Observation of warm water inflows in the western Baltic Sea between September 2022 and February 2023

Hydrographic data collected in the western Baltic Sea were used to study the hydrodynamics of the region between September 2022 and February 2023. The amount of volume change and salt transport into the Baltic Sea were quantified directly from observations and were estimated using an operational model. Salinity, temperature, dissolved oxygen concentration, and current velocities were measured at several locations: Fehmarn Belt buoy (FEB), Darss Sill station (DAR), Arkona Basin buoy (ARK), and Bornholm Basin station (BB). Using sea level height (SLH) data two inflow events were characterized by saline and warm near-bottom water with high dissolved oxygen concentrations between 14 September-10 October 2022, and 5 December 2022-15 January 2023. The unusually warm winter event was the result of a southwesterly wind regime featuring the transport of warm Channel Water through the North Sea towards the Kattegat. The first event transported 126 $\mathrm{km^3}$ of water into the Baltic Sea, corresponding to a salt transport of about 0.55 Gt. The second event transported about 140 $\mathrm{km^3}$ of water and added 1.1 Gt of salt to the Baltic Sea. While the water column properties at the DAR did not show the typical characteristics of a major Baltic inflow (MBI) during either event, the amount of salt transport, especially during the second inflow event, was similar to that of a weak MBI. Both events left their distinct mark on the temperature of the deep Bornholm Basin and the second event at least led to deep water ventilation in this basin.