First Evidence of Coherent Bands of Strong Turbulent Layers Associated with High-Wavenumber Internal-Wave Shear in the Upstream Kuroshio

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.

Phytoplankton growth and consumption by microzooplankton stimulated by turbulent nitrate flux suggest rapid trophic transfer in the oligotrophic Kuroshio

“…Several studies have pointed out that those steep topographic features stir and modify the water column through upwelling (Hasegawa et al, 2004(Hasegawa et al, , 2008 and turbulent mixing (Tsutsumi et al, 2017;Nagai et al, 2017). Compared with the turbulent nitrate fluxes reported in previous studies, the fluxes observed in the Tokara Strait were 1 order of magnitude higher than those reported in the Kuroshio Extension front (Kaneko et al, 2012(Kaneko et al, , 2013Nagai et al, 2017), which is greater than those at other oceanic sites, and equivalent to those at coastal sites (Cyr et al, 2015). The turbulent nitrate flux in the downstream Kuroshio Current near the Tokara Strait is similar in magnitude to our estimates (Nagai et al, 2019b).…”

“…The Kuroshio nutrient stream characterized by an intense core of nutrient flux at subsurface contributes substantially to productivity in the euphotic zone of the Kuroshio in a manner similar to the contribution of the nutrient stream along the Gulf Stream (Komatsu and Hiroe, 2019). Turbulence around the Kuroshio appears to be an important mechanism that supplies nutrients via upward movement of deeper waters into the Kuroshio (Nagai et al, 2019a). Frontal disturbances also contribute to the supply of nutrients into the euphotic zone of the Kuroshio (Kuroda, 2019).…”

“…These nutrient supplies have been hypothesized to stimulate biological productivity in the Kuroshio. Within the wide path of the Kuroshio, the supply of nutrients by these mechanisms can be particularly efficacious around the Tokara Strait because of the extensive frontal disturbances (Nakamura et al, 2006) and strong turbulence (Tsutsumi et al, 2017;Nagai et al, 2017Nagai et al, , 2019b in that area.…”

How the Kuroshio Current Delivers Nutrients to Sunlit Layers on the Continental Shelves With Aid of Near‐Inertial Waves and Turbulence

“…Turbulent eddy vertical diffusivity K = ∕N 2 is obtained using measured TKE dissipation rate and buoyancy frequency N = √ −g( ∕ z)∕ o , where and o are density of water and its reference, respectively, g is gravitational acceleration, assuming = 0.2 for the mixing efficiency factor (Osborn, 1980). To avoid aliasing, the vertical shear of the lateral flow computed for each component is back-rotated with respect to the reference time t 0 (time at the Kuroshio axis) assuming rotation with the inertial frequency, similar to Nagai et al (2017). Inertial-wave velocity and shear rotate clockwise with time, which may introduce temporal aliasing when the observations last more than equal to an inertial time scale.…”

“…On the other hand, in the downstream of these nutrient streams, several microstructure surveys have observed relatively strong turbulence in the thermocline (Nagai et al, 2009(Nagai et al, , 2012Nagai, Tandon, et al, 2015); Inoue et al 2010), which induces nutrient diffusive fluxes of O(0.1 mmol N·m −2 ·day −1 ) (Inoue et al, 2010;Kaneko et al, 2012Kaneko et al, , 2013. Recent microstructure surveys have found very strong turbulence in the Tokara Strait (Nagai et al, 2017;Tsutsumi et al, 2017), some of which are associated with the high-vertical-wavenumber near-inertial internal waves. Recent microstructure surveys have found very strong turbulence in the Tokara Strait (Nagai et al, 2017;Tsutsumi et al, 2017), some of which are associated with the high-vertical-wavenumber near-inertial internal waves.…”