Large-Scale Atmospheric Dynamics and Sahelian Precipitation

Observations show a broad band of precipitation across northern Africa, with maxima evident in some analyses on either side of the continent. A low-resolution GCM with simple boundary conditions produces such a band and, by producing a double-maximum structure, suggests the operation of distinct mechanisms for generating rainfall in the east and west. The precipitation, moisture convergence, and low-level wind convergence anomalies are very similar, indicating that an understanding of the low-level dynamics is essential for understanding the precipitation perturbation over the land surface. A linear model analysis shows that the anomalous low-level convergence is primarily forced by condensational heating in the middle and upper troposphere over East Africa. Low-level condensation and dry convection are also important for driving convergence in the west. Understanding the response of the low-level flow is key for understanding how inhomogeneity at the surface is communicated into the precipitation field. Midtropospheric condensational heating stretches vortex columns and induces a positive vorticity tendency in the lower troposphere. To establish a climatology, the low-level dynamics must adjust to balance this tendency in a way that maintains moisture convergence. The balance is accomplished by the meridional advection of low absolute vorticity air from the south and frictional effects.