Dahomey Gap
Encyclopedia
In West Africa
, the Dahomey Gap refers to the portion of the Guinean forest-savanna mosaic
that extends all the way to the coast in Benin
, Togo
and Ghana
, thus separating the forest zone
that covers much of the south of the region into two separate parts. The forest region west of the gap is called the Upper Guinean forests
or Guinean forest zone, and the portion east of the gap is called the Lower Guinean forests
, Lower Guinean-Congolian forests, or Congolian Forest Zone.
The major city in the Gap is Accra
. Several other cities, such as Kumasi
, exist on the fringe of the Gap.
belt on all sides, and no mountains block moisture. Yet, Accra in the heart of the Gap receives only 720 millimetres (28 inches) of rainfall per year — less than half the amount needed to sustain tropical rainforest (which would be expected at a latitude of 6° N).
The cause of the dryness of the Dahomey Gap can simply be explained thus:
, dry conditions due to a much colder Atlantic Ocean
(aided by extensive cold currents from ice sheets in Europe
and North America
) have meant that the present-day forest zone has supported very little or no rainforest. In interglacial
periods, however, rainfall throughout West Africa has often been so heavy that the Gap has become wet enough to support rainforest, thus eliminating the savanna.
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:...
, the Dahomey Gap refers to the portion of the Guinean forest-savanna mosaic
Guinean forest-savanna mosaic
The Guinean forest-savanna mosaic is an ecoregion of West Africa, a band of interlaced forest, savanna, and grassland running east to west and dividing the tropical moist forests near the coast from the West Sudanian savanna of the interior.-Setting:...
that extends all the way to the coast in Benin
Benin
Benin , officially the Republic of Benin, is a country in West Africa. It borders Togo to the west, Nigeria to the east and Burkina Faso and Niger to the north. Its small southern coastline on the Bight of Benin is where a majority of the population is located...
, Togo
Togo
Togo, officially the Togolese Republic , is a country in West Africa bordered by Ghana to the west, Benin to the east and Burkina Faso to the north. It extends south to the Gulf of Guinea, on which the capital Lomé is located. Togo covers an area of approximately with a population of approximately...
and Ghana
Ghana
Ghana , officially the Republic of Ghana, is a country located in West Africa. It is bordered by Côte d'Ivoire to the west, Burkina Faso to the north, Togo to the east, and the Gulf of Guinea to the south...
, thus separating the forest zone
Forest zone
In West Africa, the forest zone refers to the southern part of the region once covered by tropical rainforest. Sometimes this region is referred to as Guinea to distinguish it from the grassland-covered Sudan, drier Sahel and perarid Sahara.-Extent:...
that covers much of the south of the region into two separate parts. The forest region west of the gap is called the Upper Guinean forests
Upper Guinean forests
The Upper Guinean forests is a tropical moist forest region of West Africa. The Upper Guinean forests extend from Guinea and Sierra Leone in the west through Liberia, Côte d'Ivoire and Ghana to Togo in the east, and a few hundred kilometers inland from the Atlantic coast. A few enclaves of montane...
or Guinean forest zone, and the portion east of the gap is called the Lower Guinean forests
Lower Guinean forests
The Lower Guinean forests is region of coastal tropical moist broadleaf forest extending along the coast of the Gulf of Guinea from eastern Benin through Nigeria and Cameroon....
, Lower Guinean-Congolian forests, or Congolian Forest Zone.
The major city in the Gap is Accra
Accra
Accra is the capital and largest city of Ghana, with an urban population of 1,658,937 according to the 2000 census. Accra is also the capital of the Greater Accra Region and of the Accra Metropolitan District, with which it is coterminous...
. Several other cities, such as Kumasi
Kumasi
Kumasi is a city in southern central Ghana's Ashanti region. It is located near Lake Bosomtwe, in the Rain Forest Region about northwest of Accra. Kumasi is approximately north of the Equator and north of the Gulf of Guinea...
