The sea water density depends on temperature, salinity and pressure. The maximum density of fresh water is at 4 °C temperature. The more saline the water is, the lower the temperature of the maximum density is. This means that the increase of salinity decreases the temperature of the maximum density. For example, when salinity is 12 ‰, the temperature of the maximum density is 1,45 °C. Because the denser water gets down, the density’s dependence on temperature and salinity has a big significance on the stratification of the Baltic Sea.

The border between two layers is called a cline. There, the salinity or the temperature changes drastically. The layer where the salinity changes is called halocline and the layer of temperature change is called a thermocline.

Thermal stratification

The thermal stratification changes with the seasons. Because cold water is denser than warm water, in the autumn when the surface water cools, it gets heavier and sinks deeper. This creates a thermal vertical circulation that ends when the whole surface layer has reached the temperature of its maximum density. After that the cooling of the surface does not lead to circulation anymore, but the cooled water stays on the surface. Nevertheless, the surface water continues to cool down due to the winds mixing the water deeper. Then the warmth in the deeper water gets to the surface is released to the atmosphere.

In the spring the sea starts to receive more heat than it releases. The sun warms first the surface layer that as denser water tends to go down. When the temperature has risen over the temperature of the maximum density, the surface water does not sink deeper, but the heat is transferred deeper due to wind mixing. A warm layer of 10 to 20 meters is formed to the surface layer of the sea. In deeper, near the bottom, where the depth is more than 70 meters the temperature changes much less during the year and slower than in the surface. Deep waters reach their maximum temperature only in late autumn. Between the warm surface layer and the colder bottom layer there is a 5 to 10 meters thick summer thermocline, where the temperature may cool down even 10 degrees in a few meters. In the summertime, beneath the warm surface layer is the coldest water layer, old winter water, that does not warm much even during the summer. In upwelling, the old winter water can rise to the surface, and cool it quickly and effectively.

In the Baltic Sea the surface layer´s water warms until late July and starts to cool at the latest in the second week of August. In the northern parts of the Bay of Bothnia the water is at its warmest at only 14 to 15 degrees on average, and in the southern parts 16 to 18 degrees. In calm, sunny days the surface temperature can rise to over 20 degrees all around the Baltic Sea.

Salinity stratification

The Baltic Sea has a strong two-layer structure of salinity. Salt increases water´s density, so the more saline water is usually under the more fresh water. In the Baltic Sea the fresh water brought by rivers and saline water from Atlantic meet, so that the heavier saline water sinks beneath the lighter fresh water. The salinity stratification is permanent, but also variable. For example, in the springtime, when the snow and ice melts, there’s more fresh water on the surface than during other times of a year. The halocline is at a depth of 40 to 80 meters in the Baltic Sea and is 10 to 20 meters thick. In the halocline the salinity increases significantly when going deeper. The halocline prevents the fresher and thus lighter water of the surface layer mixing with the more saline and thus heavier water of the bottom layer.

There are rivers all over the coasts of the Baltic Sea, but the northern parts get most fresh water. This is why these are less saline than the southern Baltic Sea. The salinity of the northern parts of the Gulf of Bothnia and the Gulf of Finland is less than 1/3 of the salinity in the southern Baltic Sea. The Baltic gets new fresh water from the rivers and rain and more saline water from the North Sea. The saline water comes through the shallow and narrow Danish Straits in the southwest corner of the Baltic Sea. Usually, the water changes also there in the surface layer, which means that the incoming water is not very saline. Saline water has to rise against the flow in a sense, which makes it difficult for notable amounts of saline water to reach the Baltic Sea. Only with very specific weather conditions the saline water can rise from the deep waters of Skagerrak over the sills of the Danish Straits to the Baltic Sea. This phenomenon is called a Major Baltic Inflow.