The random nature of drought: Mathematical and physical causes

Annual and monthly rainfall records for three regions of the United Kingdom and five locations in Australia have been examined with specific regard to the frequencies of droughts and dry spells. It is shown that plots of cumulative totals of precipitation tend to obey are sine law behaviour and the excursions above and below the × axis fall within the limits of patterns to be expected by chance. Runs of successive values of negative anomalies from the monthly median were counted and their mean and theoretical recurrence times computed. Chi‐square tests suggested that the frequencies of runs were no more nor less than might be expected from chance expectation in the United Kingdom. In Australia the frequencies in the Perth, W. A., Adelaide, S. A., and Hobart, Tasmania records showed similar characteristics to those in the United Kingdom. However in Melbourne and Sydney the frequencies of runs of negative anomalies from the monthly median showed high significance > 1% that they were more frequent than would be expected. This higher than expected frequency of droughts and dry spells is attributed to the ENSO factor. It is suggested that overall the occurrence of precipitation events is a chance mechanism although some regions, notably southeast Australia, northeast Brazil and the Sahel are affected by climatically forced mechanisms such as sea surface temperature anomalies, blocking, over grazing etc. A discussion is offered of the meaning of “naturally” occurring drought in terms of physical interactions described by nonlinear mathematical equations and alternatively in terms of casino probability.