Why do summer droughts in the Southern Great Plains occur in some La Niña years but not others?

Abstract

AbstractDroughts in the Southern Great Plains (SGP) have been attributed to the cold phase of El Niño–Southern Oscillation or La Niña. While La Niña events have been clearly linked to winter droughts, their link to summer droughts has remained unclear. We analyze the difference in precipitation between dry and nondry summers over the SGP during La Niña years. Anomalously high geopotential height and subsidence over the SGP occur in spring, along with an intensified northward moisture flux that advects moisture away to the northern plains and Midwest. The dependence of SGP drought on La Niña is statistically significant only in winter and becomes insignificant in spring and summer. The drought development in La Niña years is related to an anomalous warming over the tropical North Atlantic in spring and an anomalous negative North Atlantic Oscillation (NAO) in summer, both of which suppress precipitation by strengthening the anomalous high over the SGP and displacing the subtropical jet streams. In years with relatively large precipitation anomalies (i.e., the 27% driest and wettest La Niña years over the SGP), up to 45% of the variances of summer precipitation can be explained by a linear combination of the Niño 3.4 sea surface temperature (SST), Pacific Decadal Oscillation (PDO), tropical North Atlantic SST, and NAO indices. An anomalously strong dry summer appears to be largely a result of a superposition of these factors.