Effects of internal waves on bottom thermal structures and turbulent mixing in the Xisha Islands

Abstract

Using velocity and high-resolution temperature mooring data from the fore-reef slope of Yongxing Island in the northwest South China Sea (water depth of 69 m), we examine the effects of internal waves on the temporal variations in temperature, bottom mixed layer (BML) and turbulent mixing. The diurnal tide is found to be the dominant tidal force and the baroclinic tide is highly active, which would account for the 11 d abnormal spring-neap cycle in the barotropic tidal current. During the ebb period (tidal elevation decreases), the bottom diurnal baroclinic current transports cold water upslope, resulting in a decrease in temperature, and vice versa during the flood period. The BML thickness $H_{bml}$ widely varies around $1.5m$, approximately 2% of the water depth. The bottom turbulent mixing is not so active, indicated by the bulk dissipation $E_{\varepsilon }$ of 10 $\mathrm{mW}{m}^{-2}$ and turbulent diffusivity ${\kappa }_z$ of $2\times 10^{-4}$ $m^2{s}^{-1}$. Both $H_{bml}$ and $E_{\varepsilon }$ approximate a log-normal distribution, demonstrating strong intermittency. The high-frequency $\left(\omega \right)$ internal bores can increase $H_{bml}$ by four times and enhance the turbulent mixing by one order, which should be responsible for the slow cascade of $H_{bml}$ $(\sim {\omega }^{-1.5})$ and $E_{\varepsilon }$ $(\sim {\omega }^{-1.0})$. In the downslope phase, $H_{bml}$ is about 30% thicker, and turbulent mixing is enhanced by 3 times stronger than those in the upslope phase. It is revealed that under the forcing of internal waves, turbulent mixing corresponds to a thick BML with $H_{bml}\sim {〈{\kappa }_z〉}^{0.25}$, and stratification ($N$) has a significantly negative correlation with the development of BML, $H_{bml}\sim {〈N〉}^{-1.8}$.