Throughout the Congo Basin mean annual rainfall exceeds 1250 mm, but it exceeds 1500 mm over most of the region. Within this central region are three rainfall maxima, within which mean annual rainfall exceeds 2000 mm in at least some areas. The rainfall maxima coincide with maxima in MCS activity and lightning frequency.
The seasonal cycle of rainfall in the region is traditionally assumed to be bimodal, with peak rainfall being associated with the twice‐annual equatorial transit of the ITCZ. Detailed analyses show this scenario to be inadequate. For one, there is no discrete low‐level convergence zone in the region. On the contrary, divergence and subsidence prevail at low levels over much of the region. Moreover, the pattern of seasonality is complex and varies significantly over relatively short distances.
While there is no distinct ITCZ, there is a well‐defined rainbelt and it does move seasonally, reaching its northernmost position in the boreal summer and its southernmost position in the boreal winter. The factors producing this broad region of rainfall are not yet well understood, but they likely include the atmospheric energy content (e.g., moist static energy) and total atmospheric moisture, in conjunction with low‐level circulation.
Over most of the region the seasonal cycle is weak and only 30–40% of the rainfall is concentrated in the wettest portion of the year. The two rainy seasons are generally considered to be MAM and SON, with the latter being the more important season, and the main dry season being JJA. This generalization holds mainly in the latitudes 0° to 5°S, but the second peak tends to fall in ON. The relative importance of the two seasons changes cross the east–west extent of the basin, with MAM becoming the dominant season further east. To the north, the dry season is DJF and the bimodality is weak, with only a slight reduction evident in the boreal summer. In the northernmost region of the basin, in the latitude span of 5° to 10°N, the pattern becomes unimodal with a peak in the boreal summer, generally in August. In the southernmost region, in the latitude span of 5° to 10°S, the seasonality tends to be bimodal but with only one pronounced maximum.
3.8.3. Spatial and Temporal Variability
One of the very unusual characteristics of the rainfall regime over the Congo is the extremely low spatial variability. Elsewhere in Africa rainfall variability on interannual time scales is coherent over large areas. Over the Congo, variability is localized and the correlation between individual stations is exceedingly low in both rainy seasons.
Because of the paucity of gauge data in the Congo Basin, it is difficult to provide a detailed and reliable picture of the interannual variability of rainfall. Six sectors were examined, three of which fall largely within the Congo Basin. In most of the regions a shift to drier conditions occurred around 1970. It was apparent in MAM in all areas but the central Congo Basin, in annual rainfall in the northern and southern basin, and in ON rainfall over the northern basin and in Cameroon. This was roughly synchronous with the major decline in Sahel rainfall c. 1968 that was evident in all months between April and November (Nicholson et al., 2018b, c). Abrupt changes occurred in the rainfall regime along the Guinea Coast as well around the same time. The changes observed in those regions and in the Congo Basin appear to be associated with a quasi‐global change in teleconnections to the global oceans.
3.8.4. Conclusions
There are several questions concerning the meteorology of the Congo Basin that require further study. Examples are the reasons for the low spatial coherence of rainfall variability, the reasons for the drier atmospheric conditions compared to the Amazon and other equatorial regions, the inter‐relationships between large‐scale factors such as the Walker Circulation and more local factors such as orographic circulations, and the reasons why different factors appear to control the intensity of storms versus the rainfall produced.
Concern about climate change and its impact on the Congo rain forest has already been raised, in light of a decline in forest productivity (Zhou et al., 2014). Analyses presented in this paper indicate a long‐term decline in rainfall that could be linked to global climate change. Hence, better projections of future climate would be desirable. Unfortunately, projections made by climate models cannot yet be used with confidence (Creese et al., 2019, Crowhurst et al., 2020). A better understanding of the meteorological processes controlling rainfall and convection over the Congo Basin could contribute towards improving these models.
ACKNOWLEDGMENTS
This project is supported by a grant from the National Science Foundation, Number 1854511. The author would like to thank Douglas Klotter for the production of figures and Liming Zhou and Adam Hartman for providing several figures used in the chapter.
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