Changing impact of large-scale atmospheric circulation variability on the water mass exchange and circulation of the Baltic Sea during winters 1950–2022

The winter climate of northern Europe is influenced by the variability of large-scale pressure systems that control the airflow over the region, namely NAO+, NAO-, SCAND, and the Atlantic Ridge. The positive phase of the North Atlantic Oscillation (NAO+) is associated with westerly winds that bring mild and moist air to the Baltic Sea region, while the negative phase (NAO-) is characterised by easterly winds and the advection of cold, dry air. Both phases are linked to shifts in the centres of action and changes in storm track pathways. The third mode, the East Atlantic or Atlantic Ridge pattern, is structurally similar to the NAO but features a north–south dipole with anomaly centres shifted southward. This pattern reflects variations in the north–south positioning of the NAO. The fourth dominant mode, the Scandinavian, Eurasian, or Blocking pattern, influences the Baltic region by generating high-pressure conditions over Scandinavia. These conditions are typically accompanied by dry, cold weather and weak easterly winds. Severe winters in the Baltic, characterised by extensive ice cover, are often associated not only with NAO- conditions but also with blocking situations.This study examines the relationship between changes or shifts in large-scale atmospheric winter conditions and their impact on regional-scale variability over the Baltic Sea area from 1950 to 2022. The primary objective is to identify the dominant large-scale atmospheric circulation patterns, specifically North Atlantic winter climate regimes, on monthly and seasonal timescales that drive the development of regional atmospheric weather types over the Baltic Sea area. These weather types, in turn, are linked to different Baltic Sea circulation patterns and the exchange of water masses with the North Sea.Using cluster analysis, we identify the prevailing large-scale atmospheric patterns that directly affect regional climate variability in the Baltic Sea area, as determined through weather type classification. In the next step, typical barotropic circulation patterns in the Baltic Sea are associated with dominant regional weather types. This approach enables the downscaling of large-scale atmospheric circulation variability to barotropic circulation variability within the Baltic Sea basin.