On the Mechanisms that Control the Rainy Season Transition Periods in the Equatorial Congo Basin

Abstract. The Congo Basin equatorial region (2° S–2° N, within the watershed), experiences semi-annual rainy seasons (RSs) in boreal spring and fall. Previously, the mechanisms driving the transitions to these RSs have not been investigated systematically. We show that both RS transitions begin with increases in the low-level atmospheric moisture transport from the Atlantic Ocean into the region, ~1.5 months prior to the spring RS and ~2 months prior to the fall RS. Evapotranspiration contributes the most to atmospheric moisture but does not change significantly throughout both transition periods. Sharp precipitation jumps 10 days before the start of the spring and fall RSs result from boundary layer moisture increases orographically uplifted to the lower troposphere by the East African Rift and Congo Basin Cell ascending branch. This destabilizes the lower free troposphere, lowering the level of free convection and decreasing convective inhibitive energy. Meanwhile, the African Easterly Jet-North and westerly return flow of the Congo Basin Cell induce vertical shear for the spring and fall RSs, respectively. Mid-tropospheric convergence strengthens due to return flow at ~700 hPa from shallow meridional cells that direct low-level moisture towards the West African Heat Low prior to the spring RS, and to the Congo Air Boundary prior to the fall RS. Therefore, the RS onsets over the equatorial region are driven by seasonal changes in large-scale atmospheric circulation, contrasting with the pivotal role of increasing evapotranspiration in driving the transition to the RS onset over the southern Congo Basin.