Intertropical Convergence Zone

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.

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

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 Convergenze 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.

South Pacific convergence zone

Omega-500-july-era40-1979
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.[4] 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.[5]

Effects on weather

ITCZ january-july
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.[6] 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).[7]

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

ITCZ jun 25 2010
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.[8] 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 come.

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)[9] 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.

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. 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 and music

The doldrums are notably described in Samuel Taylor Coleridge's poem The Rime of the Ancient Mariner (1798), in Patrick O'Brian's novel Desolation Island (1978), and in Laura Hillenbrand's non-fiction book Unbroken: A World War II Story of Survival, Resilience, and Redemption (2010). Additionally, the Doldrums are a fictional place in Norton Juster's novel The Phantom Tollbooth (1961).

Noel Gallagher used the term to describe a woman stuck in the decade of the 1960s in the song "Holy Mountain": “get out of the doldrums, baby now!”

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.
  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. ^ E. Linacre and B. Geerts. Movement of the South Pacific convergence zone Retrieved on 2006-11-26.
  5. ^ 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.
  6. ^ Djurić, D: Weather Analysis. Prentice Hall, 1994. ISBN 0-13-501149-3.
  7. ^ 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
  8. ^ 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.
  9. ^ "Q & A Turbulences" 1.June.2009 The Guardian

External links

Asymmetry of the Intertropical Convergence Zone

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

Chemical equator

The chemical equator concept was coined in 2008 when researchers from University of York discovered a distinct divide between the polluted air over Indonesia from the largely uncontaminated atmosphere over Australia.

The divide of the atmosphere of the northern hemisphere from the atmosphere of the southern hemisphere is different from that of the Intertropical Convergence Zone.

Climate of Colombia

The Climate of Colombia is characterized for being tropical and isothermal as a result of its geographical location near the Equator presenting variations within five natural regions and depending on the altitude, temperature, humidity, winds and rainfall. Each region maintains an average temperature throughout the year only presenting variables determined by precipitation during a rainy season caused by the Intertropical Convergence Zone.

Cyclone Nigel

Severe Tropical Cyclone Nigel was the second of two tropical cyclones to affect Northern Vanuatu and the Fijian islands during January 1985. The system was first noted as an ill-defined low-pressure area ("low") located within the intertropical convergence zone near the Cape York Peninsula. Over the next few days the low moved eastwards and increased in strength; it was named Nigel on January 16 as it developed into a tropical cyclone.

Harmattan

The Harmattan is a season in the West African subcontinent, which occurs between the end of November and the middle of March. It is characterized by the dry and dusty northeasterly trade wind, of the same name, which blows from the Sahara Desert over West Africa into the Gulf of Guinea. The name is related to the word haramata in the Twi language. The temperature is cold in most places, but can also be hot in certain places, depending on local circumstances.The Harmattan blows during the dry season, which occurs during the lowest-sun months. In this season the subtropical ridge of high pressure stays over the central Sahara Desert and the low-pressure Intertropical Convergence Zone (ITCZ) stays over the Gulf of Guinea. On its passage over the Sahara, the harmattan picks up fine dust and sand particles (between 0.5 and 10 microns).

Hippotion aporodes

Hippotion aporodes is a moth of the family Sphingidae. It is known from forests from Ivory Coast to the Congo Basin and Uganda. In West Africa it is a strong migrant, moving regularly north and south with the Intertropical Convergence Zone.The length of the forewings is 28–34 mm. It is very closely allied to Hippotion celerio. It differs in being consistently darker, in the oblique band of the forewings being more evenly curved, buff instead of silvery and enclosing three narrow but distinct, dark brown parallel lines. The hindwings are as in Hippotion celerio, but the crimson patch at the base is deeper in tone, the blackish markings are better developed, invading most of the wing with the exception of the base. The inner margin and a series of intemervular post-median spots are crimson.

Hurricane Kenneth (2005)

Hurricane Kenneth was the strongest and longest-tracked hurricane of the 2005 Pacific hurricane season. The eleventh named storm and fifth hurricane of the season, Kenneth developed from a disturbance in the Intertropical Convergence Zone to the southwest of Mexico on September 14. It quickly attained peak winds of 135 mph (215 km/h) on September 18, before weakening due to increased wind shear and turning to a southwest drift. After weakening to tropical storm status, Kenneth attained a steady west-northwest motion and encountered favorable enough conditions for it to gain power and attain hurricane status on September 25. The cyclone again weakened as its motion halted, and on September 30 Kenneth dissipated a short distance off the Big Island of Hawaii. The remnants of Kenneth produced one of the highest rainfall totals in Hawaii, reaching up to 12 inches (305 mm) on Oahu. The rainfall caused flooding, though no major damage was reported.

