Variability of coherent and incoherent features of internal tides in the north South China Sea

Bingtian Li,Juan Li,Xianqing Lv,You Fu,Zexun Wei,Qingjun Fu,Xinyi Wang

doi:10.1038/s41598-020-68359-7

Abstract

The coherent and incoherent features of internal tides (ITs) in the north South China Sea (SCS) are investigated based on observations and numerical simulations. The 11-month (from May 2011 to March 2012) moored current observations indicate that coherent semidiurnal ITs are obviously amplified, which can be attributed to the interference of ITs. Interference enhances coherent motions of semidiurnal ITs, but weakens those of diurnal ITs. Moreover, observations also show that semidiurnal ITs are more incoherent than diurnal ITs. Variations of vertical stratification and surface tide forcing can hardly affect the incoherence of ITs. The increase of incoherent signal is largely due to the influence of mesoscale eddies. Mesoscale eddies affect both amplitude and phase of ITs, making them more incoherent. Mesoscale eddies not only increase the intensity of background currents, but also induce horizontal variations of density. Variations of horizontal density and the influence of background currents lead to the increase of incoherent signals. And semidiural ITs are more sensitive to the influence of mesoscale eddies, making them more incoherent than diurnal counterparts. Incoherent ITs, which induce strong current shear, play essential roles in cascading tidal energy to small-scale motions, and contribute to turbulent mixing eventually. The findings help to better understand ITs and may offer reference for the improvement of parameterization of ocean turbulent mixing in the northern SCS.

Highlights

The results indicate that variability of both coherent diurnal and semidiurnal internal tides (ITs) can be largely explained by the barotropic tidal forcing at the Luzon Strait (LS)
Numerical simulations indicate interference of baroclinic tides weakens the conversion of baroclinic diurnal energy but enhances that of semidiurnal ITs
Whereas resonance of semidiurnal ITs can effectively enhance the conversion of semidiurnal ITs by engendering the density perturbation more in phase with the barotropic tide

Summary

Introduction

In the northern SCS, semidiurnal ITs are more incoherent than diurnal ITs. For diurnal ITs, time-averaged KE of barotropic tide at the LS is 2.0 J/m3, and the corresponding KE of the coherent baroclinic signals at the mooring site is 5.6 J/m3.

Results

Conclusion