Drivers of the Annual Cycle of Rainfall over Central Africa: The Role of Water Vapor and the Mid-Tropospheric Meridional Circulation

Abstract

The Intertropical Convergence Zone (ITCZ), with its twice-annual passage over central Africa, is considered as the main driver of the rainfall seasonality. But recently, this paradigm was challenged. To find out what are the main drivers of the annual cycle of rainfall over central Africa, we present a simple comprehensive paradigm with both local forcings and regional-scale processes playing crucial role. Due to the local evaporative cooling effect, the foot of the ascending branch of Hadley cells occurs where the temperature is the warmest, indicating a thermal low. This distorts the southern Hadley cell by developing its bottom-heavy structure. As result, both shallow and deep Hadley cells coexist over central Africa year–round. The deep mode is associated with poleward branches at upper levels that transport the atmospheric energy. The shallow mode is characterized by a meridional return flow in the mid-troposphere that transports the water vapour instead of lower branches as widely reported. This favours the building-up of the mid-tropospheric moisture flux convergence with a limited contribution of the midlevel easterly jet, conducive to deep convection. Embedded in this strong rising branch of Hadley cells at midlevels, the intense convective rainfall, and with it the rainfall maximum position, is seasonally controlled by the dynamics of the midlevel shallow meridional return flow. This highlights the interhemispheric rainfall contrast over central Africa and outlines its unimodal seasonality. On the other hand, forced by the Congo basin cell, the precipitable water regulates the deep convection from the vegetated surface of Congo basin, acting as a continental sea. This nonlinear mechanism separates the rainfall into three distinct regimes – (i) the moisture-convergence-controlled regime, with convective rainfall exclusively occurring in the rainy season and (ii) the local evaporation-controlled regime with drizzle and (iii) the precipitable-water-controlled regime, with exponential increase of rainfall that both occur during the dry season.