Forced and Natural North Atlantic SST Impacts on U.S. Hydroclimate in Observations and CMIP5 Models

The frequency and severity of droughts across North America has been modulated by the phase of the Atlantic Multidecadal Variability (AMV) over the historical period. The decadal oscillations in U.S. West hydroclimate (associated with ENSO) reach extreme severity during the warm and neutral phases of AMV, such as in the 1930s and the 1950s when the U.S. Great Plains and the Southwest experienced the extremely dry conditions of the Dust Bowl and the persistent Texas drought, respectively. While when AMV was in its cold phase in the early 1900s and from 1965 to 1995 droughts were less frequent or severe. The recent increases in drought severity in the Central U.S. concurs with the cold phase of the Pacific Decadal Oscillation (PDO) but also with the transition to the warm phase of AMV. In this study, we attribute the historical droughts over North America due to anthropogenic forcing (including greenhouse gases and aerosols), AMV, and PDO using both the Palmer Drought Severity Index (PDSI) and the Standardized Precipitation Evapotransiration Index (SPEI) computed for CMIP5 models and observations from 1850 to 2014. The results illustrate the potential US drought predictability on decadal and multi-decadal time scales based on the forced and natural SST variability over the Atlantic.