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 explanation of the seasonal cycle is not applicable to the central equatorial Africa (i.e., the Congo Basin).
Equatorial Africa extends between the Atlantic and Indian Oceans, suggesting that both oceans would influence the climatology of the Congo Basin, particularly its moisture supply. However, there is some disagreement about the region’s moisture sources. Using water tracers in an Earth system model, Dyer et al. (2017) found that the Indian Ocean and local evaporation were the dominant moisture sources in the Congo Basin. The recycling ratio was found to be 25%. The Atlantic source was comparatively small, as moisture transported from the Atlantic into the basin is recirculated back to the Atlantic. The Indian Ocean source was found to become particularly important in wet years. Sori et al. (2017), using the Lagrangian FLEXPART model, estimated a recycling ratio of 50% and found that it increases/decreases in wet/dry years. They also found that the eastern equatorial Atlantic and land areas east of the Congo Basin are important sources of moisture for the basin. The sources appear to vary seasonally (Balagizi et al., 2018), with the equatorial Atlantic serving as a source in summer (Neupane, 2016).
The region is strongly influenced by the Walker circulation, a pattern of east–west oriented vertical circulation cells, with a primary cell in the Pacific and secondary cells primarily over the Atlantic and Indian Oceans. However, there is considerable disagreement about the seasonal development of the component cells, especially over equatorial Africa. Hastenrath (2007) claims that the cell over the Indian Ocean and eastern Africa exists only during the boreal autumn. Cook and Vizy (2016) conclude that a Walker‐type cell exists over central equatorial Africa only during the boreal summer, while Neupane (2016) and Washington et al. (2013) provide evidence of Walker‐type overturning during the boreal spring and autumn as well. Dezfuli et al. (2015) showed a Walker‐type overturning in the boreal winter, but in the south of the Congo Basin (0° to 10°S).
Figure 3.2 Schematic of the Congo Basin cell, the “pseudo” Central Africa cell, and the Walker cells over the Atlantic and Indian Oceans
(adapted from Longandjo & Rouault, 2020; Nicholson et al. 2018a. ©American Meteorological Society. Used with permission).
Longandjo and Rouault (2020) pointed out that the Congo Basin lies in between the Atlantic and Indian Ocean Walker cells, the resultant circulation being what they termed the “pseudo” Central Africa cell (see also Pokam et al., 2014). They further suggested that, in addition to the main Walker cells, a low‐level Walker‐type cell exists over equatorial Africa. They term this cell, which is capped at roughly 750 hPa, the Congo Basin cell. Figure 3.2 is a schematic illustrating these cells.
The Congo Basin cell is also seen in the mean vertical motion for October–November (ON) and for March‐to‐May (MAM) (Figure 3.3). The Congo Basin cell is seen as a peak in rising motion from the surface to 750 hPa at ~35° to 40°E and low‐level subsidence over the continent just to the west. Overriding this cell is a strong area of ascent, with a maximum around 300 hPa. To the east lies the descending branch of the Indian Ocean cell, which extends to eastern equatorial Africa in ON but appears to be limited to the western Indian Ocean in MAM.
Two other features appear to play a role in determining the characteristics of the rainfall regime over the Congo Basin. These include a mid‐level jet stream and topography. The African Easterly Jet‐South (AEJ‐S) is the Southern Hemisphere counterpart of the better‐known AEJ over West Africa (Nicholson & Grist, 2003). This jet (Figure 3.4), with a core around 600 hPa, is a response to the temperature gradient between the tropical rainforest and the woodlands to the south. Consequently, the jet is seasonal, being well developed at the end of the dry season
