Characteristics of global and regional drought, 1950-2000: Analysis of soil moisture data from off-line simulation of the terrestrial hydrologic cycle

Drought occurrence is analyzed over global land areas for 1950-2000 using soil moisture data from a simulation of the terrestrial water cycle with the Variable Infiltration Capacity ( VIC) land surface model, which is forced by an observation based meteorological data set. A monthly drought index based on percentile soil moisture values relative to the 50-year climatology is analyzed in terms of duration, intensity and severity at global and regional scales. Short-term droughts (<= 6 months) are prevalent in the Tropics and midlatitudes, where inter-annual climate variability is highest. Medium term droughts ( 7-12 months) are more frequent in mid- to high-latitudes. Long term ( 12+ months) droughts are generally restricted to sub-Saharan Africa and higher northern latitudes. The Sahel region stands out for having experienced long-term and severe drought conditions. Severe regional drought events are systematically identified in terms of spatial coverage, based on different thresholds of duration and intensity. For example, in northern Europe, 1996 and 1975 were the years of most extensive 3- and 12-month duration drought, respectively. In northern Asia, severe drought events are characterized by persistent soil moisture anomalies over the wintertime. The drought index identifies several well-known events, including the 1988 US, 1982/83 Australian, 1983/4 Sahel and 1965/66 Indian droughts which are generally ranked as the severest and most spatially extensive in the record. Comparison with the PDSI shows general agreement at global scales and for these major events but they diverge considerably in cooler regions and seasons, and especially in latter years when the PDSI shows a larger drying trend.