Linking Variability in Baltic Sea Volume to Atmospheric Circulation

Water exchange between the Baltic Sea and the North Sea is constrained by narrow straits and substantial freshwater input, resulting in predominantly outflow conditions. Episodic saline inflows are therefore crucial for maintaining the Baltic Sea salt balance, ventilating deep basins, and sustaining ecosystem functioning. These inflows are closely linked to large volume changes (LVCs) and reflect variability in synoptic‑scale atmospheric circulation.

Here, we focus on the characteristic two‑phase evolution of LVCs. A prolonged preconditioning phase, dominated by easterly circulation types, promotes export of water from the Baltic Sea, followed by a forcing phase with strong westerly flow that enables inflow through the Danish Straits. This evolution is clearly manifested in sea surface elevation changes at Landsort: easterly circulation types prevail prior to sea level minima, whereas westerly and northwesterly types dominate prior to maxima, highlighting the asymmetric synoptic structure of LVCs.

We derive objective atmospheric circulation classifications and perform sensitivity experiments that vary the classification method, spatial domain, pressure level, and temporal aggregation. This synoptic‑climatological framework allows us to assess the robustness of circulation–inflow relationships and to identify which methodological choices most strongly influence the detected links.

Our results show that LVCs are not associated with a single synoptic pattern but instead emerge from characteristic sequences of circulation types and their temporal succession, emphasising the importance of cumulative and antecedent atmospheric forcing. Spatially, coherent synoptic‑scale forcing extends across the Baltic Sea region, with similar circulation characteristics over Central Estonia and the Danish Straits. Enhanced cyclonic activity over Central Scandinavia and the Norwegian Sea during LVCs suggests preferred synoptic pathways that strengthen basin‑scale pressure gradients. Finally, circulation anomalies are evident throughout the year preceding LVCs, with particularly strong signals in summer and at mid‑tropospheric levels, underscoring the importance of multilevel and multitemporal analyses in the synoptic climatology of extreme inflow events.