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 an isolated dome of high pressure centered over the region. Applying self-organizing map techniques to North American Regional Reanalysis reanalysis and historical and future Coupled Model Intercomparison Project version 5 (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 United States: a dynamical relationship that increases confidence in regional projections of summer drought risk over the SC United States. Plain Language Summary Water is already a scarce resource in the South-Central United States. The region's frequent droughts pit its agriculture community against the energy industry, ecosystem managers, and growing municipalities in competition for the region's increasingly over-allocated water resources. Our study identifies the daily weather patterns associated with historical drought and examines how these are likely to change as the world gets warmer. We find that the main weather pattern associated with drought, an isolated dome of high pressure centered over the South-Central region, is likely to become stronger and more frequent by the late 21st century under human-induced climate change. We also find that this appears to be related to self-reinforcing ocean-atmosphere interactions: in other words, the warmer the world gets, the stronger this pattern becomes. This is not good news for the region, as it establishes a physical mechanism that explains why projected summer rainfall is expected to decrease, and droughts to become stronger and longer, as climate continues to change.