, exist on the fringe of the Gap.
Causes of dryness
The dryness of the Dahomey Gap is unusual, given that it lies surrounded by a very wet 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...
belt on all sides, and no mountains block moisture. Yet, Accra in the heart of the Gap receives only 720 millimetres (28 inches) of rainfall per year — less than half the amount needed to sustain tropical rainforest (which would be expected at a latitude of 6° N).
The cause of the dryness of the Dahomey Gap can simply be explained thus:
- In northern winter, high pressure centred on the SaharaSaharaThe 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...
sends dry northeasterly trade winds over West Africa, creating a general dry season, including over the Gap.
- In northern summer, an enormous low pressure system known as the monsoonMonsoonMonsoon 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...
forms over the huge Africa-Asia-Europe landmass. Centred over approximately RajasthanRajasthanRājasthān the land of Rajasthanis, , is the largest state of the Republic of India by area. It is located in the northwest of India. It encompasses most of the area of the large, inhospitable Great Indian Desert , which has an edge paralleling the Sutlej-Indus river valley along its border with...
and prevented by the huge barrier of the Himalayas from moving further west, it extends a strong trough over West Africa. This trough sends warm and saturated westerly winds over West Africa, creating a wet season peaking in June on the coast (as the trough moves north) and in August inland.
- The coast in the region of the highest rainfall (GuineaGuineaGuinea , officially the Republic of Guinea , is a country in West Africa. Formerly known as French Guinea , it is today sometimes called Guinea-Conakry to distinguish it from its neighbour Guinea-Bissau. Guinea is divided into eight administrative regions and subdivided into thirty-three prefectures...
, Sierra LeoneSierra LeoneSierra Leone , officially the Republic of Sierra Leone, is a country in West Africa. It is bordered by Guinea to the north and east, Liberia to the southeast, and the Atlantic Ocean to the west and southwest. Sierra Leone covers a total area of and has an estimated population between 5.4 and 6.4...
, LiberiaLiberiaLiberia , officially the Republic of Liberia, is a country in West Africa. It is bordered by Sierra Leone on the west, Guinea on the north and Côte d'Ivoire on the east. Liberia's coastline is composed of mostly mangrove forests while the more sparsely populated inland consists of forests that open...
) slopes from southeast to northwest. This aspectAspect (geography)In physical geography, aspect generally refers to the horizontal direction to which a mountain slope faces. For example, a slope on the eastern edge of the Rockies toward the Great Plains is described as having an easterly aspect...
means that the moist westerlies are forced to shed enormous quantities of moisture, allowing rainforest to thrive despite a distinct dry seasonDry seasonThe dry season is a term commonly used when describing the weather in the tropics. The weather in the tropics is dominated by the tropical rain belt, which oscillates from the northern to the southern tropics over the course of the year...
.
- In the Dahomey Gap, however, the coast slopes in such a way (northeast to southwest) that the rain-bearing westerlies flow parallelParallel (geometry)Parallelism is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. The assumed existence and properties of parallel lines are the basis of Euclid's parallel postulate. Two lines in a plane that do not...
to the coast. This means they are not forced to give up nearly so much moisture, and thus the climate is much drier, with only a brief rainy season in May and June. Consequently, an open savannaSavannaA savanna, or savannah, is a grassland ecosystem characterized by the trees being sufficiently small or widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of C4 grasses.Some...
prevails adapted to the moderate rainfall and high evaporationEvaporationEvaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
in the hot temperatures.
Geological history
The Dahomey Gap has existed in its present form for only about four thousand years. For most of the QuaternaryQuaternary
The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...
, dry conditions due to a much colder Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
(aided by extensive cold currents from ice sheets in Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
and North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
) have meant that the present-day forest zone has supported very little or no rainforest. In interglacial
Interglacial
An Interglacial period is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age...
periods, however, rainfall throughout West Africa has often been so heavy that the Gap has become wet enough to support rainforest, thus eliminating the savanna.