We have examined in detail the modeled effect of North Atlantic (NA) extratropical SST anomalies on the North Atlantic Oscillation (NAO) and the synoptic time-scale transient circulation features of the NA storm track (Magnusdottir, Deser and Saravanan 2002). The spatial structure of the SST anomalies was the same in each 60--80 year experiment. The difference between the experiments was only associated with the amplitude and polarity of the anomaly. The spatial structure of the anomaly was that of the 40-year (1954--1994) trend in SST from the GISST2 dataset, and it has the structure of the familiar NA SST tripole. The anomalies were scaled to correspond to the centennial and bicentennial, both positive and negative, trend. We find that both polarities of the trend result in significant, but entirely different, circulation changes. In particular the negative trend (warm SST anomaly in the subpolar western NA) projects very strongly onto the NAO with a negative index. The positive trend results in the strengthening of the downstream stationary eddies and an interesting extension of the storm track into Europe.

Here, we compare the results of the aforementioned study to the results of applying tropical Atlantic (TA) SST anomalies in the same AGCM. Again, we use the GISST2 dataset to define the SST anomalies. The first EOF of SST variability in the TA is the familiar "SST dipole", straddling both sides of the equator. We split the dipole into two monopoles, one north and the other south of the equator. Our experiments consist of various combinations of the two monopoles and their polarity. We carry out six experiments in all, identified by +n, -n, +s, -s, +n-s and -n+s, where the + sign indicates a positive SST anomaly and the - sign a negative SST anomaly, and n (s) stands for SST anomaly north (south) of the equator. Each experiment is run for 80 years. In all cases, we normalize the SST anomaly to 1.5 standard deviations. We analyze the effects of these anomalies on the extratropical NA circulation, paying special attention to storm-track response and the means by which the effects of the TA SST anomalies get translated into the extratropics.