The spinning of the Earth turns the sea currents to the right of their flow direction. The topography of the basins guides the currents thus creating whirls of different sizes. The currents in the Baltic Sea depend on weather and are therefore varying, so there are no such permanent currents like in the oceans. Southwestern winds are dominant in the Baltic Sea and together with the spinning of the Earth they create a counterclockwise circular movement in the Baltic Sea that can be observed in the long time series.

Surface currents

The momentary flow velocities in the surface layer are typically 5–10 cm in a second, but in the hard storms they can be up to 50 cm in a second. In the narrow straits between the basins the currents can be even faster, over 1 meter in a second. The velocity of sea currents is usually 1% of the wind speed. The mass of the seawater is much bigger than that of air, and when it starts to move, it does not stop quickly. That’s why the sea can flow quite fast even after the wind has settled.

Deeper currents

Currents closer to the bottom depend on local topography and are usually slower than the surface layer currents. The velocities of deep currents are some few centimeters in a second. The topography strengthens the currents sometimes.