Spatial and temporal variability of turbulent mixing in the near-field of the Changjiang River

Abstract

Based on field hydrological, microstructural, and shipboard Acoustic Doppler Current Profiler data, we quantified the spatial and temporal variability of turbulent mixing in the near-field Changjiang (Yangtze) River plume. The surface dissipation rate (e) changed by three orders of magnitude from near-field (10-4 W/kg) to far-field (10-7 W/kg) plumes, indicating a decrease with distance from the river mouth. Below the river plume, e changed with depth to 10-8 W/kg, and increased to 10-6 W/kg at the layer where the Taiwan Warm Current (TWC) intruded. Thus, e in the near-field plume showed three layers:surface layer in the river plume, middle layer, and lower TWC layer. In the river plume, the strongest e and turbulent diffusivity (Kz) were greater than 10-4 W/kg and 10-2 m2/s, respectively, during strong ebb tides. A three-orders-of-magnitude change in e and Kz was observed in the tidal cycle. The depth of the halocline changed with tidal cycles, and stratification (N2) varied by one order of magnitude. Stratification in the TWC layer followed the distribution of the halocline, which is opposite to the dissipation structure. Tidal currents led to intrusion and turbulent mixing in the TWC layer. During ebb tides, e and Kz were as strong as those measured in the river plume, but did not last as long. The structure of the velocity shear was similar to the dissipation rate in both the river plume and TWC layer, whereas the velocity shear in the TWC layer did not match the stratification structure. In the high dissipation rate area, the gradient Richardson number was smaller than the critical value (Rig < 1/4). The Rig structure was consistent with shear and dissipation distributions, indicating that turbulent mixing in the near-field plume was controlled by a combination of shear induced by the discharged river flow and tidal current.