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

Anne Takahashi and Toshiyuki Hibiya, The University of Tokyo, Department of Earth and Planetary Science, Graduate School of Science, Tokyo, Japan
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.