  Understanding droughts in a climate context remains a major challenge. Over
the United States, different choices of observations and metrics have often
produced diametrically opposite insights. This paper focuses on understanding
and characterizing meteorological droughts from station measurements of
precipitation. The Standardized Precipitation Index is computed and analyzed to
obtain drought severity, duration and frequency. Average drought severity
trends are found to be uncertain and data-dependent. Furthermore, the mean and
spatial variance do not show any discernible non-stationary behavior. However,
the spatial coverage of extreme meteorological droughts in the United States
exhibits an increasing trend over nearly all of the last century. Furthermore,
the coverage over the last half decade exceeds that of the dust bowl era.
Previous literature suggests that climate extremes do not necessarily follow
the trends or uncertainties exhibited by the averages. While this possibility
has been suggested for droughts, this paper for the first time clearly
delineates and differentiates the trends in the mean, variability and extremes
of meteorological droughts in the United States, and uncovers the trends in the
spatial coverage of extremes. Multiple data sets, as well as years exhibiting
large, and possibly anomalous, droughts are carefully examined to characterize
trends and uncertainties. Nonlinear dependence among meteorological drought
attributes necessitates the use of copula-based tools from probability theory.
Severity-duration-frequency curves are generated to demonstrate how these
insights may be translated to design and policy.
