Motivated by arguments connecting North Atlantic SST variability to critical changes in global climate, Bjerknes (1964) was the first to provide insight into the shifting hierarchy of the ocean-atmosphere system on interannual and interdecadal time scales. In his investigation of SST and SLP variability, Bjerknes showed that a slow warming of Atlantic SST south of 50šN, centered on the year 1920, coincided with a positive SLP anomaly at 30šN (the Azores anticyclone), and a negative anomaly north of 50šN. This is the pattern already described as the classic signature of the NAO. Based on this finding, whereby interdecadal warming corresponded to a strengthening of the overlyinig westerlies (ocean dominates) and interannual warming corresponds to a weakening of the westerlies (atmosphere dominates), Bjerknes (1964) concluded that ocean-dominated SST interdecadal warming is linked to a slow intensification of the Gulf Stream and North Atlantic Current in response to intensified circulation of the AH.
Kushnir (1994) strengthened the idea that a complex, dynamic relationship exists between the ocean and atmosphere based on investigations of the interannual and interdecadal frequency dependence of North Atlantic SST anomalies. This study demonstrated that the ocean and atmosphere in the North Atlantic display at least two different modes of interaction: 1) SST anomalies on an interannual time scale show a consistent, local relationship to surface circulation, and 2) SST anomalies on an interdecadal time scale show a dynamic, basin-wide relationship suggestive of changes in the large scale oceanic circulation (Kushnir, 1994; Kushnir and Wallace, 1989) .
Interdecadal variations in the North Atlantic SST's revealed that middle and high latitude SST's display a long-term fluctuation with negative anomalies before 1920, and during the 1970's and 1980's (Deser and Blackmon, 1993) . Positive SST conditions prevailed from about 1930 to 1960 (as first noted by Bjerknes (1964)). The principle warming and cooling anomalies are seen in the vicinity of Iceland, in the Labrador Sea, and northeast of Bermuda (Deser and Blackmon, 1993; Houghton, 1996; Kushnir, 1994) . A similar trend exists in the Mediterranean region, with a minimum SST anomaly at roughly 1910 and a maximum in 1940 and 1965 (Metaxas et al., 1991; Repapis and Philandras, 1988).