Hurricane Liza (1968)

Hurricane Liza was the third hurricane of the 1968 Pacific hurricane season. Forming from an area of the Intertropical Convergence Zone (ITCZ) on August 28 and reaching tropical storm strength in the same day, Liza meandered generally westward over the Pacific Ocean, reaching hurricane strength on August 29 while far from land. It maintained that intensity until September 2, when the hurricane was downgraded to a tropical storm, but avoided tropical depression status despite the presentation seen by an Air Force reconnaissance plane. After weakening, the storm moved northwestward, weakening to a depression on September 4, when it began a turn to the east. There is a possibility that the depression completed a small loop between the downgrade and its dissipation on September 6.

Although it remained far from land, the waves triggered by Liza were able to reach California, where they combined with high tide, threatening beachfront homes that had weakened foundations after a previous tide. The hurricane was responsible for sweeping hundreds of Labor Day swimmers out into the ocean in Zuma Beach and Newport Beach, all of whom were saved by lifeguards. The waves also tore off a group of sundecks estimated at $5,000 (1968 USD) near Laguna Beach.

Hurricane Pauline (1968)

Hurricane Pauline was the deadliest hurricane of the 1968 Pacific hurricane season. Forming from a disturbance in the Intertropical Convergence Zone on September 26 and becoming a tropical storm on September 29, the hurricane strengthened to a peak of 85 mph (137 km/h) as a Category 1 hurricane on the Saffir-Simpson Hurricane Scale on September 30 before making landfall just east of La Paz, Baja California Sur, near Ciudad Constitución in Mexico, exiting over the Gulf of California. Before making a second landfall on the state of Sonora on October 3, Pauline lost tropical characteristics. The storm continued inland, passing directly over Ciudad Obregón before dissipating south-southeast of Sierra Vista, Arizona.

There is very little information as to the effects of Pauline on Mexico, but during the passage of the hurricane over Magdalena Bay, a boat disappeared with its five occupants. Despite a large-scale search for the boat or any possible remnants, no trace was ever found. The remnants of Pauline triggered showers over the southwest United States and may have been responsible for a damaging tornado which occurred near Glendale, Arizona.

Hurricane Uleki

Hurricane Uleki, also referred as Typhoon Uleki, was a long-lived tropical cyclone in August–September 1988 that had minimal effects on land. Originating from a disturbance in the Intertropical Convergence Zone in late-August, Uleki was identified as a tropical depression well to the southeast of Hawaii on August 28. Steady organization ensued as it moved west, becoming a tropical storm on August 30 and a hurricane on August 31. Rapid intensification took place thereafter and the storm reached its peak intensity on September 2 as a Category 3 on the Saffir–Simpson hurricane wind scale. Hurricane Hunters investigating the cyclone found peak winds of 125 mph (205 km/h) and a barometric pressure of 957 mbar (hPa; 28.26 inHg). Thereafter, Uleki stalled for two days to the southwest of Hawaii, resulting in heavy surf across the state. The dangerous swells killed two people on Oahu.

Unfavorable environmental conditions caused weakening of the hurricane by September 4 as it resumed a west-northwest course away from Hawaii. Conditions later became favorable and Uleki acquired winds of 105 mph (165 mph) on September 7, constituting its secondary peak. The hurricane crossed the International Dateline on September 8 and was reclassified as typhoon. Remaining well away from land, the cyclone steadily weakened to a tropical storm by September 12. Gradually turning north and later east, the degrading cyclone transitioned into an extratropical cyclone four days later and ultimately dissipated on September 17 near the International Dateline.

Mesoscale convective system

A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and Mesoscale Convective Complexes (MCCs), and generally form near weather fronts. The type that forms during the warm season over land has been noted across North America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.

Forms of MCS that develop within the tropics use either the Intertropical Convergence Zone (ITCZ) or monsoon troughs as a focus for their development, generally within the warm season between spring and fall. One exception is that of lake-effect snow bands, which form due to cold air moving across relatively warm bodies of water, and occurs from fall through spring. Polar lows are a second special class of MCS which form at high latitudes during the cold season. Once the parent MCS dies, later thunderstorm development can occur in connection with its remnant mesoscale convective vortex (MCV). Mesoscale convective systems are important to the United States rainfall climatology over the Great Plains since they bring the region about half of their annual warm season rainfall.

