African easterly jet

Last updated

The African easterly jet is a region of the lower troposphere over West Africa where the seasonal mean wind speed is at a maximum and the wind is easterly. The temperature contrast between the Sahara Desert and the Gulf of Guinea causes the jet to form to the north of the monsoon trough. The jet's maximum wind speeds are at a height of 3 kilometres (1.9 mi). The jet moves northward from its south-most location in January, reaching its most northerly latitude in August. Its strongest winds are in September while it begins shifting back towards the equator. Within the easterly jet, tropical waves form. Convective complexes associated with these waves can form tropical cyclones. If the jet is south of its normal location during August and September, tropical cyclogenesis is suppressed. If desertification continues across Sub-Saharan Africa, the strength of the jet could increase, although tropical wave generation probably would decrease, which would decrease the number of tropical cyclones in the Atlantic basin.

West Africa westernmost region of the African continent

West Africa is the westernmost region of Africa. The United Nations defines Western Africa as the 16 countries of Benin, Burkina Faso, Cape Verde, The Gambia, Ghana, Guinea, Guinea-Bissau, Ivory Coast, Liberia, Mali, Mauritania, the Niger, Nigeria, Senegal, Sierra Leone and Togo, as well as the United Kingdom Overseas Territory of Saint Helena, Ascension and Tristan da Cunha. The population of West Africa is estimated at about 362 million people as of 2016.

Gulf of Guinea The northeasternmost part of the tropical Atlantic Ocean between Cape Lopez in Gabon, north and west to Cape Palmas in Liberia

The Gulf of Guinea is the northeasternmost part of the tropical Atlantic Ocean between Cape Lopez in Gabon, north and west to Cape Palmas in Liberia. The intersection of the Equator and Prime Meridian is in the gulf.

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.

Contents

Formation and characteristics

During January, the African easterly jet lies at 3,000 metres (9,800 ft) above sea level at five degrees north latitude. Winds within it increase in speed from 30 km/h (19 mph) in January to 40 km/h (25 mph) in March. Shifting northward in April to the seventh parallel, winds within the jet increase to 45 km/h (28 mph). By June, it shifts northward into northwest Africa. [1]

From June into October, the thermal low over northern Africa leads to a low-level westerly jet stream to the south of the Intertropical Convergence Zone. [2] The mid-level African easterly jet develops because heating of the West African land mass during this time of year creates a surface temperature and moisture gradient between the Gulf of Guinea and the Sahara Desert. The atmosphere responds by generating vertical wind shear to maintain thermal wind balance. The jet reaches its zenith in August, lying between the 16th and 17th parallels. In September, winds maximize near 50 km/h (31 mph) between the 12th and 13th parallels. The easterly jet weakens and drops southward during October and November. [1]

Thermal low

Thermal lows, or heat lows, are non-frontal low-pressure areas that occur over the continents in the subtropics during the warm season, as the result of intense heating when compared to their surrounding environments. Thermal lows occur near the Sonoran Desert, on the Mexican plateau, in California's Great Central Valley, the Sahara, over north-west Argentina in South America, over the Kimberley region of north-west Australia, the Iberian peninsula, and the Tibetan plateau.

Intertropical Convergence Zone

The Intertropical Convergence Zone (ITCZ), known by sailors as the doldrums or the calms, is the area encircling Earth near the Equator, where the northeast and southeast trade winds converge.

Wind shear

Wind shear, sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal wind shear. Vertical wind shear is a change in wind speed or direction with change in altitude. Horizontal wind shear is a change in wind speed with change in lateral position for a given altitude.

Impact

The mid-level African easterly jet is considered to play a crucial role in the southwest monsoon of Africa, [3] and helps form the tropical waves that move across the tropical Atlantic and eastern Pacific oceans during the warm season. [4] The jet exhibits both barotropic and baroclinic instability, which produces synoptic scale, westward-propagating disturbances in the jet known as African easterly waves or tropical waves. A small number of mesoscale storm systems embedded in these waves develop into tropical cyclones after they move from west Africa into the tropical Atlantic, mainly during August and September. When the jet is south of normal during the peak months of the Atlantic hurricane season, tropical cyclone formation is suppressed. [5]

Atlantic hurricane season tropical cyclone season

The Atlantic hurricane season is the period in a year when hurricanes usually form in the Atlantic Ocean. Tropical cyclones in the North Atlantic are called hurricanes, tropical storms, or tropical depressions. In addition, there have been several storms over the years that have not been fully tropical and are categorized as subtropical depressions and subtropical storms. Even though subtropical storms and subtropical depressions are not technically as strong as tropical cyclones, the damages can still be devastating.

