Seasonal variability and long-term winter shoaling of the upper mixed layer in the southern Baltic Sea

The upper ocean mixed layer plays a crucial role in regulating the exchange between the ocean and the atmosphere. The Mixed Layer Depth (MLD) is a key parameter affecting the air-sea exchanges of momentum and heat and determining the upper ocean temperature. Numerous previous studies have investigated MLD variability in the global ocean or regional seas but no such studies were carried in the Baltic Sea. In this study, we present the first observational assessment of the MLD and its properties in the Southern Baltic Sea including quantification of its seasonal and long-term changes and identification of the multi-year winter shoaling. We calculated monthly maps of MLD in the southern Baltic Sea using a large number of historical CTD profiles collected in 1995 -2021 from a combination of different data sets. To test the robustness of the results we compared the MLDs calculated using different threshold methods. Throughout the southern Baltic Sea, across its three basins, a distinct seasonality is evident in the MLD, with values varying from 12 m in July to 60 m in December -March. During winter the water column is well mixed down to the upper halocline depth and the MLD reaches about 45 m in the Bornholm Basin, 50 m in the Slupsk Furrow, and 60 m in the Gdansk Basin. The observed global warming and decadal changes in the salty inflows from the North Sea to the Baltic have had an impact on stratification by increasing water densities in the intermediate and deep layers. Consequently, density gradients have strengthened with depth while the upper ocean mixing has weakened during the winter season. The results reveal a significant winter shoaling of the mixed layer by 4 m per decade, driven by the increased stratification due to rising temperatures and salinity. These changes could have significant impacts on the dynamics and productivity of marine ecosystems.