Time series of rainfall departures for the four regions shown in the inset map (solid lines) compared to the time series for East Africa as a whole (dashed lines). Values are expressed as a percentage standard departure from the long-term mean.

tion anomaly occurs that is sufficiently great to override that local forcing.

In southern Africa, a region with rainfall teleconnections to the Sahel (see above) but without strong interannual persistence of rainfall anomalies, the interannual variability of surface fluxes is smaller and less spatially coherent. The rain-bearing systems in that region are also less sensitive to surface processes than are those affecting the Sahel.

This might suggest the following hypothetical scenario for explaining the unusual pattern of the 1970s: below-normal rainfall in Sahelo-Saharan regions and above-normal rainfall in corresponding sectors of Southern Hemisphere Africa. The factors producing drought commencing in 1968 were reinforced in the Sahel by land-atmosphere interaction that caused the negative anomalies to persist without interruption throughout the 1970s. In the mid-to-late 1970s, some factors in the general atmospheric circulation produced wet conditions in southern Africa and also enhanced rainfall in Sahelo-Saharan regions. In the latter, however, they were insufficient to override the drought-promoting feedback and resulted in less extreme negative anomalies in years such as 1974 to 1978, but not in rainfall exceeding the long-term mean.

It is interesting to note that when only high-frequency fluctuations are considered, the rainfall fluctuations in the Sahel bear a much stronger similarity to those in the Kalahari (Figure 15) during the past 40 years. The correlation between the resulting "Sahelian" series and the rainfall series for the northern Kalahari is 0.5, just slightly lower than the correlation (0.57) between the series for the northern and southern Kalahari in Figure 8.

The results shown in Figure 15 are consistent with the idea that higher-frequency fluctuations, or yearly departures from a decadal or multi-decadal mean, are forced by similar factors in Sahelian and southern Africa. but that additional causal mechanisms of lower-frequency variability must be sought in the Sahel. A similar contrast in forcing is apparent between various parts of the Sahel and between the early

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