See also

Related Research Articles

Jet stream Fast-flowing atmospheric air-current

Jet streams are fast flowing, narrow, meandering air currents in the atmospheres of some planets, including Earth. On Earth, the main jet streams are located near the altitude of the tropopause and are westerly winds. Their paths typically have a meandering shape. Jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including opposite to the direction of the remainder of the jet.

Geography of Nigeria

Nigeria is a country in West Africa. Nigeria shares land borders with the Republic of Benin in the west, Chad and Cameroon in the east, and Niger in the north. Its coast lies on the Gulf of Guinea in the south and it borders Lake Chad to the northeast. Noted geographical features in Nigeria include the Adamawa highlands, Mambilla Plateau, Jos Plateau, Obudu Plateau, the Niger River, River Benue and Niger Delta.

Horse latitudes

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.

Wind speed

Wind speed, or wind flow velocity, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Note that wind direction is usually almost parallel to isobars, due to Earth's rotation.

Hurricane Alberto (2000) Category 3 Atlantic hurricane in 2000

Hurricane Alberto was among the longest-lived tropical cyclones on record in the Atlantic Ocean. The third tropical cyclone, first named storm, and first hurricane of the 2000 Atlantic hurricane season, Alberto developed near the western coast of Africa from a tropical wave on August 3. Initially a tropical depression, it strengthened into Tropical Storm Alberto early on August 4. While briefly turning westward on August 6, Alberto attained hurricane status. The cyclone continued to track west-northwestward, and by early the following day, reached an initial peak with winds of 90 mph (150 km/h). Shortly thereafter, Alberto re-curved northwestward and began encountering increased wind shear. As a result, Alberto weakened back to a tropical storm on August 9. However, the system quickly re-strengthened as winds became more favorable, and early on August 10, Alberto became a hurricane again. The storm gradually curved northward and north-northeastward between August 11 and August 12; Alberto attained its peak intensity with winds of 125 mph (205 km/h) during that time.

1975 Atlantic hurricane season hurricane season in the Atlantic Ocean

The 1975 Atlantic hurricane season featured the first tropical storm to be upgraded to a hurricane based solely on satellite imagery – Hurricane Doris. The season officially began on June 1 and lasted until November 30. These dates conventionally delimit the period of each year when most tropical cyclones form in the Atlantic basin. The season was near average, with nine tropical storms forming, of which six became hurricanes. Three of those six became major hurricanes, which are Category 3 or higher on the Saffir–Simpson scale. The first system, Tropical Depression One, developed on June 24. Tropical Storm Amy in July caused minor beach erosion and coastal flooding from North Carolina to New Jersey, and killed one person when a ship capsized offshore North Carolina. Hurricane Blanche brought strong winds to portions of Atlantic Canada, leaving about $6.2 million (1975 USD) in damage. Hurricane Caroline brought high tides and flooding to northeastern Mexico and Texas, with two drownings in the latter.

Tropical wave type of atmospheric trough

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.

2001 Pacific hurricane season hurricane season in the Pacific Ocean

The 2001 Pacific hurricane season was a near average season. The most notable storm that year was Hurricane Juliette, which caused devastating floods in Baja, California, leading to 12 fatalities and $400 million worth of damage. Two other storms were notable in their own rights, Hurricane Adolph became the strongest May Hurricane until 2014 when both records set by Adolph and Juliette were broken by Hurricanes Amanda and Odile. Tropical Storm Barbara passed just north of Hawaii, bringing minimal impact. The season officially began on May 15, 2001 in the eastern Pacific, and on June 1, 2001 in the central Pacific, and lasted until November 30, 2001. These dates conventionally delimit the period of each year when most tropical cyclones form in this part of the Pacific Ocean. The first storm developed on May 25, while the last storm dissipated on November 3.

1981 Pacific typhoon season typhoon season in the Pacific Ocean

The 1981 Pacific typhoon season was a slightly above average season that produced 29 tropical storms, 13 typhoons and two intense typhoons. The season ran throughout 1981, though most tropical cyclones typically develop between May and October. The season's first named storm, Freda, developed on March 12 while the final storm, Lee, dissipated on December 29. Tropical cyclones only accounted for 12 percent of the rainfall in Hong Kong this season, the lowest percentage for the protectorate since 1972.

