African Easterly Jet
Encyclopedia
The African easterly jet, is a region of the lower troposphere over West Africa
where the seasonal mean wind speed is maximum and easterly. Forming due to the temperature
contrast between the Sahara
and the Gulf of Guinea
, maximum wind speeds are located at a height of 3 kilometres (1.9 mi) to the north of the monsoon trough
. The jet marches northward from its southern location in January, reaching its most northerly latitude in August, and its strongest winds in September while shifting back towards the equator. Within the easterly jet, tropical wave
s form. Convective complexes associated with these waves can form tropical cyclone
s. If the feature is suppressed south of its normal location during August and September, tropical cyclogenesis
is suppressed. If desertification continues across Sub-Saharan Africa
, the strength of this feature would increase, though tropical wave generation is expected to decrease which could decrease the number of tropical cyclone
s in the Atlantic basin.
over northern Africa leads to a low-level westerly jet stream from June into October to the south of the Intertropical Convergence Zone
. The jet develops because heating of the West African land mass during the Northern Hemisphere summer creates a surface temperature and moisture gradient between the Gulf of Guinea and the Sahara, and 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 (31.1 mph) between the 12th and 13th parallels. The easterly jet weakens and drops southward during October and November.
of Africa, and helps form the tropical wave
s which march across the tropical Atlantic and eastern Pacific oceans during the warm season. 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 wave
s. A small number of mesoscale storm systems embedded in these waves develop into tropical cyclone
s after they move from west Africa
into the tropical Atlantic, mainly during August and September. When it lies south of normal during the peak months of the Atlantic hurricane season
, tropical cyclone formation is suppressed.
West Africa
West Africa or Western Africa is the westernmost region of the African continent. Geopolitically, the UN definition of Western Africa includes the following 16 countries and an area of approximately 5 million square km:-Flags of West Africa:...
where the seasonal mean wind speed is maximum and easterly. Forming due to the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
contrast between the Sahara
Sahara
The Sahara is the world's second largest desert, after Antarctica. At over , it covers most of Northern Africa, making it almost as large as Europe or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean...
and the Gulf of Guinea
Gulf of Guinea
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....
, maximum wind speeds are located at a height of 3 kilometres (1.9 mi) to the north 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 jet marches northward from its southern location in January, reaching its most northerly latitude in August, and its strongest winds in September while shifting back towards the equator. Within the easterly jet, 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 form. Convective complexes associated with these waves can form 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. If the feature is suppressed south of its normal location during August and September, 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...
is suppressed. If desertification continues across Sub-Saharan Africa
Sub-Saharan Africa
Sub-Saharan Africa as a geographical term refers to the area of the African continent which lies south of the Sahara. A political definition of Sub-Saharan Africa, instead, covers all African countries which are fully or partially located south of the Sahara...
, the strength of this feature would increase, though tropical wave generation is expected to decrease which could decrease the number 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 in the Atlantic basin.
Formation and characteristics
During January, it lies at 3000 metres (9,842.5 ft) above sea level at the latitude of five degrees north. Winds within it rise from 30 km/h (18.6 mph) in January to 40 km/h (24.9 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. The mid-level African easterly jet occurs during the Northern Hemisphere summer between 10°N and 20°N above West Africa, and the nocturnal poleward low-level jet occurs in the Great Plains of east and south Africa. The formation of the thermal lowThermal low
Thermal lows, or heat lows, are non-frontal low-pressure areas that occur over the continents in the subtropics such as near the Sonoran Desert, the Mexican plateau, Sahara, South America over northwest Argentina, Australia, the Iberian peninsula, and Tibetan plateau during the warm season as the...
over northern Africa leads to a low-level westerly jet stream from June into October to the south of the Intertropical Convergence Zone
Intertropical Convergence Zone
The Intertropical Convergence Zone , known by sailors as The Doldrums, is the area encircling the earth near the equator where winds originating in the northern and southern hemispheres come together....
. The jet develops because heating of the West African land mass during the Northern Hemisphere summer creates a surface temperature and moisture gradient between the Gulf of Guinea and the Sahara, and the atmosphere responds by generating vertical 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...
to maintain thermal wind
Thermal wind
The thermal wind is a vertical shear in the geostrophic wind caused by a horizontal temperature gradient. Its name is a misnomer, because the thermal wind is not actually a wind, but rather a wind shear.- Physical Intuition :...
balance. The jet reaches its zenith in August, lying between the 16th and 17th parallels. In September, winds maximize near 50 km/h (31.1 mph) between the 12th and 13th parallels. The easterly jet weakens and drops southward during October and November.
Impact
The low-level easterly African jet stream is considered to play a crucial role in the southwest monsoonMonsoon
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...
of Africa, and helps form the 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 which march across the tropical Atlantic and eastern Pacific oceans during the warm season. The jet exhibits both barotropic
Barotropic
In meteorology, a barotropic atmosphere is one in which the pressure depends only on the density and vice versa, so that isobaric surfaces are also isopycnic surfaces . The isobaric surfaces will also be isothermal surfaces, hence the geostrophic wind is independent of height...
and baroclinic instability, which produces synoptic scale, westward propagating disturbances in the jet known as African easterly waves, or 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. A small number of mesoscale storm systems embedded in these waves develop into 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 after they move from west Africa
West Africa
West Africa or Western Africa is the westernmost region of the African continent. Geopolitically, the UN definition of Western Africa includes the following 16 countries and an area of approximately 5 million square km:-Flags of West Africa:...
into the tropical Atlantic, mainly during August and September. When it lies south of normal during the peak months of the Atlantic hurricane season
Atlantic hurricane 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 cyclone formation is suppressed.
Role during climate changes
If sub-Saharan Africa becomes a desert, the African easterly jet is expected to increase in strength. However, related tropical wave activity is expected to decrease, as the African easterly jet should decrease in latitude during the summer months due to the drier conditions in western Africa.Further reading
- Cook, Kerry H.: Generation of the African Easterly Jet and Its Role in Determining West African Precipitation.
- Holton, James R.: An Introduction to Dynamic Meteorology, 2004. ISBN 0-12-354015-1