A coupled three-basin sea level model for the Baltic Sea

Sea level variations in the Baltic Sea on time-scales 2 days and longer are studied using a coupled three-basin quasi-stationary model. The three large sub-basins (Baltic proper, Bothnian Sea and Bothnian Bay) are coupled in series by two short and narrow straits. A third strait connects the Baltic proper with the Kattegat. Currents are forced through the connecting straits by the sea level difference. The mean sea levels in the sub-basins change due to freshwater supply and the water exchanges through the straits. Wind stress and horizontal variations of air pressure and density maintain sea-level slopes in the sub-basins. With the model, 85% of the total variance of the recorded sea levels can be explained. The unexplained variance is partitioned among different period bands, and for periods longer than 2 weeks 20% or less of the variance is unexplained. For shorter periods, 30–60% of the variance is unexplained. The relative importance of different forcings is also investigated. For periods longer than a few months the forcing provided by the sea level variations in the Kattegat is dominating but horizontal variations of air pressure and density also contribute. For short periods (2 days–2 weeks) the wind stress is most important, especially during winter. Estimated barotropic water transports between the subbasins are investigated, and it is found that major Baltic outflows have the same intensity and magnitude as major Baltic inflows.