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Abstract:
Winter (December-March) temperatures in most Europe is strongly correlated with the zonal circulation index of the Atlantic sector, North Atlantic Oscillation (NAO). The existence of a distinct multi-decade component 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. This means that purely statistical methods may be unable to distinguish the influence of the greenhouse gas induced trend from natural multidecadal variability.
Because of the prevailing western circulation, wintertime temperature in Europe should depend on the intensity of the western circulation as well as the North Atlantic sea surface temperature (SST) 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 study uses the data on SST and pressure fields as well as the NAO and AMO, together with an index of temperatures of Poland 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.
