Abstract
The upstream Kuroshio flows through Okinawa Trough and the Tokara island chain, the region near the continental shelf of the East China Sea and shallow seamounts, where the Kuroshio can induce strong mixing over the shallow topography. Also, tidal currents over the rough topography may produce internal tides, and associated turbulence. The previous observations show energetic high vertical wavenumber near-inertial wave shear in the Kuroshio thermocline, which implies strong turbulent mixing. However, direct turbulence measurements in this region are very scarce. Using high lateral resolution (1–2 km) direct turbulence measurements, we show here, for the first time, that strong turbulent layers form spatially coherent banded structures with lateral scales of >O(10 km), associated with bands of near-inertial wave/diurnal internal tide shear of high vertical wavenumber in the upstream Kuroshio. The turbulent kinetic energy dissipation rates within these turbulent layers are >O(10−7 W kg−1), and estimated vertical eddy diffusivity shows >O(10−4 m2 s−1) on average. These results suggest that the high vertical wavenumber near-inertial waves propagating in the upstream Kuroshio could have large impacts on the watermass modifications, momentum mixing, nutrient supply, and associated biogeochemical responses in its downstream.
Highlights
The upstream Kuroshio flows through Okinawa Trough and the Tokara island chain, the region near the continental shelf of the East China Sea and shallow seamounts, where the Kuroshio can induce strong mixing over the shallow topography
While a number of previous studies have reported that the beams of the M2 internal tide emanated from the rough topography induced strong turbulence in close proximity to canyons and seamounts[21,22], recent shipboard and lowered Acoustic Doppler Current Profiler (ADCP) measurements in the upstream Kuroshio showed widespread large amplitude near-inertial internal wave shear of high vertical wavenumber in the Kuroshio and regions between continental margins and the Kuroshio[23]
A recent extensive microstructure time-series survey at low latitude[44,45] reported that the strong turbulence coincided with the relatively large strain caused by wind-induced near-inertial waves of anomalously low frequency in the anticyclonic vorticity field[33], it is not known if near-inertial internal waves form the spatially coherent strong turbulent layers in ocean fronts
Summary
The upstream Kuroshio flows through Okinawa Trough and the Tokara island chain, the region near the continental shelf of the East China Sea and shallow seamounts, where the Kuroshio can induce strong mixing over the shallow topography. Using high lateral resolution (1–2 km) direct turbulence measurements, we show here, for the first time, that strong turbulent layers form spatially coherent banded structures with lateral scales of >O(10 km), associated with bands of near-inertial wave/diurnal internal tide shear of high vertical wavenumber in the upstream Kuroshio. While a number of previous studies have reported that the beams of the M2 internal tide emanated from the rough topography induced strong turbulence in close proximity to canyons and seamounts[21,22], recent shipboard and lowered Acoustic Doppler Current Profiler (ADCP) measurements in the upstream Kuroshio showed widespread large amplitude near-inertial internal wave shear of high vertical wavenumber in the Kuroshio and regions between continental margins and the Kuroshio[23]. The observations in this study suggest that propagating high vertical wavenumber near-inertial internal waves form spatially coherent banded layers of strong turbulence in the upstream Kuroshio, with O(100 m) and >O(10 km) vertical and lateral scales, respectively
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