Energetic Stratified Turbulence Generated by Kuroshio-Seamount Interactions in Tokara Strait

Generating mechanisms and parameterizations for enhanced turbulence in the wake of a seamount in the path of the Kuroshio are investigated. Full-depth profiles of finescale temperature, salinity, horizontal velocity, and microscale thermal-variance dissipation rate up- and downstream of the similar to 10-km-wide seamount were measured with EM-APEX profiling floats and ADCP moorings. Energetic turbulent kinetic energy dissipation rates epsilon similar to O(10-7-10-6)Wkg-1 and diapycnal diffusivities K similar to O(10-2)m2s-1 above the seamount flanks extend at least 20 km downstream. This extended turbulent wake length is inconsistent with isotropic turbulence, which is expected to decay in less than 100 m based on turbulence decay time of N-1 similar to 100 s and the 0.5 m s(-1) Kuroshio flow speed. Thus, the turbulent wake must be maintained by continuous replenishment which might arise from (i) nonlinear instability of a marginally unstable vortex wake, (ii) anisotropic stratified turbulence with expected downstream decay scales of 10-100 km, and/or (iii) lee-wave critical-layer trapping at the base of the Kuroshio. Three turbulence parameterizations operating on different scales, (i) finescale, (ii) large-eddy, and (iii) reduced-shear, are tested. Average epsilon vertical profiles are well reproduced by all three parameterizations. Vertical wavenumber spectra for shear and strain are saturated over 10-100 m vertical wavelengths comparable to water depth with spectral levels independent of epsilon and spectral slopes of -1, indicating that the wake flows are strongly nonlinear. In contrast, vertical divergence spectral levels increase with epsilon.