Atlantic Niño in some circumstances.

There are a number of explanations of the asymmetry of the Intertropical Convergence Zone (ITCZ), known by sailors as the Doldrums.

The Tropical Atlantic Variability (TAV) is influenced by internal interaction and external effects. TAV can be discussed in different time scales: seasonal and interannual.