[A-OS16] Frontiers of Ocean Mixing Research

[AOS16-11] Energetic turbulence and nonlinear internal waves in Tokara Strait

*Anne Takahashi¹, Ren-Chieh Lien¹, Eric L Kunze², Hirohiko Nakamura³, Ryuichiro Inoue⁴, Eisuke Tsutsumi⁵ (1.Applied Physics Laboratory, University of Washington, 2.NorthWest Research Associates, 3.Kagoshima University, 4.Japan Agency for Marine-Earth Science and Technology, 5.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:Turbulence, Internal waves, Kuroshio

As the Kuroshio passes through Tokara Strait, it interacts with a chain of islands and seamounts to generate enhanced turbulence. Arrays of 10 EM-APEX floats measuring full-depth profiles of finescale horizontal velocity (u, v), temperature T, salinity S and microscale temperature-variance dissipation rate χ were deployed upstream of O(10 km) wide Hirase seamount and carried downstream into its wake. Finescale vertical velocities w are inferred following Cusack et al. (2016). Hirase’s topographic Froude number Nh/U >> 1 and Rossby number U/(fL) ~ O(1) so saturated lee-wave generation near its ~200-m deep summit and vertically-decoupled vortex wakes at greater depths are expected. The float horizontal velocity profiles indicate the intense Kuroshio flow confined to the upper 200-m depth and complicated banded structures below. Vertical wavenumber spectra of shear and strain are an order-of-magnitude above the canonical Garrett-Munk model, have spectral slopes of ~–1 and a spectral rolloff at ~100-m vertical wavelengths, which is a significant fraction of the ~600-m water depth. These saturated spectra are not consistent with weakly nonlinear internal wave fields. Turbulent diapycnal diffusivities K ~ O(10^–2 m² s^–1) were observed above the seamount and extending at least 20 km downstream in a layer spanning 100-300 m depth below the surface mixed-layer. High K is associated with gradient Froude numbers exceeding 2, suggesting that the turbulence is generated by shear instability of the large-scale wave modes. It is also correlated with finescale (vertical wavelengths λ_z < 30 m) vertical velocity variance, consistent with turbulent large-eddy (e.g., Moum 1996) and stratified turbulence parameterizations (e.g., D’Asaro and Lien 2000).