Finescale parameterizations are powerful tools for estimating the global distribution of turbulent kinetic energy dissipation rates ε. However, they tend to overestimate ε in the Antarctic Circumpolar Current (ACC), where bottom-reaching mesoscale eddies coexist with energetic internal waves such as wind-induced near-inertial waves and bottom-generated internal lee waves. In this study, we explore the reason for such overestimations by analyzing simultaneous microstructure and finestructure measurements in the ACC. Finescale parameterizations overestimate ε where vertical wavenumber spectra of internal wave energy are distorted from the canonical Garrett–Munk (GM) spectrum by a spectral hump at ~0.01 cpm wavenumbers. Since shear and strain spectra are integrated over the noise-free wavenumbers lower than the rolloff wavenumber, the spectral levels are overestimated for such spectra. These distorted shear spectra are mainly located in the upper ocean, while distorted strain spectra near the seafloor. Correlation analyses suggest that shear and strain spectral humps are caused by near-inertial and lee wave packets superposed on a GM-like internal wave field, and these wave packets are generated or amplified by geostrophic shear flows.

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). One possible reason for this overestimation is that vertical wavenumber spectra of internal wave energy are distorted from the canonical Garrett–Munk spectrum by a spectral hump at low wavenumbers (~0.01 cpm). 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 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, 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.