Synoptic variability of the monsoon flux over West Africa prior to the onset

This study investigates the synoptic variability of the monsoon flux during the establishment of the West African monsoon using observations and ECMWF analyses (see Couvreux et al., QJRMS, 2010). It highlights variability at a 3-5 days time scale, characterized by successive northward excursions of the monsoon flux. Their characteristics and climatology prior to the monsoon onset are presented. These penetrations follow a maximum of intensity of the heat low (extension and minimum of pressure) and are concomitant with an acceleration of the low-level meridional wind. Some penetrations are stationary whereas others propagate westward simultaneously with African Easterly waves. Both types are investigated in more details by case studies. This enables to distinguish the boundary-layer mechanisms involved in such penetrations. A similar conceptual model holds for both. It is argued that the heat-low dynamics is a major driver of these synoptic penetrations, pointing to a predominantly continental nature of this phenomenon. In turn, the heat low can be partitioned by the penetrations. Horizontal advection is the main process that eventually accounts for these surges; nevertheless turbulence mixing also plays a significant role by vertically redistributing moisture, and in more subtle ways, by its contribution to the shaping of the low-level synoptic environment within which the surges take place.