Abstract: Energetics of mixing in a stratified basin

 

 

Authors: Peter Holtermann

The seasonal variability of deep-water mixing processes in the central Baltic Sea is investigated using data from an extensive field campaign, including long-term moored instrumentation, ship-born turbulence microstructure measurements, and observations of the spreading of an inert tracer (SF5CF3) injected in September 2007.  Budgets of heat, salt, and potential energy show strongly enhanced mixing during the winter season with dissipation rates (inferred from the increase of background potential energy and from shear-microstructure profiling, respectively) pointing at the importance of boundary mixing. Internal mixing is weak but significant, and consistent with the observed fine structure. Spectral analysis of the kinetic and potential energies from the central mooring reveals two significant energy peaks, the first around the inertial frequency and a second broadband peak in the sub-inertial range that is interpreted as the signal of basin-scale topographic wave modes. The near-inertial wave activity peaks with a substantial delay after strong wind pulses, pointing at their remote generation at the basin boundaries. The temporal variability of the vertical energy flux from the near-inertial waves is strongly correlated with the occurrence of wind pulses with a clear dominance of downward propagating energy. The inferred dissipation rates are compared to the observed turbulence levels, as well as to the near-bottom energy dissipation associated with the topographic wave motions.