Inertial Instability as a Mechanism for Latitudinal Shifts in the West African Rain Belt

Interannual variability of rainfall in the West African Sahel is regulated by changes in the intensity of the West African monsoon and changes in the latitudinal location of the rainfall maximum, termed the rainbelt. Case studies of several individual years showed that the latitude of the maximum during August, the month of peak Sahel rainfall, is strongly linked to inertial instability. When the criteria for inertial instability are satisfied, a low-level westerly jet stream develops along the Guinea Coast, displacing the African Easterly Jet northward. This in turn, displaces the rain-producing systems northward. In this paper, the mechanism is further examined, considering additional months of the rainy season and also examining the potential role of inertial instability on decadal time scales. It is also considered in the context of the so-called mid-season “jump” in the rainbelt. Results show that in years in which this abrupt latitudinal shift does not occur, the criteria for inertial instability over West Africa are not satisfied.