Drought Prediction

Even if all of these methods of dealing with drought, famine and desertification were to be initiated immediately, the results would be a long time coming. In Africa, this means that the existing and recurring problems of drought and famine must receive continuing and immediate attention. To be effective such aid requires an early warning of the problem, fast response and timely delivery of relief. The last two are essentially socio-economic elements, but the first is physical and it has led to numerous attempts by climatologists in recent years to devise a method by which drought may be predicted.

Drought is not the sole cause of famine or desertification, but it is certainly a major cause, often initiating the problem only to have it intensified by other factors. Prevention of drought is not feasible at present, nor would it necessarily bring about an end to famine and desertification if it was. If drought could be predicted, however, responses could be planned, and the consequences therefore much reduced. The simplest approach is the actuarial forecast, which estimates the probability of future drought based on past occurrences. To be successful, actuarial forecasting requires a lengthy sequence of data for analysis (Schneider 1978). In many areas, including sub-Saharan Africa, the record is simply too short to provide a reliable prediction. Problems with the homogeneity of the meteorological record may also reduce the significance of the results.

An extension of the actuarial approach is the linking of meteorological variables with some other environmental variable which includes a recognized periodicity in its behaviour (Oguntoyinbo 1986). One of the most commonly cited links of this type is the relationship between sunspot activity and precipitation (see Figure 3.13). In North America, drought on the plains has been correlated with the minimum of the 22-year double sunspot cycle. The drought years of the mid-1970s, for example, coincided with a period of minimum sunspot activity. The previous drought, some 20 years earlier, in the mid-1950s, also fitted into the cycle. Close as such a correlation may seem, it applies less well outside the western United States. Furthermore, the relationship remains a statistical one, and, as Schneider (1978) has pointed out, there is no physical theory to explain the connection between the two phenomena.

In the search for improved techniques of

Figure 3.13 Drought and sunspot cycles in western North America

Figure 3.13 Drought and sunspot cycles in western North America

Ecological Drought
Source: After Schneider and Mtesirow (1976), Lockwood (1979)

drought prediction much time and effort has gone into the study of the physical causes of drought. The immediate causes commonly involve changes in atmospheric circulation patterns. Drought in the western prairies of Canada and the United States, for example, is promoted by a strong zonal airflow, which brings mild Pacific air across the western mountains. As it flows down the eastern slopes, it warms up, and its relative humidity decreases, causing the mild, dry conditions in winter and the hot, dry conditions in summer, which produce drought (Sweeney 1985). Seasonal drought in the Sahel was long linked to the failure of the ITCZ to move as far north as normal during the northern summer (see Figure 3.14). This is no longer generally accepted as sufficient to explain the lengthier dry spells, however, and the Sahelian drought is now being examined as part of the broader pattern of continent-wide rainfall variability, associated with large-scale variations in the atmospheric circulation (Nicholson 1989).

This type of knowledge is, in itself, of limited help in predicting or coping with drought, since, by the time the patterns are recognized, the drought has already arrived. It is necessary to move back a stage to try to find out what caused the circulation change in the first place. Over North America, for example, circulation patterns seem to be related to changing sea surface temperatures in the North Pacific, which cause the course of the upper westerlies to be altered, creating a zonal flow across the continent. The failure of the ITCZ to move as far north as normal into the Sahel has also been linked to a strengthening of the westerlies in the northern hemisphere. This prevents the poleward migration of the sub-tropical anticyclone lying over the Sahara, and it remains in place to block the rain-bearing winds which would normally flow over the area from the Atlantic (Bryson 1973). The drought in Africa has also been

Figure 3.14 Variations in the northward penetration of the monsoon rains in the Sahel 1950-72

Figure 3.14 Variations in the northward penetration of the monsoon rains in the Sahel 1950-72

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