Hurricane Helene (2006) Category 3 Atlantic hurricane in 2006

Hurricane Helene was the ninth tropical storm, fourth hurricane, and strongest hurricane of the 2006 Atlantic hurricane season, tied with Gordon.

Western Disturbance continuous rainfall and flood

A Western Disturbance is an extratropical storm originating in the Mediterranean region that brings sudden winter rain to the northwestern parts of the Indian subcontinent. It is a non-monsoonal precipitation pattern driven by the westerlies. The moisture in these storms usually originates over the Mediterranean Sea and the Atlantic Ocean. Extratropical storms are a global phenomena with moisture usually carried in the upper atmosphere, unlike their tropical counterparts where the moisture is carried in the lower atmosphere. In the case of the Indian subcontinent, moisture is sometimes shed as rain when the storm system encounters the Himalayas.

Meteorological history of Hurricane Ivan

The meteorological history of Hurricane Ivan, the longest tracked tropical cyclone of the 2004 Atlantic hurricane season, lasted from late August through late September. The hurricane developed from a tropical wave that moved off the coast of Africa on August 31. Tracking westward due to a ridge, favorable conditions allowed it to develop into Tropical Depression Nine on September 2 in the deep tropical Atlantic Ocean. The cyclone gradually intensified until September 5, when it underwent rapid deepening and reached Category 4 status on the Saffir-Simpson Hurricane Scale; at the time Ivan was the southernmost major North Atlantic hurricane on record.

Tropical Storm Arthur (2002) Atlantic tropical storm in 2002

Tropical Storm Arthur was the first tropical cyclone of the relatively quiet 2002 Atlantic hurricane season. The origins of the storm are believed to have been from a decaying cold front in the Gulf of Mexico, which dropped light to moderate rainfall across the southeastern United States. Developing on July 14 near the coast of North Carolina, Arthur tracked quickly east-northward through much of its duration as a tropical cyclone. It reached peak winds of 60 mph (95 km/h) on July 16, though as it interacted with a mid-level cyclone and cooler waters it transitioned into an extratropical cyclone. The remnants of Arthur passed over Newfoundland with gusty winds and rainfall, where one person drowned.

North American Monsoon california monsoons

The North American monsoon, variously known as the Southwest monsoon, the Mexican monsoon, the New Mexican monsoon, or the Arizona monsoon, is a pattern of pronounced increase in thunderstorms and rainfall over large areas of the southwestern United States and northwestern Mexico, typically occurring between July and mid September. During the monsoon, thunderstorms are fueled by daytime heating and build up during the late afternoon-early evening. Typically, these storms dissipate by late night, and the next day starts out fair, with the cycle repeating daily. The monsoon typically loses its energy by mid-September when drier and cooler conditions are reestablished over the region. Geographically, the North American monsoon precipitation region is centered over the Sierra Madre Occidental in the Mexican states of Sinaloa, Durango, Sonora and Chihuahua.

Climate of the Philippines

+The Philippines has five types of climates: tropical rainforest, tropical savanna, tropical monsoon, humid subtropical, and oceanic characterized by relatively high temperature, oppressive humidity and plenty of rainfall. There are two seasons in the country, the wet season and the dry season, based upon the amount of rainfall. This is also dependent on location in the country as some areas experience rain all throughout the year. Based on temperature, the warmest months of the year are March through October; the winter moonton brings hotter air from November to February. May is the warmest month, and January, the coolest.

Upper tropospheric cyclonic vortex

An upper tropospheric cyclonic vortex is a vortex, or a circulation with a definable center, that usually moves slowly from east-northeast to west-southwest and is prevalent across Northern Hemisphere's warm season. Its circulations generally do not extend below 6,080 metres (19,950 ft) in altitude, as it is an example of a cold-core low. A weak inverted wave in the easterlies is generally found beneath it, and it may also be associated with broad areas of high-level clouds. Downward development results in an increase of cumulus clouds and the appearance of circulation at ground level. In rare cases, a warm-core cyclone can develop in its associated convective activity, resulting in a tropical cyclone and a weakening and southwest movement of the nearby upper tropospheric cyclonic vortex. Symbiotic relationships can exist between tropical cyclones and the upper level lows in their wake, with the two systems occasionally leading to their mutual strengthening. When they move over land during the warm season, an increase in monsoon rains occurs.

