Observations of turbulent mixing and vertical diffusive salt flux in the Changjiang Diluted Water

Based on microstructure measurements from a repeated sampling station southwest of the Jeju Island during summer, we studied the hydrography, pycnocline turbulence, and vertical salt flux in the Changjiang Diluted Water (CDW). The water column was well stratified with the CDW occupied the surface similar to 20 m. Most of the large turbulent kinetic energy dissipation rate (epsilon) were found in the bottom boundary layer. Interestingly, intermittent strong turbulence (epsilon>10(-6) W/kg) occurred in the pycnocline, which may induce strong mixing events and increase the vertical diffusive salt flux at the base of CDW by one order of magnitude. The daily-mean vertical diffusive salt flux could reach 4.3 (2.1, 8.9)x10(-6) m/s. Both moored velocity measurements and associated wavelet analysis showed the presence of velocity fluctuations when there was strong pycnocline turbulence. The moderate resolution imaging spectroradiometer (MODIS) satellite images further suggest that the velocity fluctuations are induced by the prevailing internal solitary waves (ISWs) which are mainly generated at the shelf break of the East China Sea or the topographic features surrounding Jeju Island. The calculated gradient Richardson number denote the occurrence of shear instability in the pycnocline when strong turbulence presents. The presented results have strong implications for the importance of ISWs in influencing the vertical diffusion of CDW and changes in other properties.

Automated summary

Based on microstructure measurements from a repeated sampling station southwest of the Jeju Island during summer, this study investigated the hydrography, pycnocline turbulence, and vertical salt flux in the Changjiang Diluted Water (CDW). The findings suggest that pycnocline turbulence plays a significant role in influencing the vertical diffusion and other properties of CDW.