3Vol.:(0123456789) Scientific Reports | (2024) 14:16577 | https://doi.org/10.1038/s41598-024-67328-8 www.nature.com/scientificreports/ reached the German North Sea coast on 21 December at 22:00. The strong winds had west to southwest (W-SW) direction at the Kattegat and in the western Baltic Sea (Fig. 2a–d). The maximum wind speed of 24.8 ms?1 was recorded at the Darss Sill Station (DAR) on 21 December at 23:00 (Fig. 3a). This was a favourable wind situation for the generation of an inflow event into the Baltic Sea, particularly following the relatively strong easterly winds which had caused an outflow in early December (Fig. 3a). After the storm had passed, a series of again relatively strong easterly winds of 18ms?1 were observed at the DAR from 2 to 9 January 2024. Changes in sea level height (SLH) at the Landsort Norra station in the western Gotland Basin, which reflect the water level of the central Baltic Sea, are shown in Fig. 3b. As the wind system developed, the SLH at Land- sort Norra began to rise from ? 6 cm on 15 December to +51 cm on 29 December 2023. This resulted in a SLH increase of +57 cm in 14 days, implying an average SLH increase rate of 4 cm d?1 in the Baltic Sea for this event. Due to the very small cross-sectional area of the Sound (0.1 km2 , 7 m sill depth), the variations in SLH were quite pronounced there, reaching up to +130 cm during this period (Fig. 3c). Compared to the MBI of December 2014, the December 2023 inflow was characterized by a shorter duration and a weaker intensity in the SLH variation. In the earlier event, a SLH variation of +103 cm was observed within 23 days26, resulting in an average SLH increase rate of 4.5 cm d?1 . The inflow at DAR was observed in the velocity profile illustrated at Fig. 3d–e, where strong northeastward flow (positive v- and u-components) was recorded during the inflow period, indicating the propagation of the inflowing water into the Arkona Basin. The along channel speed of the ocean currents reached values of up to 0.6 ms?1 in the whole water column during that time. Seawater characteristics at the Darss Sill The water salinity at DAR was about 8–9 psu in the entire water column from 10 December to 16 December 2023 (Fig. 4a). As the sea level increased, the more saline water from the Kattegat and North Sea reached the DAR and the seawater salinity increased to 13 psu on 20 December 2023. The salinity continuously increased to its maximum of 16.98 psu at 19 m water depth on 25 December and the stratification remained low (Fig. 4a). Afterward (on 28 December), the surface salinity decreased to values below 10 psu, while the water column below 7 m was still filled with saline water ( S > 13 psu). The deeper layers showed a salinity of more than 15 psu for a period of about 12 days from 20 December 2023 to 2 January 2024 13:00. On 3 January 2024, the salty water disappeared from the station and the seawater salinity returned to values below 10 psu (Fig. 4a). During the inflow period and specifically when the inflow core with S > 15 psu reached the DAR, the tem- perature of the seawater decreased from 6 °C on 21 December to 4.9 °C on 23 December 2023 (Fig. 4b). The dissolved oxygen concentration of the inflowing water at the bottom was about 8 ml l?1 at the onset of the event and decreased to 7.5 ml l?1 during the majority of the inflow period (Fig. 4c). This is equivalent to a saturated oxygen level between 95 and 92% during the inflow period while the surface oxygen saturation showed higher Figure 2. Atmospheric conditions during storm Zoltan in the North Sea and western Baltic Sea. Panels (a)?(d) show 10 m wind gust (background colour) and 10 m wind speed (black arrows) on 21 December 2023 12:00, on 21 December 2023 23:00, on 22 December 2023 09:00, and on 22 December 2023 18:00 respectively. The data were extracted from the reanalysis ERA532, where they are provided in hourly resolution.