The position of the monsoon trough over North West Africa has a profound impacton the agriculture of the region and the representation of the tropics in global models. The meridional movement of the trough and the moisture laden monsoon air varies on many time scales, from seasonal to diurnal. Results from analysis of data from the HAPEX-Sahel campaign show a pronounced diurnal cycle in the wind fields over northern Africa during the dry season (October-May). The diurnal cycle persists into the wet season (June-September) although with a weaker intensity. Mechanisms for this change in intensity of the diurnal cycle will be presented and discussed. During the summer, the monsoon winds in the Sahel typically increase overnight, and are dramatically reduced by convective turbulence during the day.

Evidence will be presented linking localised boundary layer properties with soil moisture patterns caused by antecedent rainfall. Furthermore, aircraft observations over such soil moisture heterogeneity indicate coherent, thermally-directmesoscale circulations associated with the boundary layer anomalies. Modellingresults examining the role of the land surface in the dynamics of the monsoon will also be discussed.