A model of the deep water flow into the Baltic Sea

Deep water renewal in the Baltic basins is a consequence of gravitationally driven bottom currents, transporting water from the North Sea area towards the deeper parts of the estuary. The dense currents have to pass several topographic constrictions, one of them being the Bornholm Channel. In this study, a numerical two-layer, limited area model has been applied to the southern Baltic Proper, in which the Bornholm Channel is located. The mathematical problem was solved as an initial value problem with rigid boundaries. The results were used to obtain a simplified formula, determining the dense bottom water flow as a function of the stratification in the upstream Arkona Basin. This relation makes it possible to calculate the deep water inflow in detail, since hydrographic observations are frequent, whereas direct transport measurements are scarce in the area. The calculated flow characteristics in the Bornholm Channel are in accordance with observations. The volume flow was found to vary between 0.9 × 10⁴ and 2.9 ×10⁴ m³ s^-1, using upstream basin values of density differences and pycnocline depths close to the observed mean magnitudes. The model indicates that inertia, Coriolis and frictional forces along with the upstream stratification and topography control the channel flow. Quasi-steady state is obtained within 24 h, whereas the time-scale for a water volume to pass the channel is approximately 2.3 days.