6Vol:.(1234567890) Scientific Reports | (2024) 14:16577 | https://doi.org/10.1038/s41598-024-67328-8 www.nature.com/scientificreports/ days compared to DAR. For the presence of the core of the inflowing water ( S > 15 psu), the time lag between ARK and DAR further increased to 3.4 days. Measurements at the FEB, (Supplementary Fig. 1) showed that the salinity of the inflowing water varied between 22 and 23 psu and was diluted at a rate of 5.5 psu per 100 km along its pathway at the Darss Sill (115 km), and arrived at the DAR with a mean salinity of 16 psu. Salinity data was not available for the passage through the Sound. If we assume a similar dilution rate along this pathway as was calculated for Darss Sill, the salinity of the Sound Sea can be estimated to have been about 27–28 psu. Using these values for salinity the salt transport during the inflow event in December 2023 can be estimated to have been between 1.7 and 1.8 Gt. Despite the lower salinity observed at the DAR, the salt transport through the Belt Sea (1.36 Gt) was signifi- cantly higher than that from the Sound Sea (0.35 Gt) with a ratio of about 8:2. Regarding the uncertainty of the estimate, the calculation of the salt transport into the western Baltic Sea in this event exhibits a sensitivity of up to 4.5% to a 1-psu salt variation at the inlets, with 4% attributed to the Belt Sea and 0.5% to the Sound Sea. Using the intensity index QFM9619, the inflow of December 2023 was classified as a moderate inflow ( QFM96 = 17.5). Compared to previous events, the intensity of the December 2023 event was similar to the event in January 200325,26. This was, however, the strongest MBI in the Baltic Sea since 2016 ( Supplementary Fig. 2). Since only very strong MBIs are able to ventilate the central basin of the Baltic Sea23,26,33,34, it is likely that storm Zoltan, alone, despite its high wind speeds, did not bring enough saline water into the western Baltic Sea to reach the central basins. Earlier observations during the stagnation period from the beginning of 1995 to the end of 1996 in the deep waters of the Gotland and F?rö Deeps showed that a medium-range MBI in November 1996 did not ventilate these basins significantly. The stagnation period was only interrupted from February to May 1997 due to a series of small inflows.35. It is thus essential to note the significance of smaller events, particularly those occurring just before and after an MBI event, which are important for the central deep basin ventilation. This has been demonstrated through the comparison of MBIs in 2003 and 2014, where the earlier event, despite its weaker strength, had a greater impact on the ventilation in the Gotland Basin8. Whether the event of December 2023 will contribute to the ventilation of the central basins remains unknown yet and will need to be verified by observations and numerical simulations in the future. Methods Time series of wind speed and direction, ocean currents, temperature, salinity, and dissolved oxygen were col- lected from three permanent autonomous stations (FEB, DAR and ARK) of the German Marine Monitoring Network (MARNET, Fig. 1). Two stations (DAR and ARK) were operated and maintained by the Leibniz Institute Figure 5. Vertical profiles of salinity (a) and temperature (b) from 1 December 2023 to 10 January 2024 at the Arkona Basin bouy (ARK). The dissolved oxygen concentrations at 7 m and 40 m are depicted in (c). The isohalines of 13, 15, and 17 psu are given in the salinity time series in (a) and the isohaline of 15 psu is shown again in (b). Hourly averages of the original higher resolution measurements were used here. The vertical axes in (a) and (b) not only indicate the depth, but also show the position of the instruments in the water column. Salinity at 45 m was not available due to a lack of measurements.