Evolving Impacts of Multiyear La Niña Events on Atmospheric Circulation and U.S. Drought

Wintertime precipitation over the southern US is known to decrease with interannual cooling of the equatorial Pacific associated with La Niña, which often persists two years or longer. Composite analysis based on a suite of observational and reanalysis datasets covering the period 1901-2012 reveals distinct evolution of atmospheric teleconnections and US precipitation anomalies during multi-year La Niña 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. US 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 SST anomalies of broader meridional extent. Given the extended climatic impacts, accurate prediction of La Niña duration is crucial.