Space-time trends in U.S. meteorological droughts

The Contiguous United States and the nine climatologically homogeneous regions therein. Understanding droughts in a climate context remains a major challenge. Over the United States, the commonly used drought indices suggest a decreasing trend in severity with no significant trend in the spatial coverage of droughts. While previous studies show hydroclimate extremes, including droughts, do not necessarily follow the comparable trends exhibited by their averages, this study for the first time distinctly delineates trends in the spatial coverage of extreme meteorological droughts independently from the mean. Multiple observational data sets, as well as years indicating possibly anomalous droughts, are carefully examined to characterize data-dependent uncertainties and trends. The Standardized Precipitation Index is computed at a time scale of six months to capture the seasonality of drought. The nonlinear dependence among drought attributes necessitates the use of a copula-based tool from probability theory, leading to the development of severity-duration-frequency curves. The paper finds spatial coverage of extreme meteorological drought in the recent years (post-2010) exceeds that of the iconic droughts of the 1930s (the Dust Bowl era), and the 1950s. These results are in contrast with trends in spatial variance that does not exhibit any statistically significant trend. In addition, we find drought persistence remains relatively stationary over the last half century. The findings can inform drought monitoring and planning, and improve future drought resilience.