Polar High

The polar highs are areas of high atmospheric pressure around the north and south poles; the north polar high being the stronger one because land gains and loses heat more effectively than sea. The cold temperatures in the polar regions cause air to descend to create the high pressure (a process called subsidence), just as the warm temperatures around the equator cause air to rise to create the low pressure intertropical convergence zone. Rising air also occurs along bands of low pressure situated just below the polar highs around the 50th parallels of latitude. These extratropical convergence zones are occupied by the polar fronts where air masses of polar origin meet and clash with those of tropical or subtropical origin. This convergence of rising air completes the vertical cycle around the polar cell in each latitudinal hemisphere. Closely related to this concept is the polar vortex.

Surface temperatures under the polar highs are the coldest on Earth, with no month having an average temperature above freezing. Regions under the polar high also experience very low levels of precipitation, which leads them to be known as "polar deserts".

Air flows outwards from the poles to create the polar easterlies in the arctic and antarctic areas.

South Pacific convergence zone

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 (160W, 20S). 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.

South Saharan steppe and woodlands

The South Saharan steppe and woodlands is a deserts and xeric shrublands ecoregion of northern Africa.

The ecoregion covers 1,101,700 square kilometers (425,400 sq mi) in Algeria, Chad, Mali, Mauritania, and Sudan. It extends east and west across the continent in a band, forming a transition between the hyper-arid Sahara Desert to the north and the Sahel grasslands and savannas to the south.

Movements of the equatorial Intertropical Convergence Zone (ITCZ) bring summer rains during July and August which average 100 to 200 mm, but vary greatly from year to year. These rains sustain summer pastures of grasses and herbs, with dry woodlands and shrublands along seasonal watercourses.

Thermal equator

The thermal equator (also known as "the heat equator") is a belt encircling the Earth, defined by the set of locations having the highest mean annual temperature at each longitude around the globe. Because local temperatures are sensitive to the geography of a region, mountain ranges and ocean currents ensure that smooth temperature gradients (such as might be found if the Earth were uniform in composition and devoid of surface irregularities) are impossible, the location of the thermal equator is not identical to that of the geographic Equator.

The term is less frequently used to describe the belt of maximum temperatures surrounding the globe which migrates roughly between the Tropic of Cancer and the Tropic of Capricorn. This region is known as the Intertropical Convergence Zone where the Earth orbits the Sun. This zone is the result of trade winds from the northern and southern part of the hemisphere eventually joining together.Still another definition states that the thermal equator is the latitude at which insolation is identical throughout the year. This is not the same as the astronomical equator because the Earth reaches perihelion (the minimum distance from the Sun in its orbit) in early January and is at aphelion (maximum distance) in early July. Therefore, insolation is somewhat higher at 0° latitude in January than in July even though the height of the Sun (at noon) and the length of day (from sunrise to sunset) is essentially the same. At a few degrees north of the equator the perihelion/aphelion factor is balanced by the fact that the angle of the Sun is slightly more direct, and the days are slightly longer, at the time of the summer solstice for the Northern Hemisphere (most commonly on June 21), making the level of insolation virtually the same in both "summer" and "winter."

Chennai, India, at 13 degrees north is considered to lie on the thermal equator at its longitude (80 degrees east).

Tropical climate

A tropical climate in the Köppen climate classification is a non-arid climate in which all twelve months have mean temperatures of warmer than 18 °C (64 °F). In tropical climates there are often only two seasons: a wet season and a dry season. Tropical climates are frost-free, and changes in the solar angle are small. In tropical climates, the temperature remains relatively constant (hot) throughout the year. Sunlight is intense.

Tropical instability waves

Tropical instability waves, often abbreviated TIW, are a phenomenon in which the interface between areas of warm and cold sea surface temperatures near the equator form a regular pattern of westward-propagating waves. These waves are often present in the Atlantic Ocean, extending westward from the African coast, but are more easily recognizable in the Pacific, extending westward from South America. They have an average period of about 30 days and wavelength of about 1100 kilometers, and are largest in amplitude between June and November. They are also largest during La Niña conditions, and may disappear when strong El Niño conditions are present.Tropical instability waves are not related to tropical waves, which are atmospheric disturbances that propagate westward along the Intertropical Convergence Zone, sometimes giving rise to tropical storms.

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 moves from east to west across the tropics, causing areas of cloudiness and thunderstorms. West-moving waves can also form from the tail end of frontal zones in the subtropics and tropics, and may be referred to as easterly waves, but these waves are not properly called tropical waves; they are a form of inverted trough sharing many characteristics with fully tropical waves. All tropical waves form in the easterly flow along the equatorward side of the subtropical ridge or belt of high 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.

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