The vertical structure of internal lee wave-driven benthic mixing hotspots

In global ocean circulation and climate models, the bottom-enhanced turbulent mixing is often parameterized by assuming that the vertical decay scale of the energy dissipation rate ζ is universally constant at 500 m. In this study, using a non-hydrostatic two-dimensional numerical model in the horizontal-vertical plane that incorporates a monochromatic sinusoidal seafloor topography and the Garrett-Munk (GM) background internal wave field, we find that ζ of the internal lee wave-driven bottom-enhanced mixing is actually variable depending on the magnitude of the steady flow U₀, the horizontal wavenumber k_H, and the height h_T of the seafloor topography. When the steepness parameter (Sp=Nh_T/U₀: N is the background buoyancy frequency near the seafloor) is less than 0.3, internal lee waves propagate upward from the seafloor while interacting with the GM background internal wave field to create a turbulent mixing region with ζ that extends further upward from the seafloor as U₀ increases, but is nearly independent of k_H. In contrast, when Sp exceeds 0.3, the inertial oscillations (IOs) gradually develop at heights not far above the seafloor topography, inhibiting the upward propagation of the bottom-generated internal lee waves. By interacting with the background IOs, the upward propagating internal lee waves dissipate some of their energy, but simultaneously contribute the rest of their energy to amplify the IOs. The oscillatory flow, consisting of the superposition of the steady flow and the IOs, efficiently generates upward propagating internal lee waves during the period centered on the time of its maximum, when it becomes transiently stationary.