Microstructure observations of three-layered mixing on the shelf of the northwestern South China Sea

Abstract

This study presents microstructure observations on the northwestern South China Sea (nwSCS) shelf, where energetic turbulence drove a high dissipation rate on the order of 10−5 W kg−1 in May 2021. Together with the hydrography and current observations, the results show a typical three-layered structure of dissipation on the shelf and reveal different mixing regimes for different layers. Below the mixed layer, mixing over the shallow shelf displays periodically varying characteristics, indicating low-mode internal tides. Strong mixing occurred in the stably stratified layer, reaching an approximate magnitude of 1 × 10−5 W kg−1. Our analysis suggests that the strong mixing in this layer is attributed to the shear behavior of the low-mode internal waves because the observed dissipation rate increases with increasing vertical shear and stratification as described by the MacKinnon and Gregg parameterization. In the bottom boundary layer, mixing by tidal shear is dependent on the Richardson number and trends with velocity and vertical shear. In addition, our analysis also reveals that mixing continuously contributed to downward heat transport to the deep layers. This study can improve our understanding of the vertical structure of fine-scale mixing and its regimes, and their effects on the shelf of the nwSCS.