Surface controls on the seasonality of the West African monsoon system are investigated using observational analyses and applied to predict rainfall along the Guinean coast and in the Sahel on seasonal time scales. Processes that determine the meridional progression of rainfall across West Africa are contrasted with those of the Atlantic marine ITCZ to the west and the continental interior to the east. An understanding of the important physical processes is used to develop indices that can be monitored to predict the beginning and end of the rainy seasons in the two regions.

During April through June, highest precipitation rates are located along the Guinean coast. During this time, the processes that maintain the rainfall maximum are similar to those that maintain rainfall in the Atlantic marine ITCZ. However, while the Atlantic marine ITCZ moves north of 5N in June, the Guinean coast precipitation maximum lingers in the south. The development of inertially unstable flow is instrumental in controlling the location of the precipitation maximum during this time. An instability index is developed to aid in prediction, and the observational requirements for its specification are discussed. When the precipitation maximum resumes its northward progression, on average during the first week in July, it moves northward quickly in response to thermal conditions over the continent. In contrast to its northward progression, the southward retreat of the rainfall maximum is smooth because the development of inertial instability and air/sea interactions do not play the same role in delaying its meridional movement. Important predictors for the end of the rainy season in the Sahel are related to Indian Ocean and North Atlantic SSTs, as well as the strength of the northerly flow from the Mediterranean region.