Source: After Bryson and Murray (1977)

blamed on the intensification of the tropical Hadley cell. The descending arm of that cell is responsible for the general aridity in the region, and any increase in its strength or extent is accompanied by further suppression of precipitation, particularly in areas peripheral to the desert. The synchronicity of wet and dry years, north and south of the Sahara, supports this explanation (Nicholson 1981).

It has also been suggested that human activities have caused 'degradation induced drought' by contributing to the process by which the Hadley cell is intensified. Charney (1975) proposed that overgrazing and wood cutting, in the Sahel, increased the surface albedo, and disrupted the regional radiation balance. Surface heating declined as more solar radiation was reflected, and this in turn caused some cooling of the atmosphere. This cooling encouraged subsidence, or augmented existing subsidence, and helped to reduce the likelihood of precipitation by retarding convective activity. With less precipitation, vegetation cover decreased, and the albedo of the surface was further enhanced. This process was described as a biogeophysical feedback mechanism. It received considerable attention in the 1980s because it seemed to fit the observation that drought in the Sahel was more persistent than elsewhere in Africa. While such persistence may appear to be a function of the positive feedback mechanism central to the theory, it cannot be taken as confirmation of it (Hulme 1989).

As with many of the efforts to explain drought in Africa, it was difficult to develop the theory of degradation-induced drought further, because of the lack of empirical evidence. Following a study of long-term changes in African rainfall, Nicholson (1989) concluded that the human cause of drought, as espoused by Charney, did not seem feasible, but allowed that surface changes—whatever the cause—might feed back into the system to reinforce the drought. Establishing the nature and extent of these surface changes has not been easy. The most obvious approach would be the use of satellite remote sensing to estimate both vegetation destruction and changes in the surface albedo. However, the controversy over the interpretation of satellite data in assessing the extent of desertification has yet to be resolved, and problems still remain in the use of satellite derived data in this type of modelling (Thomas and Henderson-Sellers 1987). The existing data record is also short. Hulme (1989), in his assessment of the theory, estimated that changes of the magnitude proposed by Charney—a 14 to 35 per cent increase in albedo—would require vegetation destruction on at least a subcontinental scale over a period of as much as 20 years. As yet there is no evidence of this, but that may be in part a function of the inadequacy of the data, and he concluded that the theory is at best 'not proven'. Since then, Balling (1991) has calculated that desertification may actually have caused surface air temperatures to increase (rather than decrease as Charney's theory requires), although Hulme and Kelly (1993) have suggested that Balling's estimates of warming are too high.

While such studies may ultimately provide a better understanding of the problems of drought, and the mechanisms involved, they are insufficient to provide a direct forecasting mechanism. Researchers have re-examined certain relationships in the earth/atmosphere system in search of something more suitable. Their approach is based on the observation that the various units in the system are interconnected in such a way that changes in one unit will automatically set in motion changes in others, through autovariation. Since many of the changes are time-lagged, it should be possible to predict subsequent developments if the original change can be recognized. This forms the basis of the concept of teleconnection, or the linking of environmental events in time and place.

In recent years the search for a drought forecasting mechanism involving teleconnection has centred on changing conditions in the world's oceans. Sea-surface temperatures (SSTs) have been examined as possible precursors of the circulation patterns which cause drought in the Sahel, and a correlation between global SSTs and drought has been established (Folland et al. 1986; Owen and Ward 1989). When southern hemisphere ocean temperatures exceed those in the northern hemisphere, rainfall in the Sahel is below average. The warmer southern oceans may reduce the strenghth of the ITCZ leading to less uplift and therefore less precipitation. When the world began warming in the 1980s, the southern oceans warmed first and fastest, and the years with record global temperatures—1983, 1987 and 1990—were also years in which the rains failed. Only 1988 did not fit the pattern, and Pearce (1991b) has drawn attention to the possibility that if global warming continues as expected, drought in the Sahel might only get worse.

The links established between SSTs and drought in sub-Saharan Africa have allowed the UK Meteorological Office, through the Hadley Centre for Prediction and Research, to issue rainfall forecasts for the area (Owen and Ward 1989). These involve multi-stage predictions. To be useful, the forecasts have to be supplied by April, but it is the SSTs in June and July that correlate most closely with the rainfall. Thus the precipitation forecast is based on SSTs predicted two months ahead, and any changes between April and June will reduce the quality of the forecast. Apart from 1988, when a strong La Niña caused a major cooling of the tropical Pacific and ruined the forecast, the predictions have been remarkably accurate (Pearce 1991b). Although the lead time is short, the approach is one of the most promising yet developed for drought prediction.

ENSO events in the south Pacific have also received considerable attention as potential precursors of drought. It has long been known that following an El Niño changes occur in wind fields, sea surface temperatures and ocean circulation patterns. The large shifts of air and water, associated with these developments, cause major alterations to energy distribution patterns. Zonal energy flow replaces the meridional flow which is normal in tropical Hadley cells, and, because of the integrated nature of the atmospheric circulation, the effects are eventually felt beyond the tropics. Reduced rainfall has been noted in a number of semi-arid areas following such episodes and teleconnections have been established between ENSO events and precipitation in areas as far apart as Brazil, India, Indonesia and Australia. Drought in northeastern Brazil commonly occurs in conjunction with an El Niño event, and India receives less monsoon rainfall during El Niño years. The relationship is well-marked in India, where monsoon rainfall over most of the country was below normal in all of the 22 El Niño years between 1871 and 1978 (Mooley and Parthasarathy 1983). Similarly, in Australia 74 per cent of the El Niño events between 1885 and 1984 were associated with drought in some part of the interior of the continent (Heathcote 1987). Such figures suggest the possibility of El Niño episodes being used for drought prediction in some parts of the world.

There is no clear relationship between drought in the Sahel and the occurrence of ENSO events (Lockwood 1986). Semazzi et al. (1988) have suggested that sub-Saharan rainfall is linked to ENSO events through the influence of the latter on SSTs in the Atlantic. However, SST anomalies do occur in years when El Niños are poorly developed or absent. Thus, although an ENSO episode may be a contributory factor in the development of drought in the Sahel, it does not seem to be a prerequisite for that development. Associated with ENSO is La Niña, which has physical characteristics completely opposite to those of El Niño. It is a cold current rather than a warm one and flows west instead of east. The climatological impact of La Niña also seems to be opposite to that of El Niño. At the time of the last major La Niña—in 1988—heavy rain caused flooding in Bangladesh, Sudan and Nigeria. The drought forecast for the Sahel that year did not come to pass. Instead, the region experienced one of its wettest years in recent decades (Pearce 1991b). There is as yet too little data to establish a link between La Niña and rainfall variability, but it is a relationship that merits additional investigation.

Teleconnection links remain largely theoretical, although most of the relationships can be shown to be statistically significant. Certainly, in the study of drougmht, it has not been possible to establish physical relationships which would allow increasing aridity to be predicted with any accuracy. However, it seems probable that future developments in drought prediction will include consideration of teleconnections, particularly those involving time-lags, established in conjunction with the improvement of general circulation models (Oguntoyinbo 1986).

The Basic Survival Guide

The Basic Survival Guide

Disasters: Why No ones Really 100 Safe. This is common knowledgethat disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.

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