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.
The Antarctic Circumpolar Current (ACC) is a wind-driven ocean current that flows in a closed circulation around Antarctica unimpeded by continents, thus linking the Atlantic, Pacific, and Indian Oceans. It is characterized by low density stratification (∼10 −6 s −2 ) and large depth extent of (1 km) as well as a huge transport of 100-150 Sv (1 Sv = 10 6 m 3 /s). The ACC region exhibits several mixing hot spots presumably caused by breaking internal waves such as near-inertial waves generated by wind stress fluctuations and lee waves generated by the ACC impinging on rough topography (e.g.,
The finescale parameterization, formulated on the basis of a weak nonlinear wave–wave interaction theory, is widely used to estimate the turbulent dissipation rate, ε. However, this parameterization has previously been found to overestimate ε in the Antarctic Circumpolar Current (ACC) region. One possible reason for this overestimation is that vertical wavenumber spectra of internal wave energy are distorted from the canonical Garrett-Munk spectrum and have a spectral “hump” at low vertical wavenumbers. Such distorted vertical wavenumber spectra were also observed in other mesoscale eddy-rich regions. In this study, using eikonal simulations, in which internal wave energy cascades are evaluated in the frequency-wavenumber space, we examine how the distortion of vertical wavenumber spectra impacts on the accuracy of the finescale parameterization. It is shown that the finescale parameterization overestimates ε for distorted spectra with a low-vertical-wavenumber hump because it incorrectly takes into account the breaking of these low-vertical-wavenumber internal waves. This issue is exacerbated by estimating internal wave energy spectral levels from the low-wavenumber band rather than from the high-wavenumber band, which is often contaminated by noise in observations. Thus, in order to accurately estimate the distribution of ε in eddy-rich regions like the ACC, high-vertical-wavenumber spectral information free from noise contamination is indispensable.
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