The Baltic Sea is full of relatively salty cold water up to a depth of 50-60 m. The salinity is between 8 and 13 psu and the temperature is 4 or 5 degrees. Above this deep water the salinity is lower (6-7 psu) with not much mixing with the underlying water. The temperature in the top layer varies with the seasons.
In the winter the temperature can vary from just under zero up to about 4 degrees. In the spring the surface water warms up and becomes less dense. This warmer, lighter water does not mix with the colder underlying water, leading to thermal stratification of the water.
During the spring and summer the surface water warms up continuously but is also mixed by the wind. The temperature reaches a maximum of about 20 degrees at the beginning of August and the surface layer reaches its maximum thickness of about 30 m.
The temperature and salinity stratification in the Bothnian Sea (5-7 psu, 0-15 degrees) and the Bay of Bothnia (2-4.5 psu, 0-15 degrees) is similar to the rest of the Baltic but both the temperatures and salinities are lower.
The Kattegat and Skagerrak
In the Kattegat and the Skagerrak the relationships are different because of the water exchange with the North Sea. The Skagerrak has a significant salty layer that starts at a depth of 10-20 metres, where the salinity is 35 psu and the temperature is 6 degrees.
The salinity of the surface water is 25-30 psu and the temperature varies seasonally between 6 and 20 degrees. The temperature layer often reaches as far down as the salty layer.
Higher temperatures along the coast
The temperature around the islands in the archipelagos and along shallow beaches is often higher than out at sea. The surface water warms more quickly as there is less mixing. In the shallow water, the sun’s rays can even warm the sea floor.