Earth rainfall climatology

Earth rainfall climatology Is the study of rainfall, a sub-field of Meteorology. Formally, a wider study includes water falling as ice crystals, i.e. hail, sleet, snow. The aim of rainfall climatology is to measure, understand and predict rain distribution across different regions of planet Earth, a factor of air pressure, humidity, topography, cloud type and raindrop size, via direct measurement and remote sensing data acquisition. Current technologies accurately predict rainfall 3–4 days in advance using numerical weather prediction. Geostationary orbiting satellites gather IR and visual wavelength data to measure realtime localised rainfall by estimating cloud albedo, water content, and the corresponding probability of rain. Geographic distribution of rain is largely governed by climate type, topography and habitat humidity. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation. On the leeward side of mountains, desert climates can exist due to the dry air caused by compressional heating. The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah climes. The urban heat island effect leads to increased rainfall, both in amounts and intensity, downwind of cities. Global warming may also cause changes in the precipitation pattern globally, including wetter conditions at high latitudes and in some wet tropical areas, and drier conditions in parts of the subtropics and middle latitudes. Precipitation is a major component of the water cycle, and is responsible for depositing most of the fresh water on the planet. Approximately 505,000 cubic kilometres (121,000 cu mi) of water falls as precipitation each year; 398,000 cubic kilometres (95,000 cu mi) of it over the oceans. Given the Earth's surface area, that means the globally averaged annual precipitation is 990 millimetres (39 in). Climate classification systems such as the Köppen climate classification system use average annual rainfall to help differentiate between differing climate regimes.

Tropical Depression Sixteen-E (2004) Pacific tropical depression in 2004

Tropical Depression Sixteen-E was the final tropical cyclone of the 2004 Pacific hurricane season. The storm developed out of a tropical wave that moved off the western coast of Africa on October 8. The wave crossed the Atlantic Ocean and entered the eastern Pacific on October 18. The system began to gradually organize, and on October 25 it was classified as a tropical depression. The storm did not significantly intensify, as wind shear prevented it from attaining tropical storm status. The short-lived depression moved northward, and made landfall in Mexico on October 26. Quickly deteriorating, the system dissipated shortly thereafter, although its remnants persisted for a couple more days. The depression had no major effects on land. However, it produced heavy rainfall in parts of Mexico, and the remnants triggered thunderstorms over the southwestern United States.

Gulf of California moisture surge

A Gulf of California moisture surge, or simply gulf surge, is a meteorological event where a pulse of high humidity air is pushed up the Gulf of California. Gulf surges bring moisture to southern Arizona during the North American Monsoon. Prior to the 1970s, the consensus of meteorologists was the moisture that fueled the central and southern Arizona monsoon resulted from the movement of the Bermuda High to a more south and west position, which in turn transported water vapor to the region from the Gulf of Mexico. However, operational meteorologists in the 1970s described episodic surges of moisture that infiltrated the area that was thought to originate in the Gulf of California. It was noted that these episodes were likely to be associated with a convective system near the tip of the Baja peninsula such as a tropical cyclone or an easterly wave.

Climate of Africa

Owing to Africa's position across equatorial and subtropical latitudes in both the northern and southern hemisphere, several different climate types can be found within it. The continent mainly lies within the intertropical zone between the Tropic of Cancer and the Tropic of Capricorn, hence its interesting density of humidity. Precipitation intensity is always high, and it is a hot continent. Warm and hot climates prevail all over Africa, but the northern part is that most marked by aridity and high temperatures. Only the northernmost and the southernmost fringes of the continent have a Mediterranean climate.The equator runs through the middle of Africa, as do the Tropic of Cancer and the Tropic of Capricorn, making Africa the most tropical continent.

References

  1. 1 2 Marcel Leroux (2001). The Meteorology and Climate of Tropical Africa. Springer. pp. 138–139. ISBN   978-3-540-42636-3 . Retrieved 5 February 2011.
  2. B. Pu and K. H. Cook (2008). Dynamics of the Low-Level Westerly Jet Over West Africa. American Geophysical Union, Fall Meeting 2008, abstract #A13A-0229. Retrieved on 8 March 2009.
  3. Kerry H. Cook. Generation of the African Easterly Jet and Its Role in Determining West African Precipitation. Retrieved on 8 May 2008.
  4. Chris Landsea. AOML Frequently Asked Questions. Subject: A4) What is an easterly wave ? Retrieved on 8 May 2008.
  5. Climate Prediction Center (November 1997). "Figure 7". National Oceanic and Atmospheric Administration . Retrieved 5 February 2011.

Further reading

International Standard Book Number Unique numeric book identifier

The International Standard Book Number (ISBN) is a numeric commercial book identifier which is intended to be unique. Publishers purchase ISBNs from an affiliate of the International ISBN Agency.