The Influence of Atlantic Sea Surface Temperature Anomalies on the North Atlantic Oscillation

The influence of Atlantic sea surface temperature (SST) anomalies on the atmospheric circulation over the North Atlantic sector during winter is investigated by performing experiments with an atmospheric general circulation model (GCM). These consist of a 30-year run with observed SST anomalies for the period 1961–90 confined geographically to the Atlantic Ocean, and of a control run with climatological SSTs prescribed globally. The simulated monthly variance of 500-mb geopotential heights over the North Atlantic attains more realistic values when observed Atlantic SSTs are prescribed. The latter are not found not to make marked changes to the structure of the model's preferred modes of variability over the North Atlantic, as determined by the leading EOFs of monthly means, and a cluster analysis of daily fields. However, circulation patterns which resemble the positive phase of the North Atlantic Oscillation (NAO), are more pronounced when observed Atlantic SSTs are prescribed; the variance of the interannual NAO index increases by five-fold over its control value. Atlantic SST variability is also found to produce an appreciable NAO-like response in the December–February time mean. The model's NAO is found to be sensitive to SST anomalies over the tropical and subtropical South Atlantic using a singular value decomposition analysis. These SST anomalies influence the summer monsoonal circulation over South America, which in turn affects the model's cross-equatorial regional Hadley circulation. These results are confirmed using an additional 30-year GCM experiment with observed SSTs confined to the South Atlantic.