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Natural Climate Variability on Decade-to-Century Time Scales
Correlation coefficient between the anomaly pattern of each year's rainfall (averaged over the regions shown in Figure 1) and the anomaly pattern corresponding to Type 2 in Figure 2. For the 99 percent confidence level, r ranges from about 0.28 to 0.31, depending in part on the number of regions with available data in a given year.
4a and 4d). Overall, the decadal rainfall fluctuations tend to occur quasi-synchronously over the continent. There is a strong tendency for the fluctuations in the subtropical latitudes of both hemispheres to be roughly in phase, as well as a tendency for the fluctuations in the equatorial and subtropical latitudes to be opposed to each other. These same characteristics are evident in the historical past.
Figures 4a and 4d clearly demonstrate that major periods of anomalous rainfall in the Sahel correspond to continental-scale anomalies as well. This is likewise true for historical fluctuations of Sahel rainfall, such as the droughts of the 1820s and 1830s and the 1910s (Figure 6). Both of these periods were times of frequent drought throughout much of the continent. A period of sufficient rainfall from about 1870 to 1895 was part of a continental pattern mirrored by the 1950s pattern (Figure 4a). In the 1950s, lake levels rose dramatically from Chad in the north to Ngami in the south, and the floods of rivers like the Niger and Nile were consistently high. Near Timbuktu, in the Niger Delta where annual rainfall is now 195 mm, wheat production thrived to such an extent that grain was exported to neighboring regions. In the semi-arid regions of North Africa rainfall probably averaged about 25 to 30 percent above the twentieth-century mean during the period 1870-1895 (Nicholson, 1978).
Interestingly, the historical patterns depicted in Figure 6 were constructed prior to the derivation of the rainfall anomaly types in Figure 2; they were based on proxy data and qualitative indicators. While this origin might render the reconstructions questionable, their reliability is supported by the clear correspondence to the major anomaly types apparent in the modern quantitative rainfall record. They also demonstrate other major characteristics of African rainfall variability established with modern records: the decadal-scale persistence of anomalies in the Sahel, and their continental scale.
In certain cases, global teleconnections to decadal-scale African rainfall fluctuations have also been demonstrated. One example is the change from "wetter" to "drier" conditions around the turn of the century. This change was seen throughout the tropics, with a major shift occurring around 1895 (see, e.g., Kraus, 1955a,b). The anomalously dry 1830s was also a period of globally anomalous climate, often considered to mark the end of the Little Ice Age. The anomalous conditions of the 1950s are also noted in other tropical and subtropical regions such as Central America, India, and Australia. These decadal teleconnections have not, however, been studied systematically.
Fluctuations on Time Scales of Centuries
The climatic fluctuations that have affected Africa on time scales of centuries are difficult to establish because
African rainfall anomalies for three historical periods. Minus signs denote evidence of drier conditions; plus signs denote evidence of above-average rainfall; zeroes denote near-normal conditions; circled symbols denote regional integrators, such as lakes or rivers. (From Nicholson, 1978; reprinted with permission of Academic Press Ltd.)