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 t o 1992. A statistical approach is adopted and the variables selected for ex amination, in addition to the SST, are the 500 hPa fields of the geopotenti al height and the dynamic storm track. Singular Value Decomposition (SVD) o f the wintertime monthly mean fields for all paired combinations of these t hree fields suggests that their intraseasonal covariability is dominated by types of recurring patterns. The first is an "North Atlantic Oscillation-l ike" variation with height and SST patterns in the form of dipolar anomalie s and a storm track pattern whose extremes connote an elongation (shortenin g) sharpening (broadening) and intensification (weakening) of the track. Th e second is akin to "strong and weak zonal flow regimes" and is characteriz ed by height and SST anomalies that are latitudinally aligned and storm tra ck variations that connote a shortening (lengthening) and northeastward (so utheastward) reorientation of the track. Lag correlation analysis of the relative phase of the flow and SST variatio ns 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-atmosph ere forcing. It is also demonstrated that the two types of SVD-derived patterns represen t a significant part of the last 30 years trends in the selected fields, an d the coupled SVD patterns also shed light on the nature and degree of coup ling of the three selected fields.