Characteristics of 20th century drought in the United States at multiple time scales

Abstract

Characteristics of drought and wet periods were analyzed in terms of areal coverage, intensity, duration, frequency, and variability at different space and time scales. This provided insight not only into the historical perspective of anomalously dry and wet conditions, but also into the long-term variation of climate in the United States. The Standardized Precipitation Index (SPI) provided the means to analyze drought and wet periods at different time scales, a perspective that is not achieved with typical drought indices. The National Climatic Data Center and the Carbon Dioxide Information Analysis Center compiled the U.S. Historical Climatology Network (USHCN) for the purpose of analyzing climate in the United States. The USHCN includes monthly precipitation data for 1,221 stations in the contiguous United States. The distribution of stations provided the means to examine the areal coverage of drought and wet events both nationally and regionally, and the climate record of the USHCN provided the means to analyze the frequency and variability of drought and wet events for the years 1911 through 1995. The contiguous United States as a whole has become wetter over the period 1911- 1995. Additionally, all nine major regions studied for the United States have also become wetter over the period. As a result, there has been a lower frequency of both short- and long-term droughts and a higher frequency of both short- and long-term wet periods during the last 25 years of the period of record. Also, for the country as a whole, the areal coverage and intensity of long-term droughts between 1911 and 1970 are unmatched by the long-term droughts of the last 25 years of the period. On the other hand, the short-term droughts of the last 25 years of the period do compare in intensity and areal coverage to short-term droughts of the first 60 years of the period. For the country as a whole, the average duration and frequency of short-term wet periods have increased at a magnitude opposite to the decreasing average duration and frequency of short-term droughts over this period. Moreover, the percentages of stations experiencing drought at all time scales have decreased at rates nearly opposite to the increasing percentages of stations experiencing anomalously wet conditions at all time scales. Nevertheless, the contiguous United States was never entirely in or out of drought at any time scale during this period. Additionally, the contiguous United States was never entirely experiencing or entirely without anomalously wet conditions. Regionally, the most dramatic increase in the frequency of long-term wet anomalies over the last 25 years of the period has occurred in regions along the Mississippi and Ohio river valleys. Despite the occurrence of a few intense short-term droughts, these regions have all experienced long-term wet periods in the 1970s, the 1980s, and again in the early 1990s. Furthermore, from 1970 through 1995, the most consistent seasonal wet anomalies for these regions have occurred in the autumn.