Turbulent mixing and its contribution to the oxygen flux in the northwestern boundary current region of the Japan/East Sea, April–October 2015

Abstract

The Japan/East Sea is well ventilated and is the most oxygen-rich region in the Pacific. However, quantitative estimates of the turbulent fluxes are missing due to a lack of observational data. To assess turbulent mixing, we employ data from the moored profiler Aqualog survey of April–October 2015 near the northwestern boundary of this region where the oxygen maximum is observed. The survey allowed observation of collocated depth profiles of conductivity, temperature, ocean current, and dissolved oxygen 8 times per day. The data were processed by using the Mixing (MX) Oceanographic Toolbox based on the fine-scale parameterization of turbulent dissipation. The dissipation rate, the eddy diffusivity and the diapycnal fluxes of heat, salt and oxygen are estimated in the depth range from 130 to 350 m throughout the profiler deployment period. The survey-averaged diffusivity increased with depth from 0.5 × 10 −5 to 4.0 × 10 −5 m 2 s −1 . The month-to-month variability in the mixing in the intermediate water is presented. Early in May 2015, a transition in mixing occurred from the winter regime with upward turbulent fluxes of both heat and salt to the summer regime with the downward mixing of heat. The turbulent mixing was elevated in June when large anticyclonic eddies passed the profiler mooring. The application of the eddy diffusivities to the profiler mooring oxygen data yields an average downward oxygen flux of roughly 8.6 mol m −2 month −1 . • 6-month survey using a moored profiler in the northern Japan/East Sea. • Collocated fine-structure data profiles in the westward coastal current. • Turbulent mixing induced by internal wave breaking was quantitatively assessed. • Turbulence was enhanced due to passage of eddies and after the atmospheric storms. • Turbulent oxygen flux was directed from the intermediate water downward in the summer.