Spectral Features of Internal Waves in the South China Sea

Abstract The close causal link between internal waves (IWs) and IW-driven mixing highlights the importance of investigating IW spectra, especially since IW spectral features vary significantly under different dynamics. In this study, we comprehensively examine the three-dimensional structures of IW spectral levels and their associated features in the South China Sea (SCS), using CTD (Conductivity-Temperature-Depth) and LADCP (Lowered Acoustic Doppler Current Profiler) measurements collected simultaneously. We find that the Luzon Strait has higher shear and strain spectral levels compared to the central SCS, but the opposite is true for the shear-to-strain ratio ( R ω ). The shear spectral level shows a more significant increasing trend with depth than the strain spectral level. This results in elevated R ω values in the deep SCS, indicating a substantial presence of near-inertial IWs (NIWs) there, which are always accompanied by strain spectra featuring a pronounced high-wavenumber peak and flatter displacement spectra. The analysis of a publicly available numerical simulation output further reveals two main regions of abundant deep-ocean NIWs in the SCS, namely the region between 11°N and 15°N and that around the Xisha Islands, mainly due to the parametric subharmonic instability of diurnal internal tides, wave-eddy interaction, and the breaking and dissipation of internal lee waves. Moreover, we obtain a relationship between R ω and the slope of displacement spectrum ( q ξ ), R ω = 10 0.83 q ξ +3.13 , which offers novel insights for improving finescale parameterization based solely on strain. These results serve as an inspiration to explicitly link the IW behavior to the finescale parameterization in different regions globally.