Low wavenumber hump in the internal wave energy spectra observed in the Antarctic Circumpolar Current region

The fine-scale parameterization (e.g., Polzin et al. 1995; Gregg et al. 2003; Ijichi and Hibiya 2015) is a powerful tool for estimating the global distribution of the turbulent energy dissipation rates ε. It has been reported, however, that the fine-scale parameterization tends to overestimate the exact ε in the Antarctic Circumpolar Current (ACC) region (Waterman et al. 2013; Takahashi and Hibiya 2019). In this study, we examine the reason for such overestimates using the datasets from the simultaneous microstructure and fine-structure measurements carried out in the ACC region.

We find that the vertical wavenumber spectra of shear and strain have humps in the low wavenumbers (vertical wavenumber m ~ 0.01 cpm) at the places where the fine-scale parameterization overestimates ε. This is because the fine-scale parameterization assumes that the shape of the internal wave energy spectrum is flat like the Garrett-Munk spectrum, and the shear and strain energy levels are generally estimated by the spectral level in low wave numbers.

In the ACC shear spectra with a hump are mainly located in the upper ocean, and the magnitude of the hump is correlated with the shear/strain ratio Rω, vertical shear of the large-scale background flow Uz, internal wave energy level Eiw and the polarization ratio CCW/CW (multiple correlation coefficient R² = 0.36). On the other hand, strain spectra with a hump are mainly located in the near-bottom, and the magnitude of the hump is negatively correlated with Rω (correlation coefficient R² = 0.38). These results suggest that a hump in shear spectra might be created by downward-propagating near-inertial wave packets trapped in the geostrophic shear, whereas a hump in strain spectra might be created by bottom-generated high-frequency internal lee wave packets.