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Figure 3.9 (a) Mean annual rainfall (mm) based on gauge data for the period 1945 to 1984 and CHIRPS2 data for the period 1981 to 2019. (b) Mean rainfall for four individual months from CHIRPS2 for the period 1981 to 2019.

      Throughout the basin mean annual rainfall exceeds 1250 mm, but it exceeds 1500 mm over most of the region. In the core of the basin, annual rainfall exceeds 1750 mm. Within that area are three maxima, located at roughly the equator and 20°E, 2°S, and 28°E, and 3°N and 28°E. CHIRPS2 shows the maximum at 2°S and 28°E to be the strongest. Mean annual rainfall exceeds 2000 mm in at least some areas. Rainfall maxima are also evident along the Atlantic coast between roughly the equator and 7°N, over the Ethiopian highlands, over the western portion of Lake Victoria, and in a small sector of western Kenya (northeast of Lake Victoria). The aridity of eastern Africa contrasts sharply with the high rainfall over the Congo Basin.

      3.4.2. Monthly Rainfall

      Figure 3.9 also shows mean rainfall during four individual months based on CHIRPS2: January, April, July, and October. The gauge analysis indicates similar results and hence is not shown. The seasonal shift in the tropical rainbelt over the region is clearly evident. In January peak rainfall is south of the equator, with monthly rainfall on the order of 100 to 200 mm. It exceeds 150 mm over most of that area. North of the equator monthly rainfall progressively decreases from 100 mm near the equator to less than 10 mm at around 6°N. In April the rainbelt is displaced northward. Mean rainfall is on the order of 100 to 200 mm throughout the latitude span of 10°S to 7°N, with rainfall decreasing rapidly further north. Further displacement is evident in July, with high rainfall north of the equator and a rapid decrease southward from the equator. July rainfall decreases from roughly 50 mm near the equator to less than 10 mm in the southernmost regions. In October the rain belt retreats southward and high rainfall extends throughout the basin. The rainbelt is most extensive and strongest during this month. Vast areas receive 200 to 300 mm of rainfall.

      3.4.3. The Seasonal Cycle

Compared to other regions of Africa, the seasonality of rainfall over the Congo Basin is relatively weak. Over most of the region only 30 to 40% of the rainfall falls within the wettest quarter of the year (Figure 3.10). In semi‐arid regions to the north and south, that figure is generally 60% to more than 90%. Traditionally, the seasonal pattern over the Congo Basin is considered to be bimodal, with two rainy seasons and two dry seasons. The two rainy seasons are considered to be MAM and SON (e.g., Dyer et al., 2017; Pokam et al. 2014; Washington et al. 2013,). The latter season is generally assumed to be the wettest season, June to August (JJA) the driest. These generalizations are most applicable in the central basin. Beyond that area there is considerable diversity in the seasonal cycle.

Figure 3.11 shows the seasonal cycle averaged for 5‐degree grid cells. The map on the left is based on gauge data for the period 1945 to 1984 and the map on the right is based on CHIRPS2 for the period 1981 to 2019. The Congo Basin lies roughly within the latitude span 10°N to 10°S. Just north of the equator (latitudes 0° to 5°N) a bimodal pattern is evident across the region to roughly 30°E (roughly where the East African highlands begin). However, within that sector it is well‐developed (i.e., with very dry conditions between the two maxima) only in the westernmost region. Throughout these latitudes the dry season tends to be December‐to‐February. To the south (latitudes 0° to 5°S) the bimodal pattern is well developed, with peak rainfall in March–April and ON and driest conditions during JJA.

Figure 3.10 Percent concentration of rainfall in the wettest quarter of the year, based on CHIRPS2 and averaged for the period 1981 to 2019.

Figure 3.11 The seasonal cycle of rainfall, with bar graphs indicating mean rainfall in each month. Left: from gauge data averaged for the period 1945 to 1984. Right: from CHIRPS2 averaged for the period 1981 to 2019. Data are averaged for five‐degree grid boxes.

      In both regions, the relative importance of the boreal autumn rainy season decreases across the east–west extent of the basin. Reasons for the shift appear to be related to the Walker‐type over‐turnings discussed in Section 3.2. Over central equatorial Africa (Figure 3.3), the column of ascent is stronger during the boreal autumn than in the boreal spring, consistent with stronger rainy season during autumn. Over eastern equatorial Africa, the descending branch of the Indian Ocean cell is stronger in the boreal autumn than the boreal spring, thus conditions are more conducive to rainfall in the boreal spring.

      In the latitudes further to the north and further to the south, the rainfall regime tends to become unimodal. That is clearly the case in the latitudes 5°N to 10°N, where peak rainfall occurs during June‐to‐September. In those latitudes November‐to‐February or March is generally dry. To the south (latitudes 5°S to 10°S) the dry period of the boreal summer is more pronounced, with May‐to‐September being relatively dry. Although a bimodal distribution is clear in the westernmost sectors, elsewhere there is a more or less continuous rainy season from September or October through to April.

Figure 3.12 illustrates the intensity of the dry season across the region by presenting the monthly mean rainfall during the driest consecutive three‐month period. In the central basin, within 2.5° of the equator, the monthly mean during the driest season is on the order of 60 to 140 mm. Northward and southward the dry season becomes progressively drier. From 5°N to 10°N, monthly means during the dry season range from zero to 17 mm, except over Ethiopia in the east. From 5°S to 10°S, monthly means during the dry season range are below 30 mm, except near the Tanzanian coast.

Figure 3.13 shows the wettest and driest months, based on CHIRPS2 data. The month of maximum rainfall region is generally August in the far north but October over most of the basin. Southward there is a progressive shift to November then December. However, there is an anomalous region near the Atlantic coast with an April maximum and another in the southeast with a March maximum. This pattern is evident in the NIC131 gauge data as well (not shown) and may be linked to the influence of coastal sea‐surface temperatures (Balas et al., 2007). With respect to the driest month, there is a sharp shift near the equator from December or January being the driest month to the north and June or July being the driest month to the south.

Figure 3.12 Mean rainfall (mm/mo) for the driest three‐month

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