Decadal Relationship between European Blocking and the North Atlantic Oscillation during 1978–2011. Part I: Atlantic Conditions

Abstract

Both the positive and negative phases of the North Atlantic Oscillation (NAO+ and NAO−, respectively) and atmospheric blocking in the Euro-Atlantic sector reflect synoptic variability over the region and thus are intrinsically linked. This study examines their relationship from a decadal change perspective. Since the winter-mean NAO index is defined as a time average of instantaneous NAO indices over the whole winter, it is unclear how the activity of European blocking (EB) events can be related to the variation of the positive mean NAO index. Here, this question is examined by dividing the winter period 1978–2011 into two decadal epochs: 1978–94 (P1) with an increasing and high NAO index and 1995–2011 (P2) with a decreasing and low NAO index. Using atmospheric reanalysis data, it is shown that there are more intense and persistent EB events in eastern Europe during P1 than during P2, while the opposite is true for western Europe. It is further shown that there are more NAO+ (NAO−) events during P1 (P2). The EB events associated with NAO+ events extend more eastward and are associated with stronger Atlantic mean zonal wind and weaker western Atlantic storm track during P1 than during P2, but EB events associated with NAO− events increase in western Europe under opposite Atlantic conditions during P2. Thus, the increase in the number of individual NAO+ (NAO−) events results in more EB events in eastern (western) Europe during P1 (P2). The EB change is also associated with the increased frequency of NAO− to NAO+ (NAO+ to NAO−) transition events.

Impact of Multidecadal Variability in Atlantic SST on Winter Atmospheric Blocking

“…1a). In addition, enhanced blocking is found from the Azores to Scandinavia as well as over Greenland, as many previous studies have shown (e.g., Scherrer et al 2006;Häkkinen et al 2011a;Luo et al 2015). The interannual standard deviation shows a very similar pattern and amplitude to those of the climatological mean ( Fig.…”

The role of intraseasonal variability at mid‐high latitudes in regulating Pacific blockings during boreal winter

“…Two other major regions of blocking occurrence are the Euro-Atlantic and Pacific sectors (Lejenäs and Økland, 1983;Tibaldi and Molteni, 1990). Euro-Atlantic blockings are closely related to the European precipitation (Trigo et al, 2004;Sousa et al, 2016), surface temperature (Trigo et al, 2004), and North Atlantic Oscillation (Luo et al, 2015a(Luo et al, , 2015b. Both Euro-Atlantic and Pacific blockings may influence the occurrence of sudden stratospheric warming (Nishii et al, 2009(Nishii et al, , 2011.…”

Emerging European winter precipitation pattern linked to atmospheric circulation changes over the North Atlantic region in recent decades

“…The storm track activity indicates an eastward extension relative to its mean track position toward the northern British Isles (Figure d), consistent with increased winter precipitation and enhanced AR incidence there in recent decades [ Huntingford et al , , and references therein]. The eastward extension in the storm track might be associated with a shift in the NAO toward its positive phase during the period 1980–2000 and strengthened eastward (weakened westward) component of the North Atlantic storm track [ Luo et al , ]. These changes are in line with a northward shift in the jet stream, in agreement with the stronger zonal flow over the North Atlantic in a warmer climate with an intensified and less wobbly jet stream predicted by many climate models [e.g., Barnes and Polvani , ].…”

“…Leading modes of variability, such as the NAO and AMO, often exert their impact on European winter climate through modulation of atmospheric blocking activity over the North Atlantic‐European sector [e.g., Scherrer et al , ; Croci‐Maspoli et al , ; Häkkinen et al , ; Davini et al , ; Luo et al , ]. Atmospheric blocking represents a prominent weather phenomenon in the extratropics: During a blocking situation, the large‐scale midlatitude zonal flow is impeded and meridional anomalies occur in the upper level jet; the anomalous circulation pattern remains largely stationary, generally persists for several days at a time, and is often associated with extreme events [e.g., Coumou and Rahmstorf , ].…”