Laboratory and numerical simulation of the Skagerrak circulation

Abstract

A rotating laboratory model and two, three-dimensional numerical hydrodynamic models are used to simulate the stationary circulation of the Skagerrak region. The laboratory experiment is used as a reference solution (benchmark) for numerical simulations with the same set-up and forcing, except for some minor details. The laboratory model is validated to field data for 3 different processes: the Atlantic Water circulation along the bottom slopes, the Skagen front at the mouth of the Kattegat and deep-water renewal in Oslofjorden. The benchmark includes a map of the surface currents, several current velocity sections, salinity distributions and the response of the water column outside Oslofjorden to the spreading and dilution of tracers discharged in the German Bight and the Kattegat. Wind forcing and time-dependent flows like tides and frontal instabilities are not included in the benchmark. Local wind forcing is not simulated; however, the advective boundary conditions are a result of remote, regional meteorological forcing. The main validation to field data is for the SKAGEX experiment, during which time the local wind forcing was weak. Two numerical models are used in this study. A non-hydrostatic model MIKE 3 that employs an artificial compressibility technique and a hydrostatic model, the Princeton Ocean Model (POM), which uses sigma coordinates in the vertical. The results show that both models are capable of simulating large-scale geophysical flows in a region with steep topography and large density gradients. In the former case, improved results are obtained when the artificial compressibility is chosen as a calibration parameter. In the latter case a z-level interpolation method is used to help minimize potential pressure gradient errors and reasonable results are obtained with this model. However, the different response of the two numerical models highlights the need for model improvements in order to enable the simulation of the strong horizontal current shears in these waters. The results show the importance of potential vorticity dynamics in steering the slope currents and the baroclinic jets along the coasts.