The Roles of Temperature and Precipitation in Drought: A Comparison of California and Ethiopia

Recent research indicates that the prolonged severe drought in California is characterized by both a precipitation deficit and abnormally warm temperatures. The latter has resulted in both higher evaporation rates and less snowpack in winter, providing positive feedback. Although previous multiyear droughts are present in a century long record in California, the present drought is the first one to combine a several consecutive years of abnormally warm climate with the precipitation deficit. The idea that droughts in the 21st century may be exacerbated by high temperatures is mentioned in the most recent IPCC AR5 report. In this research, we apply the ideas gleaned from the extratropical drought (California) to examine if and how temperature affects droughts in the tropics (Ethiopia). Monthly temperature and precipitation records for the period 1953 – 2013, for two regions in Ethiopia, are studied. One striking finding is a consistent large standard deviation rise in the temperatures across all calendar months from the 1950's to the present. The average increase is 4.59 standard deviations (representing a temperature increase of +3.2C), with a range of 3.86 standard deviations (September, +2.3C ) to 5.67 standard deviations (January, +3.3C). All but one calendar month have an increase in temperature of >4 standard deviations from the 1950s to the 2010s. August, a month critical for agriculture, has the largest warming at +4.1C . Given these findings, drought years are then related to individual months of temperature and precipitation anomalies in a number of regression and regression tree approaches. Results suggest that there are significant monthly temperature precursors to drought in Ethiopia. Hence, combining temperature anomalies with those for precipitation, to produce a drought risk index, may prove useful for predictive purposes.