Dahomey Gap

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.
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, which is in the heart of the Gap, receives only 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:
Geological history.
Evidence from biogeography suggests that the Dahomey Gap has had significance for up to 90 million years. Murphy and Collier, in their analysis of the Aplocheiloid fishes show a split in the African species which they attribute to the presence of an epicontinental sea between the late Cenomanian and early Eocene.
 This discontinuity had earlier been noted in plant species by White and is supported by an analysis of the "Coffea" clade by Maurin et al.
The Dahomey Gap has existed in its present form for only about four thousand years. For most of the Quaternary, 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.