Interdecadal Variations in North Atlantic Sea Surface Temperature and Associated Atmospheric Conditions

Evidence is presented for a distinct Pattern Of ocean-atmosphere relationship associated with interdecadal variability in the North Atlantic region. Using a century of surface marine observations it is shown that middle- and high-latitude sea surface temperature (SST) display a long-term fluctuation with negative anomalies before 1920, and during the 1970s and 1980s. Positive SST conditions prevailed from about 1930 to 1960. The pattern of interdecadal SST variability is constructed by subtracting the average field during 15 cold years from that during a similar interval of warm years. The early-century warming and the more recent cooling display a similar spatial pattern. In both cases the pattern is basin scale and largely of one polarity, with maxima in the vicinity of Iceland, in the Labrador Sea, and northeast of Bermuda. The corresponding differences in surface atmospheric conditions are determined by averaging and subtracting, in the same manner, fields of sea level pressure (SLP) and surface winds. The results display a circulation anomaly in the middle of the ocean basin, centered at about 45°N and 35°W. In this midocean area, an anomalous cyclonic circulation prevailed during years with warm SST, and an anticyclonic anomaly dominated during years with cold SST. These circulation anomalies are strongest during the winter months. To emphasize the distinct nature of interdecadal variability, short-term, interannual variability is examined in a similar analysis. The resulting patterns display bands of zonally elongated centers of action that are negatively correlated with one another. These anomalies underlay similarly banded features in the zonal wind distribution. The mutual relationship between interannual fluctuations of SST and surface wind conditions suggests that the former are maintained by the latter through a local thermodynamic interaction. The interdecadal patterns of SST on one hand and SLP and winds on the other lack a similar coherent relationship. This fact, and the unique horizontal distribution of interdecadal SST anomalies, suggest that interdecadal variability may be governed by a basin-scale dynamical interaction between the large-scale oceanic circulation and the atmosphere.