Increasing drought in the American Southwest? A continental perspective using a spatial analytical evaluation of recent trends

One highly popularized prediction, given the ongoing build-up of greenhouse gases, is that the American Southwest will become drier as the descending arm of the Hadley Circulation intensifies in coming decades. However, other numerical modeling studies exist showing that the region may become wetter in the future; furthermore, empirical studies have revealed both increases and decreases in soil moisture in the period of historical records depending on the selected time period, data, definition of "Southwest," and statistical methods used. In this investigation, we use a variety of spatial analytical methods to assess drought trends in the America Southwest with respect to trends throughout the rest of the conterminous United States. Using 1980 as a start of our study period, we show that a highly statistically significant trend toward increased drought has occurred in the American Southwest; the trend is particularly strong over the Colorado River Basin. The primary temporal variance in PDSI values is significantly related to two major teleconnections: the Pacific North American pattern (PNA) and the Atlantic Multidecadal Oscillation (AMO); the relationship with the teleconnections is much stronger in winter than in summer. Given societal trends expected in coming decades, water demand and water supply are certain to continue to be a major focus in the American Southwest, irrespective of future hydroclimatic conditions.