The sinkingdown occurs in the Kattegat when the inflowing Atlantic water enters the Baltic as an undercurrent which is both warmer and denser than that on the surface.
At its southern end the Kattegat is blocked by the Danish islands, and it communicates with the Baltic proper by narrow channels called the Sound, the Great Belt and the Little Belt.
In spring and summer the water from the Baltic is sufficiently abundant to inundate the whole surface of the Kattegat and Skagerrak, but in winter the sources of the Baltic current are for the most part dried up by the freezing of the land water.
All the waters which enter the Skagerrak or Kattegat as undercurrents can be found at the surface of the North Sea.
This channel is usually included in the Baltic. The part of it west of a line joining the Skaw with Christiania fjord receives the name of Skagerrak; the part east of this line is called the Kattegat.