Long term trends and variability of the mixed layer depth in the Baltic Sea

The depth of the upper mixed layer (MLD) in the Baltic Sea is a critical physical parameter that influences heat exchange, nutrient cycling, and oxygenation in this semi-enclosed basin. Variations in MLD have significant ecological consequences: a shallower mixed layer restricts the supply of oxygen and nutrients to deeper waters, potentially exacerbating hypoxia, whereas deeper mixing in previously ice-covered areas can enhance ventilation and nutrient entrainment. To investigate long-term MLD variability, we analyzed a 30-year (1993–2023) high-resolution ocean reanalysis data set across the Baltic Sea. The results reveal pronounced spatial heterogeneity in MLD trends. In the western and southern Baltic, the MLD has become shallower, driven by surface warming and intensified stratification that stabilizes the upper layer. Conversely, in the eastern and northern Baltic, MLD has deepened, coinciding with reduced winter sea-ice cover and increasing wind speeds that promote wind-driven mixing – despite concurrent warming, increased precipitation, and enhanced stratification. These region-specific trends highlight the complex interplay among warming, ice loss, and stratification in shaping upper-ocean dynamics. Our findings provide a baseline for understanding mixed-layer variability in the Baltic Sea under climate change and emphasize the need to consider regional differences in future projections.