Turbulence and Mixing: A Controller of Tracer Distributions in the East China Sea

Abstract

Vertical mixing in the ocean is a key process for the diapycnal exchange of biogeochemical materials between different layers. The data collected from observations of microstructure profiler (MSP) in the East China Sea (ECS) in the summertime show high turbulent energy dissipation rates near the bottom and within the thermocline, with typical value of O(10-7 Wkg-1) and occasionally close to 10-6 Wkg-1. Corresponding vertical eddy diffusivities reach to 10-2 m2s-1. High values of dissipation rate and vertical diffusivity are consistent with high turbidity or enhanced Chlorophyll a concentration. In addition, the data from repeated MSP castings and vessel mounted Acoustic Doppler Current Profiler (ADCP) measurements show good correlation between the enhanced bottom mixed layer and the phase of tidal current indicating that vertical mixing near the bottom occurs mainly due to the tidal stirring. During internal wave episodes or strong wind events, high-frequency fluctuations of all components of the velocity occur for almost entire water column suggesting the possibility of vertical transport of suspended materials across the pycnocline. This implies that near bottom mixing and internal mixing within the thermocline are key physical processes to control biogeochemical tracers, e.g. the subsurface Chlorophyll maximum layer, in the ECS shelfme show high turbulent energy dissipation rates near the bottom and within the thermocline, with typical value of O(10-7 Wkg-1) and occasionally close to 10-6 Wkg-1. Corresponding vertical eddy diffusivities reach to 10-2 m2s-1. High values of dissipation rate and vertical diffusivity are consistent with high turbidity or enhanced Chlorophyll a concentration. In addition, the data from repeated MSP castings and vessel mounted Acoustic Doppler Current Profiler (ADCP) measurements show good correlation between the enhanced bottom mixed layer and the phase of tidal current indicating that vertical mixing near the bottom occurs mainly due to the tidal stirring. During internal wave episodes or strong wind events, high-frequency fluctuations of all components of the velocity occur for almost entire water column suggesting the possibility of vertical transport of suspended materials across the pycnocline. This implies that near bottom mixing and internal mixing within the thermocline are key physical processes to control biogeochemical tracers, e.g. the subsurface Chlorophyll maximum layer, in the ECS shelf