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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 the most basic aspects of the hydrological regime in these two ecologically important regions, the Congo Basin and the Amazon.

The paper commences with a detailed overview of the meteorological background (Section 3.2) and the data utilized in this study (Section 3.3). The mean conditions of rainfall are described in Section 3.4. Interannual variability of rainfall is examined in Section 3.5 and the nature of the convective regime prevailing in the Congo Basin in Section 3.6. The comparison with the Amazon is presented in Section 3.7. Section 3.8 summarizes the rainfall and convective regime and suggests pressing questions about the meteorology of the region.

Figure 3.1 Depiction of topography over the Congo Basin and surrounding regions. Solid line approximates the boundary of the Congo River drainage basin. Dashed line approximates the extent of tropical rainforest.

3.2. METEOROLOGICAL BACKGROUND

      The Congo Basin, i.e., the drainage basin of the Congo River, lies primarily in the equatorial latitudes between roughly 10°N and 14°S, bounded in the north and south by highlands over the Central African Republic and Zambia, respectively (Figure 3.1). Its eastern boundary is the Rift Valley highlands, at roughly 34°E, while highlands over Cameroon and Gabon lie along its western edge around 14°E. The rainforest itself extends throughout most of the basin, generally where elevation is below about 500 meters.

      Traditionally, the rainfall regime of the Congo Basin is assumed to be associated with the twice‐annual equatorial transit of the Intertropical Convergence Zone or ITCZ, producing an annual cycle with peaks in the two transition seasons. Recent work has shown that there is little evidence of such a discrete zone of low‐level convergence over equatorial Africa during the rainy season (Nicholson, 2018; Yang et al., 2015). Moreover, low‐level divergence prevails over much of the region during those seasons.

      Clearly the factors creating the meteorological regime are much more complex. Essentially, there is a broad zone of rainfall, termed the tropical rain belt or equatorial rain belt by some authors, that moves latitudinally with the seasons. Several factors make this region conducive to the production of rainfall. Longandjo (2018), for example, shows that the rainbelt over the Congo Basin is associated with a maximum in moist static energy (MSE, a measure of bulk atmospheric energy content) with wind shear in the lower‐ and mid‐troposphere playing a role.

      The results of research on eastern equatorial Africa are also relevant here. Liebmann et al. (2017) suggested control via upper‐level winds, low‐level specific humidity, and convective available potential energy (CAPE). Their findings are consistent with those of Yang et al. (2015), who demonstrate the importance of MSE, saturation moisture static energy, and vertically integrated moisture in the lower‐ to mid‐troposphere in the development of both rainy seasons over East Africa. Yang et al. further conclude that the annual cycle of MSE is prescribed by a combination of monsoon winds and tropical Indian Ocean SSTs (sea‐surface temperatures). The monsoon winds do not extend beyond the highlands, so that their full