The Impact of the Gulf Stream on European wintertime blocking

The influence of the Gulf Stream sea surface temperature front on the onset of midlatitude European wintertime blocking events is investigated. Analysis of the composite blocking events in the NCEP-CFSR dataset reveals a strong precipitation band develops along the warm flank of the front during the days preceding blocking onset. Isentropic potential vorticity (PV) analysis shows negative upper-level parcel PV tendency in the region of the Gulf Stream precipitation band opposes the eastwards translation of the developing ridge leading to anticyclonic wave-breaking and subsequent blocking. A pair of AGCM experiments forced with observed satellite SST and smoothed SST is examined. In the presence of the observed SST front the model exhibits the composite Gulf Stream precipitation band, European blocking frequency distribution and the anticyclonic direction of wave-breaking similar to the reanalysis dataset. These blocking characteristics are not well captured in the smoothed SST experiment in which blocking frequency is about 30% less than the control experiment over central Europe. The difference in blocking frequency and distribution is shown to impact extreme cold spells, particularly over large parts of central Europe. In the control experiment, which closely reproduces the distribution found the NCEP-CFSR data, the number of winter cold spell days approximately doubles compared to the experiment with smoothed SST.