Observation of large volume change in the western Baltic Sea between September 2022 and February 2023

Hydrographic data collected in the western Baltic Sea was used to study the hydrodynamics of the region between September 2022 and February 2023. The amount of volume change and salt transported into the Baltic Sea were quantified directly from observations and estimated using an operational model. Salinity, temperature, dissolved oxygen concentration, and current velocities were measured at several stations: Fehmarn Belt, Darss Sill, Arkona Basin, and Bornholm Basin. Using sea level height data, two large volume change events were identified. The events, characterized by saline (S>20 psu) and warm near-bottom water (up to 2°C warmer than the sea surface) with high dissolved oxygen concentration (> 6 ml l^-1), were identified between 14 September and 10 October 2022, and 5 December 2022 and 15 January 2023. From September to October 2022, the first event transported 126 km³ of water into the Baltic Sea. Based on the salinity of the inflowing water, this corresponds to an estimated salt transport of about 0.55 Gt. The second event in December 2022-January 2023 transported approximately 140 km³ of water and carried 1.1 Gt of salt into the Baltic Sea. Both instances have exhibited remarkable warm water, categorizing them as significant warm water inflows into the western Baltic Sea. While the properties of the water column at the Darss Sill station did not show the typical characteristics of a major Baltic inflow (MBI) during either of the two events, the amount of salt transport, especially during the second inflow event, is equivalent to that of a weak MBI.