Abstract: Observation of boundary mixing processes due to near-inertial waves in a stratified basin



Authors: Eefke van der Lee and Lars Umlauf

Near-inertial waves form an important contribution to oceanic energy and shear spectra, and thus play a major role in mixing the ocean’s interior. Here, we focus on a different aspect: the role played by near-inertial waves in mixing close to sloping boundaries. Our study area is one of the deep basins of the Baltic Sea, which, due to the fact that tidal energy is negligible, allows us to isolate the effect of near-inertial waves that is otherwise (often) overshadowed by internal tides. The measurements presented here consist of moored ADCPs and CTD loggers in the center of the basin and on the slopes, combined with densely spaced shear-microstructure and CTD cross-slope transects. During summer stratification, a three-layer density structure, with a thermocline and a deeper halocline, was observed with clear signals of downward near-inertial energy propagation after a short wind event. These motions are interpreted as near-inertial wave modes interacting with, and being reflected at, the boundaries. At the slopes the transects reveal a periodic near-bed dissipation rate signal and a growing and decaying bottom boundary layer (BBL) thickness corresponding to the high along-shore current events at the near-inertial frequency. Near-bottom dissipation rates are greatly enhanced compared to the interior, and, due to the straining of lateral density gradients by the cross-slope velocity, mixing is rather efficient, and contributes significantly to the basin-scale mixing.