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

Abstract : This study investigates the synoptic variability of the monsoon flux during the establishment of the West African monsoon using observations and ECMWF analyses. It highlights variability at a 3-5-day 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 detail by case-studies. This enables us 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 the 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, turbulent 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
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Submitted on : Friday, September 11, 2009 - 1:44:39 PM
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F. Couvreux, F. Guichard, Olivier Bock, B. Campistron, J.-P. Lafore, et al.. Synoptic variability of the monsoon flux over West Africa prior to the onset. Quarterly Journal of the Royal Meteorological Society, Wiley, 2010, 136 (S1), pp.159-173. ⟨10.1002/qj.473⟩. ⟨hal-00415876⟩



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