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3 The Rainfall and Convective Regime over Equatorial Africa, with Emphasis on the Congo Basin
Sharon E. Nicholson
Department of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee, Florida, USA
ABSTRACT
The paper examines the character of the rainfall and convective regime over equatorial Africa, in order to better understand the meteorological processes governing rainfall and convection in this region. Emphasis is placed on the Congo Basin region. Included is a review of the meteorological factors shaping the rainfall and convective regimes around the Congo Basin. Several aspects of rainfall and its seasonality are examined, along with the interannual variability of rainfall in key regions. In several regions a downward trend is documented, with a change toward increasing aridity occurring around 1970. However, the trends are weak, are generally not significant, and do not continue to present. The nature of the convective regime, in particular the contribution of mesoscale convective systems, is also described. Finally, a comparison is made with the rainfall and convective regime over the Amazon, an equatorial region that is considerably wetter than the Congo Basin. Large areas of the Amazon receive 2500 mm to over 3000 mm per year on average, while few areas of the Congo Basin receive more than 1750 mm per year on average. Peak rainfall tends to occur in the afternoon over the Amazon, but at night or early morning over the Congo Basin. This suggests the mesoscale convective systems play a greater role in the Congo Basin. Several reasons are suggested for the relatively dry conditions over the Congo Basin, including lower moisture flux convergence and lower relative humidity.
3.1. INTRODUCTION
The Congo Basin (Figure 3.1) is the location of the world’s second‐largest expanse of rainforest. The region has much in common climatologically and ecologically with the Amazon rainforest. However, there are some hydrologic peculiarities that have not been adequately explained. One of these is the relatively low amount of annual rainfall in the region compared to the Amazon. Several papers have suggested explanations, including lower precipitable water content over the Congo (Jackson et al., 2009; McCollum et al., 2000) and low‐level subsidence (Dezfuli & Nicholson, 2013; Nicholson & Dezfuli, 2013).
This paper does not attempt to solve the controversy. Rather, it intends to use both a literature review and several new analyses to increase our understanding of the rainfall and convective regime over equatorial Africa, and the Congo Basin in particular. Secondarily, it provides a very rudimentary comparison of some of
