Congo Basin Hydrology, Climate, and Biogeochemistry. Группа авторов

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Congo Basin Hydrology, Climate, and Biogeochemistry - Группа авторов


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humid regions (Nicholson, 2011), the low‐spatial correlation between stations over the Congo further reduces the interannual variability on a regional scale. Notably, the interannual variability of convective activity over the Congo is also relatively low (Hart et al., 2019).

Schematic illustration of the coefficient of variation of annual rainfall, based on CHIRPS2 and averaged over the time period 1981 to 2019. Schematic illustration of location of six multi-region sectors for which interannual variability is assessed.

      The three Congo Basin series are shown in Figure 3.19a. Note that the record for the central basin only extends through 2005. In that sector similar variations are apparent in annual rainfall and for the two seasons. A brief, relatively dry period commences around 1979 but an abrupt change to extremely wet years occurs around 1993. Because the wet period occurred so abruptly and was so extreme, the station network was examined to determine whether or not discontinuities in the station network were associated with these major shifts. That was not the case. Moreover, the CHIRPS2 data are consistent with this. Thus, the very wet period is likely real and not an artefact of the paucity of stations in the region.

      The trends in the northern and southern portions of the basin are quite different from those in the central basin. In the south there are no obvious long‐term trends in either annual or ON rainfall. However, it is arguably drier since around 1970, with a preponderance of years with below normal rainfall since then. During MAM an abrupt shift to drier conditions is evident around 1970. In the north, negative anomalies likewise become more prevalent since around 1970 in the annual series and the season series. CHIRPS2 suggests some recovery in recent years in the MAM series.

      Rainfall in Cameroon shows the most pronounced long‐term trends. A shift to drier conditions around 1970 is apparent in annual rainfall and in both seasons. Some recovery is seen in recent years. This pattern is markedly similar to that over the Sahel (Nicholson et al., 2018c), where the seasonal cycle is similar to that over Cameroon.

      Pronounced changes are also evident over East Africa, especially during the ON season. An abrupt shift to wetter conditions occurred around 1960 and has continued to present. Annual rainfall is determined primarily by conditions during this season, although it is the secondary rainy season over East Africa (Camberlin & Philippon, 2002, Hastenrath et al., 2011; Nicholson, 2017). Thus the shift to wetter conditions around 1960 is also evident in annual rainfall but to a lesser extent than in ON. MAM rainfall does not show any abrupt changes, but drier conditions have prevailed since around 1970.

      The common denominator in rainfall variability in the six regions evaluated is a shift to drier conditions around 1970. It was apparent in March–April–May in all areas but the central Congo Basin, in annual rainfall in the northern and southern basin, and in October–November rainfall over the northern basin and in Cameroon. This coincides roughly the beginning of a major period of aridity in the Sahel that commenced in 1968 (Nicholson et al., 2018b). Notably, at about that same time a major change occurred in the relationship between Sahel rainfall and ENSO (e.g., Janicot et al. 1996, Losada et al. 2012, Camberlin et al. 2001).

      In the central basin a sequence of dry years occurred commencing around 1970, but conditions shifted abruptly to high rainfall in the late 1970s to early 1990s. That wet episode


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