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Communication incl. Poster: BibTeX citation key:  Cooka
Cook, K. H. 2009. West African monsoon onset and demise: connecting theory and observations to aid prediction. Work presented at Third International AMMA Conference, July 20—24, at Ouagadougou, Burkina Faso.
Added by: roussot 2009-09-23 11:10:39    Last Edited by: roussot 2009-10-18 17:42:21
Categories: Atmospheric processes, Monsoon system and its variability
Keywords: Atmospheric Boundary Layer
Creators: Cook
Publisher: African Monsoon Multidisciplinary Analyses (Ouagadougou, Burkina Faso)
Collection: Third International AMMA Conference

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Onset of the West African monsoon in the Sahel (also called the monsoon jump) is sudden, characterized by the movement of the precipitation maximum from the Guinean coast into the continental interior over a few days’ time. In contrast, the southward retreat of the rainfall maximum in the late summer and fall is smooth. The difference is due to the generation of inertially unstable conditions in the spring, when the formation of the low-level West African westerly jet introduces strong meridional zonal wind gradients and low-level divergence along the Guinean coast. The dynamics is different in the fall retreat of rainfall, with no source of inertial instability and no jump.
The theoretical description of monsoon onset and demise suggests that several observable features must be in place before the monsoon onset, including the West African westerly jet, the thermal low over the western Sahara, and low-level divergence along the Guinean coast. While the exact trigger for the jump may be in synoptic-scale activity, there is information to aid prediction on time scales longer than the synoptic scale through observations of these precursors to the jump. The theory also indicates that the timing of the monsoon onset is more variable than the demise on interannual and decadal times scales, since the end of the season is primarily controlled by the seasonality of the solar forcing.
Last Edited by: roussot