Synoptic wave perturbations and convective cystems over equatorial Africa

Equatorial Africa is one of the most important convective centers in the tropics. Maximum convection and the major rainfall seasons over equatorial Africa are boreal spring (March-May) and autumn (September-November). For the boreal spring season, strong quasi-periodic (5-6 day) perturbations of the convection dominate the intraseasonal variability over this region. Composite structure computed from NOAA-OLR and ERA-40 reanalysis reveals a convectively coupled Kelvin-wave pattern. Further spectral analysis of the OLR signal in the Kelvin time-space spectral domain show that such a dominance of Kelvin-waves on the synoptic variability is a unique feature of the whole equatorial area.

The mesoscale convective systems embedded into these synoptic disturbances are studied using satellite brightness temperature at higher resolution (0.5° and 3 hours). We inspect the diurnal and the wave modulations of the occurrence, the size and the life cycle of the mesoscale convective systems. These systems are generated mostly over the western slopes of the Rift Valley highlands. They propagate west southwestward over the Congo Basin where they reach their maximum size. The Kelvin-synoptic disturbance does not modify significantly the diurnal triggering of convection but favors the development and the maintenance of large organized convection over the Congo Basin. The disturbance thus perturbs mainly the lifetime and the size of the largest convective systems. This gives persisting convection even for hours when it is usually suppressed (i.e. around noon L.T.).

The equatorial position of the ITCZ over the Atlantic Ocean and over Central Africa is hypothesized to favor the development of these coupled equatorial waves compared with other tropical regions where the ITCZ is generally off the Equator. The interaction between the convection and surface processes over this continental region could also play a large role in the growing of this synoptic disturbance.