Spatial modes of drought in the Central United States

The occurrence of drought in the central U.S. during the summer growing season can have enormous economic and social impacts. However, most droughts are not ubiquitous throughout the region between the Appalachian and Rocky Mountains, but instead are focused in subregions of this domain in any given year. The purpose of this paper is to examine the regional patterns of drought occurrence on an interannual basis, and to begin to explore the disparate impacts of the various spatial modes of drought. Thornthwaite water balance deficits were estimated for the climate divisions for each growing season since 1895, and these data were then subjected to principal component analysis and clustering to identify coherent regions that experience drought simultaneously. Five regional drought types were identified in the central U.S., focused on the southeastern portion of the region, the southern Great Plains, the northern Great Plains, the upper Midwest, and the central Midwest. Time series of soil water deficits in the centers of these regions indicate a variety of histories during the century. While all the regions have deficit local maxima in the 1930s era, the shape and timing of the peak varies. The addition of unique patterns outside of the 1930s era results in considerable separation in the drought histories of the five regional drought types. The relationships between the drought state time series and other climatological factors such as temperature, precipitation, and circulation will be discussed. Finally, the differential impacts of each regional drought mode on agricultural yields in the central U.S. will be examined. Because of the variable spatial nature of drought, there are usually some geographically distributed winners and some losers in the circumstances of each drought type.