Simulation of North Atlantic low-frequency SST variability

Abstract

The role of atmospheric circulation anomalies in generating midlatitude sea surface temperature (SST) variability is investigated by means of ocean general circulation model (OGCM) experiments, in which observed winds are prescribed during the period 1950-1979. The heat flux parameterization involves atmospheric advection equivalent to specifying an interactive atmosphere, albeit a simple one. Forced by the observed wind field, the model is successful in reproducing the large-scale patterns of the observed wintertime SST in the North Atlantic. The local correlations between observed and simulated wintertime SSTs are high enough to be significant around 40° N but are not as high in summer (when the scales of the anomalies are smaller) and north of 50° N. The model results are used to understand more about the mechanisms of creating interannual SST variability in winter. During wintertime, strong northwesterly winds from the American continent cool the northwestern part of the ocean, and advective (Ekman) transports of cold water from the north intensify the SST anomaly. The influence of the sensible and latent heat fluxes is slightly higher than that of the advective transport. During this 30-yr period the atmospheric circulation and the (observed and simulated) SST also show anomalies that persist for several years. In the 1970s, a strong cyclone over Greenland was repeated in several consecutive winters, and the simulated SST was anomalously cold from 1973 to 1978. Weaker northwesterly winds around 40°-50° N in the 1950s led to an anomalously warm model SST from 1951 to 1955. The simulation is particularly close to the observation during the warm 1950s and delayed and reduced in amplitude in the cold 1970s when a great salinity anomaly was observed in the northern North Atlantic.