High-Resolution Turbulence Observations of Non-Tidal Stratified Flow Over a Steep Sill in the Baltic Sea

Julia Muchowski, Stockholm University, Geological Sciences, Stockholm, Sweden, Lars Umlauf, Leibniz Institute for Baltic Sea Research (IOW), Warnemuende, Germany, Lars Arneborg, Swedish Meteorological and Hydrological Institute, Oceanography Research Dept., Norrköping, Sweden, Elizabeth Frances Weidner, University of New Hampshire Main Campus, Durham, NH, United States, Peter Ludwig Holtermann, Leibniz Institute for Baltic Sea Research (IOW), Rostock, Germany, Jen-Ping Peng, Leibniz Institute for Baltic Sea Research (IOW), Physical Oceanography, Rostock, Germany, Christoph Humborg, Stockholm University, Baltic Sea Centre, Stockholm, Sweden and Christian Stranne, Stockholm University, Department of Geological Sciences, Stockholm, Sweden
Stratified flows over steep sills are well-known hot spots of mixing that tend to be underestimated by in-situ measurements and models due to their limited spatial resolution. Such flows are typically associated with tidal currents in the ocean. Here we present a detailed example of a non-tidal, quasi-stationary, vigorously turbulent flow of this type from the Baltic Sea.
During an expedition in the Sea of Åland in 2019, we repeatedly measured a 2 km long transect across a steep sill connecting the central and northern parts of the Baltic Sea. We continuously acquired acoustic data with a state-of-the-art broadband echo sounder (Simrad EK80) using a chirped pulse from 45-90 kHz. The remote acoustic observations were complemented by about 150 in-situ microstructure profiles along the transect, serving as ground truth. With a vertical/horizontal resolution of 0.1/1 m (for the acoustic dataset), we were able to visualize the detailed structure of a highly energetic stratified flow over the sill, including vertical overturns on the lee side (diameters of up to 100 m), lee waves, and Kelvin-Helmholtz instabilities. Preliminary results show a good agreement between the acoustic observations of turbulence and the dissipation rates from the in-situ microstructure data. Dissipation rates were among the highest ever observed in the deep layers of the Baltic Sea, suggesting that the sill region constitutes an important mixing hot spot. Characteristic parameters of the flow, derived from the geometry of the sill and upstream velocity and buoyancy frequency, suggest a transition from subcritical to supercritical flow and boundary layer separation. This is in good agreement with the observation of strong turbulence and a hydraulic jump on the lee side of the sill.