Читать книгу Congo Basin Hydrology, Climate, and Biogeochemistry - Группа авторов - Страница 42

Prior work (e.g., Nicholson et al., 2012) has examined large‐scale rainfall variability over Africa by delineating 90 regions that are relatively homogeneous with respect to interannual variability. The Congo Basin and surrounding areas include eight of these regions.

The regions in the subtropical portions of the continent (e.g., the Sahel) and in eastern equatorial Africa are highly homogeneous. That is, the individual stations correlate very highly with the region mean. The spatial coherence of rainfall variability (correlations among regions) is also strong in subtropical areas, with broad expanses of the continent experiencing drought or wetter conditions in the same years.

In comparison, the homogeneity within the eight regions in and around the Congo Basin is relatively weak (Nicholson, 1986) and the very low spatial coherence of rainfall variability on interannual time scales is unique within Africa. This is evident both in inter‐regional correlations (e.g., Dezfuli & Nicholson, 2013) and inter‐station correlations. Figure 3.15 illustrates the former by correlating rainfall in individual regions (see Nicholson et al., 2018b) with rainfall for a much larger geographical sector. For the Sahel, East Africa, southern Africa, and central Africa, correlations typically range from roughly.6 to.9, indicating strong coherence over very large areas. The Congo Basin stands out with correlations below.4.

Figure 3.15 Correlation of annual values of regional rainfall (delineated by thin lines) with rainfall averaged for the larger sector (bold solid lines) in which the regions are found. Correlations are based on the period 1930 to 1989.

Figure 3.16 shows for the same sectors the correlation between stations as a function of distance during the main rainy seasons. The extremely low correlations over the Congo Basin clearly stand out. Within 100 km they fall to roughly.26 for MAM and.13 for ON, compared to approximately.4 to.7 for other locations. Within 2 km they fall to around.2 over the Congo, while elsewhere correlations remain much higher (.4 to.5 in most cases) for inter‐station distances up to 5 km.

A full understanding of the meteorology of the Congo Basin requires an understanding of the reasons for the low spatial coherence, a characteristic that suggests very localized influences on rainfall. This is consistent with the results of other studies suggesting that links to large‐scale forcing are very weak over the Congo Basin (e.g., Dezfuli & Nicholson, 2013; Nicholson & Dezfuli, 2013; Pohl & Camberlin, 2006). A potential explanation for the low spatial coherence of rainfall variability within the Congo Basin may be the strong orographic effects (Jackson et al., 2009). These interact with the large‐scale flow to produce a complex pattern of low‐level winds and ascent, with large changes over small distances.

Other factors that might contribute include the dominant contribution of very large storms, the impact of the MJO (e.g, Berhane et al., 2015; Raghavendra et al., 2020), and the contribution of convectively coupled equatorial waves (e.g., Sinclaire et al., 2015). Zipser et al. (2006) calculated that roughly 3% of the rainfall systems provide 60 to 70% of the region’s rainfall, suggesting a strong impact of a “hit” or “miss” by one or two storms. Our preliminary work on the MJO and convectively coupled equatorial waves has shown that the contribution of these phenomena to rainfall shows substantial geographical variation over equatorial Africa. We have also shown that of the equatorially “trapped” waves, only the Kelvin wave plays a major role in the region.

Figure 3.16 Inter‐station correlation during the period 1930 to 1989 as a function of distance between stations (in km).

Source: Jackson et al., 2009. © American Meteorological Society. Used with permission.