Synergistic Effect of El Niño and Negative Phase of North Atlantic Oscillation on Winter Precipitation in the Southeastern United States

This study reveals that the significant increase of winter precipitation over the southeastern United States (SEUS) is associated with El Niño and the negative phase of North Atlantic Oscillation (NAO−). Diagnosis of large-scale dynamics shows that El Niño and NAO− have a synergistic effect on the enhancement of transient eddies and stationary waves in the eastern Pacific, the southern United States, and the North Atlantic. These enhanced transient eddies are associated with subtropical jet stream acceleration and maintenance of subtropical low from the eastern Pacific to the Atlantic, influencing stronger stationary waves propagation from the tropical Pacific to the SEUS and North Atlantic, and from the northwestern Atlantic to the SEUS and Pacific. This favors a positive phase of the meridional dipole in geopotential height anomalies between the tropics and subtropics in the Western Hemisphere (WTSD) during the co-occurrence of El Niño and NAO−. The strong positive WTSD-like pattern, accompanied by a zonally extended and southerly shifted subtropical westerly jet, induces a northward-tilted secondary circulation with ascending motion over the SEUS and Gulf of Mexico. Simultaneously, an intensified trough over the SEUS promotes moisture and warm advection from the subtropical and tropical Pacific converging with cold advection from the northwestern North Atlantic in the adjacent Gulf of Mexico, which is beneficial for front and cyclone generation, and induces precipitation in the SEUS. This study suggests that the synergistic effects of El Niño and NAO− help to understand the variability of winter SEUS precipitation.