The Baltic Sea Tracer Release Experiment: 2. Mixing processes

Deep water mixing processes in the largest basin of the Baltic Sea (Gotland Basin) are investigated here with the help of long-term moored instrumentation and turbulence microstructure measurements. Budgets for heat and salinity reveal a strong temporal variability in the deep water mixing coefficients that is shown to be coupled to the wind forcing down to time scales of individual wind events. Consistent with the results from a simultaneously conducted tracer release experiment, near-boundary turbulence was identified as the key processes for basin-scale mixing. Turbulent bottom boundary layers were found to be thick and stable in the flat, deep part of the basin, where highest dissipation rates were observed, and comparatively thin, less energetic, and highly variable on the slopes due to a permanent tendency for boundary layer restratification. In the absence of internal tides, strongly damped basin-scale topographic waves with periods of a few days, a deep rim current, and, to a smaller extent, near-inertial waves were identified as the main energy sources for boundary mixing. These results are likely to be relevant also for other medium-sized stratified basins.