The effect of water density on sea level

Watercourses that run into the Baltic add fresh water, thus reducing the salinity of the sea. The density of fresh water is lower than salt water, which affects the sea level in the Baltic.

The salinity is 3-4 psu in the Bothnian Bay, 5-6 psu in the Sea of Bothnia, 6-9 psu in the Baltic Proper and 15-30 psu in the Skagerrak and Kattegat. Since fresh water is lighter, it takes more fresh water in the Baltic to balance the sea level outside – thus mean sea level in the Baltic is higher than that outside (relative to the geiod).

The sea level in the Bay of Bothnia is about 34-40 cm higher than that in the Skagerrak. This difference is not directly apparent in the sea level curves from the measurement stations, as these are corrected to each gauge’s own annual local sea level.