Congo Basin Hydrology, Climate, and Biogeochemistry. Группа авторов

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).

As demonstrated in Nicholson et al. (2019), it is difficult to ascertain the rainfall trends over the central basin because of low station density and relatively poor performance of satellite products in the region. To produce the most reliable estimates of rainfall variability in the region, three data sets are examined: the NIC131 raw gauge data, NIC131‐gridded reconstructed data set, and CHIRPS2. Rainfall is represented by a standardized anomaly, the standard deviation divided by the long‐term mean. For the raw gauge data set, this permits the calculation of regional averages based on climatologically diverse stations and a temporally varying gauge network. Initially, year‐to‐year fluctuations for multi‐region sectors (Figure 3.18) are examined, then long‐term trends are quantified for individual regions and for the Congo Basin as a whole. The regional grouping into the six sectors is determined by linear correlation between adjacent regions (e.g., Nicholson et al., 2018b).

Figure 3.19 shows the year‐to‐year variations in rainfall in these sectors, based on both gauge data over the length of record and on CHIRPS2 between 1981 and 2019. Because the post‐1980 mean evident in the gauge data is often quite different from that of the earlier period, an adjustment is made to the CHIRPS2 anomaly, based on the ratio of the post‐1980 mean to the long‐term mean. This makes the two data sets more comparable. The three Congo sectors (Figure 3.18) are labeled Central Basin, North Basin, and South Basin. The remaining three sectors (Cameroon, Congo/Gabon, and East Africa) have much more extensive gauge coverage and satellite estimates are more reliable in these areas (Nicholson et al., 2019). Agreement with these regions increases the credibility of the estimates for the Congo Basin regions.

Figure 3.17 The coefficient of variation (standard deviation divided by the mean) of annual rainfall, based on CHIRPS2 and averaged over the time period 1981 to 2019.

Figure 3.18 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.

Figure 3.19 (a) Interannual variability of rainfall for three of the sectors shown in Figure 3.18: North Congo, Central Basin, South Congo. Rainfall is expressed as a standardized departure (y‐axis), with a value of one equivalent to one standard deviation. Top: annual rainfall. Middle: March–April–May rainfall. Bottom: October–November rainfall. The diagrams on the left are based on gauge data and cover the period for which gauge data are available in the region. The diagrams on the right are based on CHIRPS2 and cover the period 1981 to 2019.Vertical lines in the gauge diagrams indicate the start of the CHIRPS2 analysis. (b) Interannual variability of rainfall for three of the sectors shown in Figure 3.18: Cameroon, Congo/Gabon, and East Africa. Rainfall is expressed as a standardized departure (y‐axis), with a value of one equivalent to one standard deviation. Top: annual rainfall. Middle: March–April–May rainfall. Bottom: October–November rainfall. The diagrams on the left are based on gauge data and cover the period for which gauge data are available in the region. The diagrams on the right are based on CHIRPS2 and cover the period 1981 to 2019. Vertical lines in the gauge diagrams indicate the start of the CHIRPS2 analysis.

Figure 3.19b shows the rainfall series for Cameroon, the Congo/Gabon area, and East Africa. The long‐term variability of annual and MAM rainfall in the Congo/Gabon region shows some similarity to that in the northern Congo. However, the shift to drier conditions occurred a few years earlier, in the early 1960s. Driest conditions occurred in a sequence of years around 1980. Rainfall then increased to a period of wet years centered around 2007. A later decline is apparent in the gauge data and in CHIRPS2. The trends in MAM are similar. During ON a shift to wetter conditions occurred in the early 1940s but since that time no obvious trends are evident. A similar increase in annual rainfall occurred in the early 1940s.

      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