Internal hydraulic jump in the Tsugaru Strait

Abstract

Sea straits are known for important sites to modify water-mass properties and transport nutrients through vigorous vertical mixing. The first microstructure measurements ever taken in the Tsugaru Strait, between the Sea of Japan and the Northwest Pacific Ocean, showed elevated turbulent mixing. The turbulent energy dissipation rate was especially enhanced to O(10−6 W kg−1) via an internal hydraulic jump when the throughflow passed over an abrupt sill. Beyond the sill, the associated vertical eddy diffusivity reached O(10−2 m2 s−1) and the vertical turbulent nitrate flux below the subsurface chlorophyll-a maximum layer was estimated to reach 1 mmolN m−2 day−1, which could elevate the new production of phytoplankton within the layer. Large vertical velocities exceeding 0.1 m s−1 were also observed near the seafloor of the sill where the flow was hydraulically controlled. A simple advection–diffusion model suggested that the energy dissipation rate of O(10−5 W kg−1) was necessary to reproduce the observed modification of subsurface salinity maximum across the strait. Upwelling and/or vertical straining associated with the internal hydraulic jump is also believed to play an important role in the water-mass modification in the strait and could enhance new primary production for a wide area downstream.