Parsing the contributions of ocean forcings and atmospheric variability to spatially widespread droughts in the contiguous United States

Spatially widespread droughts that simultaneously affect the majority of the contiguous United States (CONUS), or pan-CONUS droughts, are an infrequent but consistent feature of North American hydroclimate over the last millennium. Events such as the 2012 drought that covered 60% of CONUS and cost an estimated $30Bn pose unique challenges because they span multiple areas that might otherwise provide redundancy in water or agricultural resources. The causes of pan-CONUS droughts are investigated using a 16-member atmospheric model ensemble forced with observed sea surface temperatures (SSTs) from 1856-2007 as a boundary condition globally or over (i) just the tropical Pacific or (ii) tropical Atlantic and climatological SSTs elsewhere. All three arrangements simulate pan-CONUS droughts with similar spatial characteristics, but the incidence of these droughts is significantly more frequent in the global ocean and tropical Pacific model ensembles. Results show that while atmospheric variability alone can produce pan-CONUS droughts, cold tropical Pacific SSTs reflecting strong La-Niña-like conditions produce an atmospheric high over the CONUS domain that shift the subtropical jets poleward and force pan-CONUS droughts. Cold tropical Pacific SSTs, however, typically bring wet conditions to the Pacific NW and for pan-CONUS droughts that specifically include this region, drying from atmospheric variability is necessary. Warm tropical Atlantic conditions also produce a detectable albeit secondary forcing of pan-CONUS droughts, but it is not clear whether the forcing is due to SST variations inherent to the Atlantic or embedded from the Pacific. Model simulations are compared with pan-CONUS drought patterns in observation-based and tree-ring estimates of the Palmer Drought Severity Index; in the case of the former, oceanic and atmospheric conditions are also considered. These comparisons indicate that the atmospheric model reproduces the spatial characteristics of pan-CONUS droughts and confirm that cold tropical Pacific and warm tropical Atlantic SSTs play primary and secondary roles, respectively, in forcing pan-CONUS droughts. Atmospheric variability is also identified as a factor that can alone cause pan-CONUS droughts or work to increase their likelihood through impacts in specific regions.