The surface layer can be up to 100 m deep. When the wind blows over the sea surface it creates waves and a surface current. The surface current is affected by the rotation of the earth which in turn causes the net current to move at a 45 degree angle to the right of the wind direction (see Ekman spirals).
The Baltic Sea has no noticeable permanent currents. The fresh water flowing into the sea from rivers travel as a thin layer above the salt water and turn to the right because of the earth’s rotation. The fresh water eventually mixes with the sea water. There is a large, slow, southerly coastal current along Sweden’s coast.
During the summer this current is at it’s weakest due to less river run-off. On the open sea, changes in sea level, wind strength and direction are the main cause of currents which thus become irregular.
Satellite pictures of algal blooms often show the paths of currents through irregular patterns and eddies in the sea surface. Currents are strongest in the sounds: –the Northern Kvark, the Åland Sea, Kalmar Sound, the Sound and the Belt Sea. The Sound and the Belt have the strongest currents as the Baltic outflow is concentrated here. Currents in the Sound can reach speeds up to 5 knots.
The Skagerrak and the Kattegat
The Skagerrak in particular has several regular and permanent current systems. The Baltic current consisting of outflowing Baltic water follows the Swedish west coast due to the earth’s rotation.
In the northernmost part of the west coast, the northward-moving Jutland Coastal Current is amplified as it moves via Skagen towards the Swedish coast. The Jutland current is saltier and therefore flows under the Baltic current. This area also experiences the strongest tidal currents in Swedish waters.