Energetic stratified turbulence generated by Kuroshio-seamount interactions in Tokara Strait

Abstract

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 ∼ 10-km wide seamount were measured with EM-APEX profiling floats and ADCP moorings. Energetic turbulent kinetic energy dissipation rates ε ∼ О(10−7 – 10−6 W kg−1) and diapycnal diffusivities K ∼ О(10−2 m2 s−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 100mbased on turbulence decay time of N−1 ∼ 100 s and the 0.5m s−1 Kuroshio flowspeed. Thus, the turbulentwake must be maintained by continuous replenishment which might arise from (i) nonlinear instability of a marginally unstable vortexwake, (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 ε 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 ε and spectral slopes of −1, indicating that the wake flows are strongly nonlinear. In contrast, vertical divergence spectral levels increase with ε.