Shear-driven vertical mixing and turbulent exchange over the continental slope in the northwestern Sea of Japan

Abstract

Using fine-scale measurements in the northwestern Sea of Japan, we estimated the vertical mixing parameters in the sea water column extended from the lower part of the thermocline downward to the near-bottom layer above the continental slope. The vertical scales of the turbulent patches were determined together with the turbulent dissipation rate and diapycnal diffusivity based on the conductivity, temperature, and depth data obtained by an Aqualog moored profiler from April through October 2015. The Thorpe-scale method was used to estimate the vertical mixing parameters as well as the vertical heat and salt fluxes. The enhanced vertical mixing, as well as enhanced upward heat flux and downward salt flux, occurred below the mixed layer despite strong density stratification. By comparing the turbulent dissipation rate and diapycnal diffusivity estimates derived via the Thorpescale method and the estimates of the same parameters obtained earlier by applying the finescale parameterization method to the same dataset in addition to the collocates of the current velocity measurements, the comparative accuracy evaluation of both methods was carried out. Finally, by compiling the vertical mixing data obtained by the Thorpe-scale method and the finescale parameterization approach, the generalized depth profile for the background diapycnal diffusivity is presented for the depth range from 70 to 350 m.