1Vol.:(0123456789) Scientific Reports | (2024) 14:16577 | https://doi.org/10.1038/s41598-024-67328-8 www.nature.com/scientificreports Observation of a moderate major Baltic Sea inflow in December 2023 Kaveh Purkiani *, Kerstin Jochumsen & Jens?Georg Fischer Just before Christmas 2023, the low?pressure system storm “Zoltan” struck Germany, resulting in widespread damage and two consecutive large storm surges on the North Sea coast in the night from Thursday 21 December 2023 to Friday 22 December 2023. Storm Zoltan brought heavy rainfall, accompanied by thunderstorms and winds ranging between 90 and 115 kmh?1 , with gusts reaching up to 140 kmh?1 along the coast which caused severe damage, particularly in northern Germany. Characteristics of the inflowing water at the Fehmarn Belt buoy (FEB), Darss Sill station (DAR) and the Arkona Basin buoy (ARK), including salinity, temperature, dissolved oxygen and ocean currents properties, were analysed to understand the impact of storm Zoltan in the western Baltic Sea. In addition to the damage along its path, following the onset of strong westerly winds associated with storm Zoltan, a large volume of water, containing saline (17–22 psu), cold (5–6 °C), and oxygen?rich 7–8 ( ml l?1 ) water from the Kattegat and the North Sea reached into the western Baltic Sea. The sea level at Landsort Norra increased by +57 cm over a period of 14 days, from 15 December to 29 December 2023. This resulted in a total volume change of 198 km3 in the entire Baltic Sea, with 169 km 3 and 29 km3 provided via the Belt Sea and the Sound Sea, respectively. Observations at the DAR indicated a significant inflow between 19 December 2023 and 1 January 2024 with salinity above 13 psu, temperature below 5.5 °C and dissolved oxygen of about 7.5–8 ml l?1 . While the maximum salinity of the bottom layer at the DAR was about 17 psu, the ARK exhibited significantly higher salinities, reaching up to 22 psu at the bottom layer. During the main inflow period, 75 km3 of highly saline water entered the western Baltic Sea. This corresponds to an average salt transport of 1.75 Gt into the western Baltic Sea (1.39 Gt from the Belt Sea and 0.36 Gt from the Sound Sea), representing more than 20% of the total annual salt import into the Baltic Sea), which places the event in the moderate range of major Baltic inflows. This event brought an amount of about 0.8 106 t oxygen into the Baltic Sea. This was the strongest inflow into the Baltic Sea since 2016. Storms in the North Sea play an essential role in shaping the dynamic environment of the region and are often associated with coastal flooding and inundation, disruptions in transportation networks, challenges in energy generation, and fatalities1–4. While these natural phenomena can be destructive, they also bring significant ben- efits by the generation of oceanic volume transport, i.e. inflow events, from the North Sea into the Baltic Sea. The connection of these shelf seas is no wide passage, but formed by several pathways along channels between islands, known as the Danish Straits (Fig. 1). The depth at Dogden Sill is only 7 m, thus limiting the water exchange. The flow through the Belt Sea is less restricted, but then has to pass over Darss Sill with a depth of 18  m5. North of the straits in the shallow Kattegat the transition zone between brackish waters from the Baltic Sea and oceanic waters from the North Sea is found6. Due to the hydrographic conditions in the Baltic Sea, the ventilation of its eastern basins mainly depends on lateral inflows5,7, as strong permanent water stratification prevents surface- bottom water exchange8. The inflows are therefore essential for oxygenating and maintaining the ecosystem of the Baltic Sea9,10. Earlier storm events during the last decade, e.g. Christian (October 2013), Xaver (December 2013), Danli, Ev and Feliz (February to March 2014) and their impacts on the volume transport into the Baltic Sea and the oxygenation of the Bornholm Basin were addressed in previous studies11,12. Just before Christmas 2023, a low-pressure system called “Zoltan” in Germany and “Pia” in Europe (hereafter Zoltan), struck Germany, resulting in widespread damage and two consecutive large storm surges on the North Sea coast in the night from 21 December 2023 to 22 December 2023 (internal report published in German13) . As given in the online report of the German Weather Service, storm Zoltan brought heavy rainfall, thunderstorms and winds between 90 and 115 kmh?1 , with gusts of up to 140 kmh?1 , along the coast14 . In addition to major rail disruptions , extreme flooding occurred in northern Germany. For instance, in Hamburg’s St. Pauli district, the sea level rose to more than 3 meters above mean high water level on 22 December 2023 . Beside causing damage along its pathway, storm Zoltan forced a large volume of water containing salty, cold, and oxygen-rich OPEN Federal Maritime and Hydrographic Agency of Germany, Hamburg, Germany. *email: Kaveh.Purkiani@bsh.de