Highly efficient boundary mixing near sloping topography in a non-tidal basin

Recent results from a tracer experiment in the deepest layers of the central Baltic Sea suggest that boundary mixing plays an essential role for the net vertical transport of dissolved substances. Here, we discuss results from an extensive field campaign, aimed at clarifying the relevance of boundary mixing in strongly stratified, non-tidal basins, taking the Baltic Sea as an example. The dataset includes high-resolution turbulence microstructure and velocity data from seven cross-slope transects, providing a detailed view of the temporal and cross-slope variability of mixing. Basin-scale motions are found to be dominated by near-inertial waves with velocities reaching 0.2 m s⁻¹. The near-bottom currents associated with these motions generate a vigorously turbulent bottom boundary layer (BBL) with a thickness of several meters, where bottom friction rather than the critical reflection of the near-inertial waves acts as the main energy source for turbulence. Most of the BBL is stably stratified, and characterized by Ozmidov scales smaller than the distance to the bottom, suggesting that near-bottom turbulence is generally not controlled by law-of-the-wall dynamics. As a consequence, the buoyancy Reynolds number is relatively small, and mixing efficiencies are high. This suggests that boundary mixing near sloping topography is more efficient than generally assumed.