Turbulent mixing within the Kuroshio in the Tokara Strait

Abstract

Turbulent mixing and background current were observed using a microstructure profiler and acoustic Doppler current profilers in the Tokara Strait, where many seamounts and small islands exist within the route of the Kuroshio in the East China Sea. Vertical structure and water properties of the Kuroshio were greatly modified downstream from shallow seamounts. In the lee of a seamount crest at 200 m depth, the modification made the flow tend to shear instability, and the vertical eddy diffusivity is enhanced by nearly 100 times that of the upstream site, to Kρ ∼ O(10−3)–O(10−2) m2 s−1. A one‐dimensional diffusion model using the observed eddy diffusivity reproduced the observed downstream evolution of the temperature‐salinity profile. However, the estimated diffusion time‐scale is at least 10 times longer than the observed advection time‐scale. This suggests that the eddy diffusivity reaches to O(10−1) m2 s−1 in the vicinity of the seamount. At a site away from the abrupt topography, eddy diffusivity was also elevated to O(10−3) m2 s−1, and was associated with shear instability presumably induced by the Kuroshio shear and near‐inertial internal‐wave shear. Our study suggests that a better prediction of current, water‐mass properties, and nutrients within the Kuroshio requires accurate understanding and parameterization of flow‐topography interaction such as internal hydraulics, the associated internal‐wave processes, and turbulent mixing processes.

Particulate Trace Metal Composition and Sources in the Kuroshio Adjacent to the East China Sea: The Importance of Aerosol Deposition

“…An upwelling feature was observed around the relatively shallow stations C8, A1, and A2, where all the parameters show an upwelling pattern in the subsurface water, including low temperature, salinity, and dissolved oxygen concentrations and elevated potential density, and phosphate and Chl a concentrations. These stations are located at the southern end of Kyushu, in the Tokara strait, where the Kuroshio turns right due to the abrupt topography shallowing and thus cause intensive turbulent mixing (Hasegawa et al, ; Tsutsumi et al, ). Indeed, abrupt shallowing of topography along the Kuroshio path is a major cause of strong vertical turbulent mixing (Chang et al, ; Tsutsumi et al, ).…”

“…These stations are located at the southern end of Kyushu, in the Tokara strait, where the Kuroshio turns right due to the abrupt topography shallowing and thus cause intensive turbulent mixing (Hasegawa et al, ; Tsutsumi et al, ). Indeed, abrupt shallowing of topography along the Kuroshio path is a major cause of strong vertical turbulent mixing (Chang et al, ; Tsutsumi et al, ). Although horizontal intrusion of coastal seawater may also play a role on the hydrographic variations within the Kuroshio, the study of Tsutsumi et al () found that vertical turbulent mixing is much more significant than the horizontal input.…”

“…Indeed, abrupt shallowing of topography along the Kuroshio path is a major cause of strong vertical turbulent mixing (Chang et al, ; Tsutsumi et al, ). Although horizontal intrusion of coastal seawater may also play a role on the hydrographic variations within the Kuroshio, the study of Tsutsumi et al () found that vertical turbulent mixing is much more significant than the horizontal input. A similar upwelling feature was also observed at stations A, B, and 1, where the Kuroshio flows through Green Island, Taiwan.…”

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

“…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.…”

Temporal Variation of Kuroshio Nutrient Stream South of Japan

“…If we apply the former to the entire Kuroshio area south of Japan that is 1,000 km long and 200 km wide, the maximum vertical nitrate flux over the entire area is 0.2 kmol/s. A recent study (Tsutsumi et al, ) showed that the vertical diffusivity in the Tokara Strait could be 100 times larger than that in the upstream region owing to flow‐topography interactions. If we use an area 300 km long and 200 km wide for the Tokara Strait and a value 100 times the vertical diffusive nitrate flux estimated by Kaneko et al (), the vertical nitrate flux could be 6 kmol/s, which is comparable to the variations in nitrate transport induced by the difference in nitrate concentration along the Kuroshio in the first layer (Table S2).…”