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

Abstract In global ocean circulation and climate models, bottom-enhanced turbulent mixing is often parameterized such 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 U 0 , the horizontal wavenumber k H , and the height h T of the seafloor topography. When the steepness parameter ( Sp=Nh T /U 0 where N is the buoyancy frequency near the seafloor) is less than 0.3, internal lee waves propagate upward from the seafloor while interacting with the GM internal wave field to create a turbulent mixing region with ζ that extends further upward from the seafloor as U 0 increases, but is nearly independent of k H . In contrast, when Sp exceeds 0.3, inertial oscillations (IOs) not far above the seafloor are enhanced by the intermittent supply of internal lee wave energy Doppler-shifted to the near-inertial frequency, which occurs depending on the sign and magnitude of the background IO shear. The composite flow, consisting of the superposition of U 0 and the IOs, interacts with the seafloor topography to efficiently generate internal lee waves during the period centered on the time of the composite flow maximum, but their upward propagation is inhibited by the increased IO shear, creating a turbulent mixing region of small ζ .