Some dynamical aspects of the precipitation regime over western equatorial Africa

The precipitation regime over western equatorial Africa is characterized by weak spatial coherence of interannual variability, extreme intensity of convection, and high lightning frequency that is maximized over the central Congo Basin. The origin of rainfall is traditionally linked to the twice-yearly passage of the ITCZ. Our analysis shows a very different picture. The diurnal and spatial patterns of rainfall and lightning suggest that the regional topography plays a major role, with large-scale mountain-valley wind systems accounting for much of the character of the regime. A second factor is the presence of a mid-level easterly jet in the southern subtropics that appears to enhance convection during one of the two rainy seasons. The jet owes its existence to the surface temperature gradients that develop at the end of the dry season in the Southern Hemisphere subtropics, i.e., during the October-November rainy season. Interestingly, easterly wave activity is present during both rainy seasons, but the origin of the waves is different in the two seasons.