Projected Changes in Summertime Circulation Patterns Imply Increased Drought Risk for the South-Central United States

Historically, extreme hot, dry summers over the South-Central (SC) United States are dominated by a dome of high pressure centered over the region. Applying self-organizing map techniques to daily geopotential height data from NARR reanalysis and historical and future CMIP5 simulations, we find that, as the world warms, this type of high pressure system is likely to become stronger and more frequent—even after removing the effect of surface warming on the expansion of the lower atmosphere. These projected changes appear to be related to self-reinforcing ocean-atmosphere interactions in a warming world. Specifically, intensified easterly winds over the Atlantic and Gulf of Mexico drive an increase in the meridional sea surface temperature gradient due to oceanic Ekman transport, which further enhances the ridge of high pressure extending across the SC U.S.: a dynamical relationship that increases confidence in regional projections of increases in summer drought risk over the SC U.S. as human-induced climate change intensifies.