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RESEARCH PRODUCT

Spring to summer changes in the West African monsoon through NCEP/NCAR reanalyses (1968–1998)

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[1] This article focuses on the spring to summer evolutions of the West African monsoon from an energetic point of view; it uses NCEP/NCAR reanalyses along with observed and simulated rainfall data over the period 1968–1998. The results show that the West African monsoon can be viewed as a coupled ocean-atmosphere-land system phased with the zenithal evolution of solar radiation which maintains its thermally direct circulation by pumping, from the surface, enthalpy mainly in spring and latent energy in summer. In this context the horizontal energy gradients in the boundary layer play a key role during the spring to summer transition. At the beginning of spring, the moist static energy (MSE) meridional gradients are the strongest over the continent and are maintained by sensible heating and cooling. Then they relax and the monsoon air mass is transported northward into the continent. However, before the wettest (driest) July to September rainy seasons the MSE meridional gradients tend to be stronger (weaker) and tend to relax later (earlier). It is assumed that these gradients could be controlled by the seasonal evolution of precipitation and soil moisture fields: indeed, the months preceding the beginning of an abnormally wet rainy season are characterized by negative rainfall and soil moisture anomalies over Sudan-Sahel. Such anomalies could maintain stronger than normal MSE and humidity gradients between the Sahelian and the Guinean areas in late spring before they relax more intensely in summer.