Contribution of the cold sector of extratropical cyclones to mean state features in winter

Recent studies have shown that midlatitude air-sea interactions are strongly modulated by synoptic variability. The present study investigates how air-sea interactions over the Gulf-Stream sea surface temperature (SST) front vary in different synoptic regimes. We focus more particularly on the variation of three atmospheric mean state features which are colocated with the SST front in winter : enhanced ascent and precipitation on the warm side of the SST front and enhanced low-level baroclinicity at the entrance of the storm track. These three fields are partitioned depending on whether they occur in the cold sector of extratropical cyclones or in any other synoptic features. The analysis is based on ERA Interim winter data covering the period 1979–2012. Results are twofold. (i) Cold sector precipitation is confined within a 5° of latitude band south of the SST front and reaches 2 mm day-1, whereas precipitation occurring outside the cold sector forms a broader spatial pattern. The same partitioning applied to vertical wind shows that the ascent on the warm side of the Gulf Stream is not a feature of the cold sector. These results mean that a significant part of the anchoring effect of the SST front on precipitation occurs via the cold sector and that the observed colocation of ascent and precipitation is not causal, in contrast to what was suggested by previous studies, but is rather the result of two different mechanisms. (ii) The surface heat fluxes and convection occurring in the cold sector restore low-level atmospheric temperature gradients within 2 days after a time maximum of meridional eddy heat flux, such that low-level baroclinicity remains largely unchanged after the passage of an extratropical cyclone. This "cold path mechanism" opens new avenues to understand how SST forces climate variability in the midlatitudes.