The influence of the Gulf Stream on Wintertime European blocking

Wintertime blocking is responsible for extended periods of anomalously cold and dry weather over Europe. In this study, the influence of the Gulf Stream sea surface temperature (SST) front on wintertime European blocking is investigated using a reanalysis dataset and a pair of atmospheric general circulation model (AGCM) simulations. The AGCM is forced with realistic and smoothed Gulf Stream SST, and blocking frequency over Europe is found to depend crucially on the Gulf Stream SST front. In the absence of the sharp SST gradient European blocking is significantly reduced and occurs further downstream. The Gulf Stream is found to significantly influence the surface temperature anomalies during blocking periods and the occurrence of associated cold spells. In particular the cold spell peak, located in central Europe, disappears in the absence of the Gulf Stream SST front. The nature of the Gulf Stream influence on European blocking development is then investigated using composite analysis. The presence of the Gulf Stream SST front is important in capturing the observed quasi-stationary development of European blocking. The development is characterised by increased lower-tropospheric meridional eddy heat transport in the Gulf Stream region and increased eddy kinetic energy at upper-levels, which acts to reinforce the quasi-stationary jet. When the Gulf Stream SST is smoothed the storm track activity is weaker, the development is less consistent and European blocking occurs less frequently.