Is the North Atlantic modulating wintertime influence of NAO on Europe temperatures?

Winter (December-March) temperatures in most Europe is strongly correlated with the zonal circulation index of the Atlantic sector, North Atlantic Oscillation (NAO), regardless of which from its several available definitions to choose. However, the variability of this index also has a distinct multi-decade component, which makes it difficult to study trends of several decades. In addition, the NAO index itself is also significantly positively correlated with the global anthropogenic forcing and with the global temperature itself. Therefore, using purely statistical methods, it is not easy to distinguish the influence of zonal circulation variability from the trend resulting from the increasing forcing of greenhouse gases when examining the variability of winter temperatures in Europe.

Because of the prevailing western circulation, wintertime temperature in Europe should dpend on the intensity of the western circulation (NAO) as well as the sea surface temperature (SST) of the ocean (indexed by AMO – Atlantic Multidecadal Oscillation). However winter is the only season when there is no statistically significant correlation of AMO and temperatures in Europe. This surprising result had no explanation until the recent discovery of the northern shift of synoptic systems correlated with AMO. This could offer a “Bjerknes compensation” type of effect where the ocean circulation modifies the atmospheric one, making the air masses arriving in winter to Europe sourced in areas of the same SST, regardless of AMO (or general warming of the North Atlantic).

This study uses the data on SST and pressure fields as well as the NAO and AMO, together with an index if temperatures of Poland (as a proxy of Central Europe) in order to throw light on the relationship. The results confirm the existence of a “Bjerknes compensation” mechanism as well as suggest a dependence of wintertime NAO on the greenhouse forcing (visible in their significant correlation), caused most probably by the recently discovered strengthening of wintertime jet stream over the North Atlantic. This relationship can have important impact on future winter temperatures in a large part of Europe and therefore its possible mechanisms should be the point of further research.

 

This work been performed as a part of the SURETY project , funded by Polish National Science Centre (NCN), contract 2021/41/B/ST10/00946.