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Natural Climate Variability on Decade-to-Century Time Scales
Geographical regions described in detail in this study.
relatively wet conditions in the 1950s and relatively arid conditions in the 1980s. The periods of anomalous conditions have not been precisely 10 years in length, nor have they commenced at the onset of each numeric decade (e.g., 1950 or 1960). Nevertheless, these periods and the transition between them are well illustrated from decadal maps and tabulations (Figure 4 and Table 1).
Fluctuations have been particularly large in Sahelian West Africa, where rainfall was about twice as great during the 1950s as during recent years (Table 1). The 1950s (Figure 4a) were also wetter than average over most of the rest of the continent, but abnormally dry in the equatorial regions. The wet episode ended abruptly in the Sahel toward 1960, after which time a change occurred in many other regions.
In the early 1960s, phenomenally wet years occurred in equatorial regions, especially in eastern Africa, while droughts affected much of southern Africa. As an example, Wajir, in northern Kenya with mean annual rainfall of 285 mm, received 612 mm in November 1961; at many Kenyan stations, rainfall that month was five to ten times the long-term mean. The levels of Lake Victoria and other Rift Valley lakes suddenly rose several meters commencing in 1961, abruptly attaining levels unmatched since the end of the nineteenth century. The 1960s as a whole (Figure 4b) was a period of relatively wet conditions throughout the equatorial latitudes and abnormally dry conditions throughout most of the subtropics of both hemispheres. The dry anomaly was particularly strong in the Southern Hemisphere.
The early 1970s were a period of drought over much of Africa (Nicholson, 1986), but later in the decade a series of tremendously wet years occurred in southern Africa, so that for the decade as a whole, a strong opposition is apparent between the two hemispheres (Figure 4c). This is not a common configuration of anomalies, and had previously occurred in only a few isolated years. A plausible explanation is presented below.
The 1980s (Figure 4d), in strong contrast to the 1950s and unlike other decades, was a period of abnormally dry conditions over most of the continent, including most equatorial regions. The decadal anomalies are largest in the Sahel, on the order of + 25 percent in the 1950s and -30 percent in the 1980s (or nearly one standard deviation). In most other areas the departures for the 1950s (Nicholson, 1989) and 1980s (Table 2) are less than 10 percent of the mean.
The continental scale of these anomalies and the abrupt change around 1971 are evident in Figure 5, which shows the linear correlation of the rainfall anomaly pattern of each year since 1901 with anomaly Type 2 in Figure 2. This anomaly type is one of negative departures throughout the continent. Most years from 1950 to 1970 are negatively correlated with it; correlations for 1950-1957, 1961, and 1962 are significant at the 99 percent level. In contrast, nearly every year since 1970 is positively and, in most cases, significantly correlated with the Type 2 pattern.
Teleconnections and Other Characteristics of Rainfall Variability: Comparison of Modern and Historical Periods
The previous section identified several characteristics of African rainfall variability on decadal-scales. One is the tendency of anomalous conditions to persist for roughly a decade. Another is the large magnitude of anomalies in the Sahel. A third is the teleconnections within Africa on decadal-scales, as illustrated by the anomaly types of Figure 2 and the decadal patterns for the 1950s and 1980s (Figures
TABLE 2 Mean Annual Rainfall (mm) at Select Sahelian Stations for Two Recent Periods