The atmospheric circulation and sea-surface temperature in the North-Atlantic area in winter: Their interaction and relevance for Iberian precipitation

The ocean surface-atmosphere relationships in the North Atlantic area in northern winter are empirically examined by canonical correlation analysis (CCA). This analysis is performed from two different points of view. First, the connection between atmospheric circulation anomalies, in terms of monthly mean sea level pressure (SLP) and monthly standard deviation of SLP (sigma(SLP)), and sea surface temperature (SST) anomalies of the Atlantic Ocean are directly examined. Second, the air-sea relationships are indirectly studied through their influence upon precipitation in an area likely to be influenced by the North Atlantic, the Iberian Peninsula. The canonical correlation analysis yields two pairs of patterns that describe the coherent variations of the combined SST-SLP fields; one pair of patterns for the SST-sigma(SLP) fields and one pair of patterns for the SLP-sigma(SLP) fields. All patterns are dominant in describing variance. A lag cross-correlation analysis of the time coefficients indicates that monthly mean SLP varies simultaneously with sigma(SLP) but is leading monthly mean SST slightly. These results are consistent with the hypothesis that anomalies of the atmospheric circulation are mainly responsible for the appearance of anomalous wintertime SST and with the notion that the intramonthly variability of the atmosphere (sigma(SLP)) is coupled to the mean flow (SLP). With respect to Iberian precipitation, one well-defined CCA pair of patterns of regional rainfall and, respectively, SLP and SST is found. Above-normal Iberian precipitation is connected with a ''high-index'' North Atlantic SLP distribution and below-normal SST in most of the Atlantic north of 20-degrees-N. The dominant process responsible for the variability of rainfall appears to be the intensity of the westerly wind and the frequency of storms imbedded in it, not the presence of regional or remote SST anomalies. It is concluded that a large-scale SLP pattern in the North Atlantic, similar to the first EOF of the SLP field, is instrumental in generating both the Iberian precipitation and Atlantic SST variability on the seasonal time scale