Observations of Turbulent Mixing During a Nonstratification Period in the Yellow Sea

Abstract

Abstract Microstructure profiling measurements collected at the continental shelf of the Yellow Sea (35°38'N, 121°20'E) from December 4 to 5, 2019, were analyzed by focusing on the characteristics of turbulent mixing in the Yellow Sea and its associated influencing factors. The vertical thermohaline structure of the water column was nonstratified during the observation period, resulting in the vertically and temporally consistent distribution of turbulence dissipation and diapycnal diffusivity. The average (in time and space) dissipation rate and diapycnal diffusivity were 2.95 × 10−8 W kg−1 and 1.86 × 10−4 m2 s−1, respectively. In the vertical distribution, intense mixing occurred near the sea surface and within the bottom layers. The temporal variation in dissipation exhibits a diurnal variation that was strongly affected by surface buoyancy flux and wind energy, and a high amount of dissipation was observed at night, with an average dissipation rate of 2.45 × 10−8 W kg−1, which was almost one order of magnitude higher than that in the daytime (3.55 × 10−9 W kg−1). The cumulative distribution functions of the dissipation rate and diapycnal diffusivity across the entire water column during the measurement period could be parameterized by a lognormal distribution model. Further analysis shows that the dissipation rate was positively related to wind speed and rotational barotropic tidal velocity. Compared with the rotating tidal current, wind-driven turbulence was able to penetrate the surface, thereby causing layer mixing throughout the entire water column (R = 0.71), and is a dominant driver of elevated turbulent mixing during wintertime.