Dynamics of the western Baltic

Abstract

The western Baltic is the southernmost part of the transition area between North Sea and Baltic Proper. The relatively shallow, strongly stratified region is subject to three kinds of external forcing: (i) local winds, (ii) barotropic pressure gradient due to sea level differences between Baltic and North Sea and (iii) baroclinic pressure gradients sustained by freshwater outflow at the sea surface and inflow of saline water near the bottom. Owing to the time scale of the weather patterns (2 to 4 days) the circulation is mostly governed by transients rather than by permanent currents. In order to elucidate the dynamics of the area four expeditions have been carried out in summer 1983, 1987, 1989 and in autumn 1984 where dense grids of CTD stations were worked in the area of the Mecklenburg Bight and Darss sill. The experimental findings reveal that easterly winds force coastal jets off the german coast associated with coastal upwelling. This response can be amplified substantially by pressure gradient forcing. Westerly winds generate eddy patterns rather than coastal jets. This can be explained by the fact that the cross-circulation forced by westerly winds is diminished by the opposite cross-circulation driven by pressure gradients, implying only a weak resulting coastal up- or downwelling. There are indications that the eddies are generated by vortex shedding at topographical features and eddy detachment at coastal irregularities.