Dahomey Gap - Causes of Dryness

Causes of Dryness

The dryness of the Dahomey Gap is unusual, given that it lies surrounded by a very wet monsoon 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 Sahara 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 monsoon forms over the huge Africa-Asia-Europe landmass. Centred over approximately Rajasthan 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 (Guinea, Sierra Leone, Liberia) slopes from southeast to northwest. This aspect means that the moist westerlies are forced to shed enormous quantities of moisture, allowing rainforest to thrive despite a distinct dry season.

• In the Dahomey Gap, however, the coast slopes in such a way (northeast to southwest) that the rain-bearing westerlies flow parallel 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 savanna prevails adapted to the moderate rainfall and high evaporation in the hot temperatures.