Mean sea surface topography in the Baltic Sea and its transition area to the North Sea: A geodetic solution and comparisons with oceanographic models

A consistent height system designed for comparisons between geodesy and oceanography is presented for the Baltic Sea area. Mean sea surface topography is computed geodetically in this height system at 32 reliable long-term sea level stations, connected by high-precision levelings, along the coasts of the Baltic Sea, the Kattegat, the Skagerrak and the adjacent part of the North Sea. Based on this, a map is drawn of the mean sea surface topography in the Baltic Sea and its transition area to the North Sea. The general agreement between our geodetic solution and recent oceanographic models turns out to be excellent; the discrepancies rarely exceed 2-3 cm. Hence the mean sea surface topography in the Baltic Sea area to a dominating extent is caused by the distribution of salinity. There are two main characteristics of the sea surface topography found here. First, there is a continuous increase of the sea surface height from the North Sea into the Baltic Sea, the height difference between the inner Dart of Gulf of Bothnia and the Skagerrak amounting to 35-30 cm. The main reason behind this is the considerable difference in salinity, close to the maximum possible one. Second, there is a steep sea level gradient in the border zone between the Kattegat and the Skagerrak, reaching 2 cm per 10 km. This reflects the salinity front there, separating the brackish Baltic Sea water from the saline North Sea water, and the associated Baltic current. A local maximum in the sea surface can be seen in the Oslo Fiord, reflecting an accumulation of low-salinity water there. An area where there is still no oceanographic model agreeing sufficiently with the geodetic solution is the Belt Sea/southwestern Kattegat.