Covariation of the mid-tropospheric flow and the sea surface temperature of the North Atlantic: A statistical analysis

The covariation of the mid-tropospheric how and the sea surface temperature (SST) of the extratropical North Atlantic is studied for the period 1962 to 1992. A statistical approach is adopted and the variables selected for examination, in addition to the SST, are the 500 hPa fields of the geopotential height and the dynamic storm track. Singular Value Decomposition (SVD) of the wintertime monthly mean fields for all paired combinations of these three fields suggests that their intraseasonal covariability is dominated by types of recurring patterns. The first is an "North Atlantic Oscillation-like" variation with height and SST patterns in the form of dipolar anomalies and a storm track pattern whose extremes connote an elongation (shortening) sharpening (broadening) and intensification (weakening) of the track. The second is akin to "strong and weak zonal flow regimes" and is characterized by height and SST anomalies that are latitudinally aligned and storm track variations that connote a shortening (lengthening) and northeastward (southeastward) reorientation of the track. Lag correlation analysis of the relative phase of the flow and SST variations suggest that the former leads the latter by the order of a month and is indicative of the flow influencing the SST configuration. However evidence adduced from consideration of lagged composite plots of strong SST patterns cautions against excluding the possibility of a nonlinear ocean-to-atmosphere forcing. It is also demonstrated that the two types of SVD-derived patterns represent a significant part of the last 30 years trends in the selected fields, and the coupled SVD patterns also shed light on the nature and degree of coupling of the three selected fields.