The West African and East African Monsoons

The West African monsoon carries moist southwesterly air off the Gulf of Guinea inland across the region each Northern Hemisphere summer, colliding along its northern edge with the hot, dust-laden Saharan Air Layer and the dry, northeasterly harmattan wind that dominates the region for the rest of the year. As the ITCZ, and the associated near-surface monsoon trough, migrates northward from its winter position near the Gulf of Guinea coast to as far as 20°N over the Sahara by August, it drags the rain-bearing monsoon flow with it, producing a single-peak rainy season that arrives progressively later, and delivers progressively less total rainfall, the further north and inland a given location lies — from over 2,000 millimeters annually along the humid Guinean coast to under 200 millimeters at the Sahara's southern margin, the Sahel, a semi-arid transitional belt stretching from Senegal and Mauritania in the west to Sudan in the east.