Observations of the deepwater flow into the Baltic Sea

It has earlier been hypothesized that the deep water flowing into the Baltic Sea forms a bottom pool in the Arkona Sea, just inside the entrance sills. The flow from the pool farther into the Baltic Sea was assumed to be baroclinic geostrophic and controlled by the pool stratification. This was supported by a realistic statistical description of the deepwater flow into the Baltic Sea, computed using historical vertical density profiles from a hydrographical station in the Arkona Sea. However, basin-wide synoptic measurements of density and currents were not available for a direct verification of the pool model. A hydrographic survey was undertaken in the Arkona Sea shortly after a major inflow event in the beginning of 1993. The conductivity-temperature-depth sections reveal a thick bottom pool of deep water separated by a halocline from the surface water of Baltic origin. Approaching the northern coastal boundary from the central parts of the pool, the halocline sank by about 20 m before hitting the bottom. The ship-mounted acoustic Doppler current profiler recorded a complicated current field. However, subtraction of the assumed barotropic part gives a current field quite similar to the baroclinic geostrophic current field computed from the density distribution. Thus a dense bottom pool including a baroclinic geostrophic boundary current along the northern flank of the pool is quite evident from our measurements.