Climate impact of the European winter blocking episodes from the NCEP/NCAR Reanalyses

A comprehensive multivariable characterisation of the climatic impacts of winter blocking and strong zonal-flow (non-blocking) episodes over the Euro-Atlantic sector is presented here, using a 40-year (1958–97) consistent dataset from NCEP/NCAR. Anomaly fields of surface or low troposphere climate variables are then interpreted based on large-scale physical mechanisms, namely, the anomalous mean flow (characterised by the 500 hPa geopotential height and the surface wind) and the anomalous eddy activity (characterised by the surface vorticity and cyclonic activity). It is shown that the lower troposphere (850 hPa) temperature patterns are mainly controlled by the advection of heat by the anomalous mean flow. However, at the surface level, the anomaly patterns obtained for maximum and minimum temperatures present important asymmetries, associated with a different control mechanism, namely the modulation of shortwave and longwave radiation by cloud cover variations. It is shown that blocking and non-blocking episodes are typically associated with important meridional shifts in the location of maximum activity of transient eddies. The influence of persistent anomaly events in precipitable water is strongly related to the corresponding anomaly fields of lower troposphere temperature. The precipitation rate, however, appears to be essentially controlled by the surface vorticity field and preferred locations of associated cyclones.