Occurrence of Winter Atmospheric Circulation Regimes in Euro-Atlantic Region and Associated Extreme Weather Anomalies in the Northern Hemisphere.

Quantitative estimates are obtained for regional features of the frequency distribution of extreme temperature, precipitation, and wind anomalies in the winter months in the Northern Hemisphere extratropical zone, associated with atmospheric circulation regimes (henceforth regimes) in the Euro-Atlantic sector. Using k-means cluster analysis for the daily geopotential height fields from the ERA5 reanalysis for 1979–2021, four most characteristic winter atmospheric circulation regimes are identified in the Euro-Atlantic sector corresponding to the North Atlantic Oscillation (NAO) in the positive (NAO+) and negative (NAO−) phases, Scandinavian blocking (SCAND), and the regime with anomalously high pressure over the North Atlantic and low pressure over Europe (ATL). Trends in the frequency of occurrence and in structures of regimes are analyzed. The NAO+ and NAO− regimes turned out to be asymmetrical in spatial structure with significantly different frequencies of occurrence (33% and 19%, respectively). The associated extreme weather anomaly fields are also asymmetric. For all regimes, an asymmetric response in the frequency of occurrence of positive and negative temperature anomalies is noted. Over the period under study, no statistically significant trends are found in the seasonal average frequency of occurrence of the regimes, or in the seasonal average fields of the regimes in the region of the geopotential height anomaly extremes, probably indicating the stability of these regimes under recent climate changes. [ABSTRACT FROM AUTHOR]