Understanding the Large Scale Driving Mechanisms of Rainfall Variability over Central Africa

Abstract

Large-scale drivers of rainfall variability over Central Africa (approximately 12°S–7°N and 15°–32°E, roughly comprising the Democratic Republic of Congo basin) are examined using rain gauge data and the NCAR-NCEP reanalysis. Research into Central Africa has been neglected comparatively to other regions of Africa primarily because of a lack of suitable data. This study focuses on how local sea surface temperatures (SSTs), the African jets and mesoscale convective systems modulate precipitation through their influence on the tropical rain belt and the inter-tropical convergence zone (ITCZ). The role of El Nino Southern Oscillation (ENSO) teleconnections on the tropospheric jets and SSTs will also be described. Central Africa has been divided into 6 homogenous regions based on the seasonal cycle in rainfall. Time series analysis from each region identified 5 extremes (wet) and 5 deficits (dry) in rainfall during the primary, and if present, secondary rainy season. These years formed the basis of composites for variables, such as SST and vector wind. The role of the jets, SSTs the ICTZ and ENSO was explored further using cross validation of the rainfall time series. Our results show that Central Africa is a very complex region, with different mechanisms influencing rainfall in each regions and season. It is also shown that the influence of the large scale drivers on rainfall is not necessarily linear, with wet and dry years affected by different factors. The loci and intensity of the tropospheric jets play a determining role in the strength and position of the rainbelt with mesoscale convective activity mostly coupled between their axes. Displacement in the ITCZ (resulting from variability in SST and land-surface gradients) is also indicated as a likely influence on rainfall,. SST and land-surface gradients can also modulate rainfall by directly influencing the strength and loci of the tropospheric jets.