Since the TAR, a variety of studies have firmly established that ENSO and SSTs in the Indian Ocean are the dominant sources of climate variability over eastern Africa (Goddard and Graham, 1999; Yu and Rienecker, 1999; Indeje et al., 2000; Clark et al., 2003). Further, Schreck and Semazzi (2004) isolated a secondary but significant pattern of regional climate variability based on seasonal (OND) rainfall data. In distinct contrast to the ENSO-related spatial pattern, the trend pattern in their analysis is characterised by positive rainfall anomalies over the northeastern sector of eastern Africa (Ethiopia, Somalia, Kenya and northern Uganda) and opposite conditions over the southwestern sector (Tanzania, southern parts of the Democratic Republic of the Congo and southwestern Uganda). This signal significantly strengthened in recent decades. Warming is associated with an earlier onset of the rainy season over the northeastern Africa region and a late start over the southern sector.
West Africa experiences marked multi-decadal variability in rainfall (e.g., Le Barbe et al., 2002; Dai et al., 2004b). Wet conditions in the 1950s and 1960s gave way to much drier conditions in the 1970s, 1980s and 1990s. The rainfall deficit in this region during 1970 to 1990 was relatively uniform across the region, implying that the deficit was not due to a spatial shift in the peak rainfall (Le Barbe et al., 2002) and was mainly linked to a reduction in the number of significant rainfall events occurring during the peak monsoon period (JAS) in the Sahel and during the first rainy season south of about 9°N. The decreasing rainfall and devastating droughts in the Sahel region during the last three decades of the 20th century (Figure 3.37) are among the largest climate changes anywhere. Dai et al. (2004b) provided an updated analysis of the normalised Sahel rainfall index based on the years 1920 to 2003 (Figure 3.37). Following the major 1982–1983 El Niño event, rainfall reached a minimum of 170 mm below the long-term mean of about 506 mm. Since 1982, there is some evidence for a recovery (see also lower panel of Figure 3.13) but despite this, the mean of the last decade is still well below the pre-1970 level. These authors also noted that large multi-year oscillations appear to be more frequent and extreme after the late 1980s than previously.
Figure 3.37. Time series of Sahel (10ºN –20ºN, 18ºW–20ºE) regional rainfall (April–October) from 1920 to 2003 derived from gridding normalised station anomalies and then averaging using area weighting (adapted from Dai et al., 2004a). The smooth black curve shows decadal variations (see Appendix 3.A).
ENSO affects the West African monsoon, and the correlation between Sahel rainfall and ENSO during JJA varied between 1945 and 1993 (Janicot et al., 2001). The correlation was always negative but was not significant during the 1960s to the mid-1970s when the role of the tropical Atlantic was relatively more important. Years when ENSO has a larger impact tend to be associated with same-signed rainfall anomalies over the West African region whereas years when the tropical Atlantic is more important tend to have a so-called anomalous ‘dipole’ pattern, with the Sahel and Guinea Coast having opposite-signed rainfall anomalies (Ward, 1998). Giannini et al. (2003) suggested that both interannual and decadal variability of Sahel rainfall results from the response of the African summer monsoon to oceanic forcing, amplified by land-atmosphere interaction.
While other parts of Africa have experienced statistically significant weakening of the monsoon circulation, analyses of long-term southern African rainfall totals in the wet season (JFM) have reported no trends (Fauchereau et al., 2003). Decreases in rainfall are evident in analyses of shorter periods, such as the decade from 1986 to 1995 that was the driest of the 20th century. New et al. (2006) reported a decrease in average rainfall intensity and an increase in dry spell length (number of consecutive dry days) for 1961 to 2000.