Two-dimensional indices of atmospheric blocking and their statistical relationship with winter climate patterns in the Euro-Atlantic region

The statistical relationship between the leading climate patterns of mid-tropospheric flow and atmospheric blocking over the Euro-Atlantic region during winter is investigated using three new two-dimensional blocking indicators. The focus is on the leading climate pattern of the 500-hPa geopotential variability, i.e. the North Atlantic Oscillation (NAO). The results indicate that the blocking-NAO relation is not restricted to the North Atlantic region, where blocking and the NAO are known to be out of phase. All three indicators show that the positive NAO phase is characterised by an enhanced occurrence of blocking-type high-pressure systems over the European mainland. The sign change of the NAO-blocking relation from west to east is well detectable with the two-dimensional blocking indicators and it is found further south than at the traditionally studied blocking latitudes near 60 degrees N. The analysis of blocking events by seasonal NAO indices leads to similar (albeit less significant) results as with a daily NAO index stratification. This indicates that the relation between the NAO and blocking is fairly insensitive to the chosen time resolution. The investigation is extended from the second to fourth pattern of the mid-tropospheric flow variability using empirical orthogonal function (EOF) patterns. It reveals that one phase of each of the major Euro-Atlantic climate patterns is collocated with the region of maximum blocking frequency. The clearest separation between positive (negative) EOF phases and blocking (no blocking) situations is found for EOF x 2 and 3 and is associated with changes from zonal to ridge-like flow, similar to the so-called northern European 'blocking signature'. This is an indication that the purely statistically defined EOF patterns are related to the physical blocking phenomenon. Copyright (C) 2005 Royal Meteorological Society.