Synoptic variability of the tropical convection over Central Africa

Spectral analysis of daily time series (1979-2005) of OLR and 3 hourly CLAUS time series (1983-1993) of Meteosat Brightness Temperature over the equatorial Central Africa reveals very striking and reproducible periodic fluctuations of the convective activity around the equinoxes with a 4-6 day period in April and with a 3-4 day period around September. Composite studies shows that April perturbations propagate eastward starting from the gulf of Guinea, whereas the September perturbations are rather stationary with two centres of activity over the Congo Basin and the Sudan.

The equatorial eastward moving April mode is associated with a westerly low-level wind perturbation consistent with a Gill-type dynamical response to the convective perturbation. The location of the main centres of the oscillation for both the April and the September modes appears also to be tied to orography. It is further hypothesized that the timing of these oscillations could be conditioned by interactions between the convective activity and the surface processes. Since these time-scales are similar to the time-scale of West African easterly waves, these surface processes could thus also play a role in the interaction between the waves and the convection (the easterly wave playing the same role as the Congo Basin in organising the convection at synoptic scale.) The good simulation of this kind of phenomenon is thus of interest for synoptic weather forecast not only over Central Africa but also for other tropical land regions.