2Vol:.(1234567890) Scientific Reports | (2024) 14:16577 | https://doi.org/10.1038/s41598-024-67328-8 www.nature.com/scientificreports/ water from the North Sea and Kattegat to enter the western Baltic Sea. Such barotropic inflows induced by storms play a significant role in the salt and oxygen budgets of the Baltic Sea and the inflow caused by Zoltan is the topic of this study. Episodic barotropic inflows of large volumes of saline and oxygenated water, the so-called Major Baltic Sea Inflows15,16 (MBI), are the most important mechanisms of central Baltic deep water ventilation. To distinguish between regular inflows and MBIs, following criteria were defined using data from the light ship “Gedser Rev”, which was approx. 50 km west of the Darss Sill5,17–19: (i) the bottom salinity must be ? 17 psu (ii) the salinity stratification defined as G = 1? Ss/Sb must be ? 0.2 (where Ss is the surface salinity and Sb is the bottom salinity) and (iii) these conditions must be persistent for at least five consecutive days. Previous analysis have shown, that easterly winds lowering the Baltic Sea sea level and following prevailing westerly winds with strong gales are a decisive factor for the occurrence of MBIs15. Statistical analyses of frequencies and intensities of inflow events5,19,20 indicate the occurrence of weak MBIs to be regular features. However, very few strong MBIs were observed in the last decades (they are: the 1993 MBI21–23, the 2003 MBI24,25 and the 2014 MBI26) that could turn the deep Gotland Basin to oxic conditions. The low frequency of strong MBIs led to long lasting stagnation periods, which is now the common state of the central deep Baltic without any strong MBI since 20148,20. The strong winds of Zoltan and the stormy conditions in the North Sea during the following week induced the formation of an MBI, which had the potential to bring saline and oxygen-rich water from the North Sea and Kattegat into the Arkona and Bornholm Basins. Depending on the intensity of the inflow event, the spreading of the water may have continued into the central basins and improved the oxygen situation in the deep waters of the Gotland Basin, which suffered again from a 10-year time span of stagnation. As a consequence of the extend periods of time between MBIs, the oxygen deficiency in the deeper layers of the Baltic Sea has already reached the highest known level27. In this study, using hydrographic observations from three locations in the Baltic Sea (Fehmarn Belt, Darss Sill and Arkona Basin), we aimed to investigate how storm Zoltan affected the hydrodynamics of the western Baltic Sea and to quantify the inflow event. Results Wind and sea level height variations Between 15 and 30 December 2023, a low-pressure system (968 hPa) with strong southwesterly (SW) wind direction developed in the North Sea and the Baltic Sea with wind speeds of more than 31 ms?1 in the German North Sea (Fig. 2). The wind gusts showed even higher wind speeds of up to 35 ms?1 . The maximum wind speed Figure 1. Bathymetric map of the southwestern Baltic Sea, along with locations where measurements were taken as red dots ( Fehmarn Belt buoy-FEB; Darss Sill station-DAR; Arkona Basin buoy-ARK). The Baltic Sea connects to the Kattegat and North Sea via shallow straits: the Belt Sea (Little and Great Belt) and the Sound. Colored arrows on the map depict the separate pathways of the inflowing salty water through these connections. Along with sills (Darss and Drogden) crucial for water exchange, depicted by dashed lines, the map also shows the Arkona Basin, one of the Baltic Sea’s first deeper basins along the pathway of the inflowing water, where depths of up to 48 meters are reached. Black squares on the map represent tide gauge stations located at Viken, Klagshamn, and Landsort Norra.