Tropical Atlantic air-sea interaction and its influence on the NAO

Abstract

An atmospheric general circulation model (AGCM) is forced with a prescribed SST dipole anomaly in the tropical Atlantic to investigate the cause of cross-equatorial SST gradient (CESG) variability and its teleconnection to the extratropics. The model response bears a striking resemblance to observations in both the tropics and extratropics. The tropical response is robust and can act to reinforce the prescribed SST anomalies through wind-induced evaporation. A new feedback mechanism involving low-level stratiform clouds in the subtropics is also identified in the model and observations. The tropical SST dipole forces a barotropic teleconnection into the extratropics that projects onto the North Atlantic Oscillation (NAO). It further induces the extratropical portion of the North Atlantic SST tripole when the AGCM is coupled with an ocean mixed layer model. CESG variability thus appears to be the centerpiece of a pan-Atlantic climate pattern observed to extend from the South Atlantic to Greenland.