Efficient boundary mixing due to near-inertial waves in a non-tidal basin: Observations from the Baltic Sea

Abstract

Diapycnal mixing in large nontidal basins is often assumed to be related to the effect of near-inertial waves. While the role of near-inertial shear for the generation of shear instabilities in the stratified interior is relatively well understood, much less is known about the impact of near-inertial motions on boundary mixing processes in nontidal systems, mainly owing to the lack of appropriate observations. Here, an extensive data set is discussed, describing the variability of boundary mixing induced by near-inertial motions near the sloping topography of one of the main basins of the Baltic Sea. These data reveal the existence of a vigorously turbulent bottom boundary layer of a few meters thickness covering the entire slope region above and below the permanent halocline that constitutes the main obstacle for vertical transport in the Baltic Sea. Near-bottom turbulence was driven by the near-inertial shear in the frictional boundary layer rather than by breaking of near-inertial waves near critical slopes. Large regions of the bottom boundary layer remained strongly stratified, and therefore, different from the traditional view of inefficient boundary mixing, mixing efficiencies reached values typical for the ocean's interior.