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