Evolving Impacts of Multiyear La Nina Events on Atmospheric Circulation and US Drought

Wintertime precipitation over the southern U.S. is known to decrease with interannual cooling of the equatorial Pacific associated with La Nina, which often persists 2 years or longer. Composite analysis based on a suite of observational and reanalysis data sets covering the period 1901-2012 reveals distinct evolution of atmospheric teleconnections and U.S. precipitation anomalies during multiyear La Nina events. In particular, atmospheric circulation anomalies strengthen and become more zonally elongated over the North Pacific in the second winter compared to the first winter. U.S. precipitation deficits also remain large, while the region of reduced precipitation shifts northeastward in the second winter. This occurs despite a significant weakening of the equatorial Pacific cooling in the second winter and suggests that the large-scale atmospheric circulation is more sensitive to tropical sea surface temperature anomalies of broader meridional extent. Given the extended climatic impacts, accurate prediction of La Nina duration is crucial.