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

The 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 (R ) 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 R , 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. Plain Language SummaryThe Southern Ocean is a special place where the Antarctic Circumpolar Current (ACC) coexists with the energetic internal waves generated by strong westerly wind as well as the ACC impinging on rough topographic features and strong turbulent mixing occurs. Because of the difficulty of direct measurements of turbulent mixing, it is common to employ parameterizations to estimate the intensity of turbulent mixing ( ). However, these parameterizations are formulated on the basis of the internal wave theory and do not take into account the existence of strong flows like the ACC. Therefore, in this study, we carry out direct measurements of turbulent mixing to assess the validity of existing parameterizations of deep ocean mixing in the ACC region. Through the comparison of the directly measured and those inferred from parameterizations, we find that the Gregg-Henyey-Polzin and Ijichi-Hibiya parameterizations can predict well at the most of the observation stations but tend to overestimate by a factor of ∼3 at the stations located on the ACC jets. These overestimates are most likely explained by the spatially anisotropic internal wavefield associated with large-amplitude monochromatic internal waves, which are generated and/or amplified by the ACC.

A parameterization of local and remote tidal mixing

“…This assessment concurs with recent studies reporting underestimated dissipation by the finestructure method in regions of rough topography or strong forcing (Bouruet-Aubertot et al, 2018a;Liang et al, 2018;Thurnherr et al, 2015). The identified low bias is not expected to be universal, however, as it depends on implementation choices of the method and regional processes at play (Hibiya et al, 2012;Kunze & Lien, 2019;Takahashi & Hibiya, 2019;Waterman et al, 2014). Profiles of Figures 12d-12f also hint at a divergence near 1.7 km depth between the present parameterization and the dissipation estimates of Whalen et al (2015).…”

Energy Sinks for Lee Waves in Shear Flow

“…The black box encloses the parameter space characterizing the Antarctic Circumpolar Currents [n 5 22.5 6 0.3 (Nikurashin and Ferrari 2010b); N/f ;5-10 (Waterman et al 2014)], where the dissipative fraction is 0.44-0.56. Southern Ocean where microstructure measurements find dissipation rates that are smaller than predicted by lee-wave generation theory and finescale parameterizations (Sheen et al 2013;Waterman et al 2013Waterman et al , 2014Cusack et al 2017;Takahashi and Hibiya 2019). Transfer of lee-wave energy back to the balanced flow through wave action E/jkUj conservation (Bretherton and Garrett 1968) can account for a factor of 2 reduction in turbulence production with the remaining portion of lee-wave power reabsorbed by the balanced flow for conditions typical of the Southern Ocean (Waterman et al 2014).…”

“…Directly measured microstructure dissipation rates also fall below inferences based on a finescale shear-andstrain parameterization (Gregg and Kunze 1991;Polzin et al 1995;Gregg et al 2003;Naveira Garabato et al 2004a,b;Kunze et al 2006;Wu et al 2011;Whalen et al 2012;Ijichi and Hibiya 2017) in Antarctic Circumpolar Currents by factors of 5 6 0.5 in Drake Passage and on the north flank of Kerguelen Plateau (Waterman et al 2013(Waterman et al , 2014 and factors of 2-3 south of Australia (Takahashi and Hibiya 2019). Brearley et al (2013) reported that the finescale parameterization was consistent with lee-wave generation predictions.…”

A Parameterization of Local and Remote Tidal Mixing

“…This assessment concurs with recent studies reporting underestimated dissipation by the finestructure method in regions of rough topography or strong forcing (Bouruet‐Aubertot et al, ; Liang et al, ; Thurnherr et al, ). The identified low bias is not expected to be universal, however, as it depends on implementation choices of the method and regional processes at play (Hibiya et al, ; Kunze & Lien, ; Takahashi & Hibiya, ; Waterman et al, ). Profiles of Figures d–f also hint at a divergence near 1.7 km depth between the present parameterization and the dissipation estimates of Whalen et al ().…”