Over the past 30 years, the Baltic Sea's surface mixed layer has changed unevenly: it is shoaling in the southwest due to warming and increased stratification, while deepening in the northeast as ice loss allows more winter mixing. These shifts may affect ecosystems by reducing deep-water ventilation and worsening hypoxia in the south, while enhancing nutrient redistribution and spring bloom potential in the north. The Baltic Sea's climate response is strongly regional.