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Why is the Baltic Sea less salty than the Atlantic Ocean?

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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 Baltic Sea is significantly less salty than the Atlantic Ocean primarily because it is nearly landlocked with a very shallow outlet, which restricts the exchange of water with the open ocean.1

Several factors contribute to its low salinity:

High Freshwater Influx: The Baltic receives a large volume of water from rivers, such as the Vistula and the Oder, as well as significant surface runoff and spring snowmelt.12
Low Evaporation: The region experiences a temperate continental climate with low evaporation rates, which prevents the concentration of salts that occurs in warmer, high-evaporation seas.12
Limited Saltwater Exchange: While a deeper countercurrent of denser salt water flows into the Baltic, the sea's restricted connection to the North Sea prevents the high-salinity levels found in the open Atlantic, where surface salinities can exceed 37 parts per thousand.12

In contrast, the North Atlantic maintains high salinity due to intense evaporation and the influence of high-salinity outflows from seas like the Mediterranean.2