Taking the pulse of the monsoon flux over West Africa prior to the monsoon onset

The West African monsoon provides most of rainfall over the Sahel. Boundary layer processes and structures play a crucial role as the monsoon flux occurs in this layer. The establishment of the monsoon flux over West Africa has not been extensively explored. Several hypotheses have been proposed to account for the abruptness of the onset of the West African monsoon underlining the role of the ocean, the role of atmospheric dynamics or both. All the previous studies have focused on relatively large time-scale (> 10 days) and spatial scales (> 2°) and have systematically removed the higher frequency signals from the different fields in their analysis partly due to the lack of observations. In this study we investigate the higher frequency fluctuations as revealed by observations and ECMWF analysis during the establishment of the monsoon. During this phase, successive northward excursions of the monsoon flux are frequent. Such pulsations are part of the intra-seasonal variability. This study focuses on their characteristics, their climatology. They are better defined during the period of installation and retreat of the monsoon whereas their characteristics are modified during the established monsoon. These penetrations have a five-day period. They follow a maximum of intensity of the Heat Low (extension and minimum of pressure) and are concomitant with an increase of the monsoon flux. A case study in June 2006 (4-7 June) enables to distinguish the boundary-layer mechanisms involved in such pulsations: they suggest that the Heat Low dynamics is a major driver of these synoptic penetrations. Using ECMWF analysis, observations acquired during the AMMA field campaign and mesoscale simulations, we highlight a fine-scale mode of intra-seasonal variability of atmospheric moisture. Our results point to a predominantly continental nature of this phenomenon. Horizontal advection of moisture is the main process that eventually accounts for these pulsations; 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 pulsations take place.