Assessment of Finescale Parameterizations of Deep Ocean Mixing in the Presence of Geostrophic Current Shear: Results of Microstructure Measurements in the Antarctic Circumpolar Current Region

Abstract

AbstractThe Southern Ocean is thought to be one of globally significant mixing hotspots. In this study, we carried out simultaneous measurements of microscale turbulence and finescale shear/strain in the south of Australia to assess the validity of the existing finescale parameterizations of deep ocean mixing in the Antarctic Circumpolar Current (ACC) region where geostrophic shear flows coexist with the background internal wavefield. It is found that turbulent dissipation rates are overall small but the internal wavefield is more energetic than the Garrett‐Munk (GM) wavefield. Finescale shear/strain ratio (Rω) well exceeds the GM value in the deep layer south of the Southern ACC Front, suggesting that the local internal wave spectra are significantly biased to lower frequencies. Through the comparison of the directly measured turbulent dissipation rates with those inferred from finescale parameterizations, we find that the Gregg‐Henyey‐Polzin and Ijichi‐Hibiya parameterizations, both of which take into account the distortion of the GM frequency spectrum in terms of Rω, can predict well the turbulent dissipation rates at most of the stations, whereas the shear‐based parameterization (the strain‐based parameterization) tends to overestimate (underestimate) the directly measured turbulent dissipation rates. However, at the observation stations located on the ACC jets, both the Gregg‐Henyey‐Polzin and Ijichi‐Hibiya parameterizations tend to overestimate the turbulent dissipation rates by a factor of ∼3. The most likely cause of the overestimates is spatial anisotropy of the internal wavefield associated with large‐amplitude monochromatic near‐inertial internal waves and/or internal lee waves.