Blending source of turbulence, upwelled nutrients and primary producers in the Kuroshio flowing through the Tokara Strait

Abstract:

The upstream Kuroshio, at the south of Kyushu, flows over many seamounts in the Tokara Strait, generating negative potential vorticity over the slope of these seamounts, near-inertial internal waves and associated very strong turbulence. The observations conducted in November 2017, using a tow-yo microstructure profiler, showed that the enhanced turbulence with the average turbulent kinetic energy dissipation rate of O(10^-7 Wkg^-1) persists along the Kuroshio at least a distance of 100 km across the Tokara Strait. Because the subsurface Kuroshio is a nutrient stream, the enhanced turbulence in the Tokara Strait can induce nutrient diffusive flux to the euphotic layer. However, because islands and seamounts in the Kuroshio can also generate wakes and eddies, it has still been unclear whether the turbulent diffusive nitrate flux is a mechanism more important than the eddy upwelling to ultimately supply nutrients to the primary producers. In this study, observation data obtained by the two-yo microstructure profiler and a tow-yo biogeochemical sensor in the south of Tokara Strait during the November 2018 cruise is analyzed. In this region, the Kuroshio turns its direction from southeast to northeast. As a result, chlorophyll-a rich shelf water is found to be subducted into the thermocline of the hairpin curving Kuroshio with very strong turbulence and high vertical wave number internal-wave shear persisting from the seamounts of the Tokara Strait. On the other hand, the results from the numerical simulation suggest that nitrate is upwelled in the submesoscale cyclonic eddies behind the islands near the Tokara Strait. These results from observations and numerical simulations suggest that the Kuroshio at the south of Kyushu could play a role in stirring the upwelled nutrients with the highly turbulent water associated with negative potential vorticity and near-inertial waves, that are further blended with the primary producers from the shelf water.