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Why is salinity highest in the western Baltic Sea?

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Map showing the physical features, major cities, and national boundaries around the Baltic Sea, North Sea, and English Channel regions of northern and western Europe. The map highlights coastal countries including the United Kingdom, Norway, Denmark, Germany, the Netherlands, Belgium, France, Sweden, Finland, Estonia, Latvia, Lithuania, Poland, and Russia, with sea depths indicated and key islands, peninsulas, and underwater trenches labeled. — Baltic Sea The Baltic Sea, the North Sea, and the English Channel.

The salinity of the Baltic Sea is highest in the western region primarily due to the influence of a deeper countercurrent of denser, saline water that flows into the Baltic from the North Sea.1

While the Baltic Sea is generally characterized by remarkably fresh water—due to its nearly landlocked nature, significant river runoff, and low evaporation rates—this deeper inflow of salt water helps maintain higher salinity levels in the west compared to the rest of the basin. In contrast, the eastern and northern reaches, such as the Gulf of Bothnia, experience much lower salinity because they are further removed from this marine influence and are heavily diluted by freshwater inputs.12