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 conductivity–temperature–depth (CTD) and lowered acoustic Doppler current profiler (LADCP) 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° 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 , , 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. Significance Statement Internal wave spectra, which reveal how the internal wave energy is distributed across different spatial and temporal scales, play a crucial role in understanding internal wave behaviors and the resulting mixing they induce. This forms the basis for parameterizing internal wave-driven mixing, which is essential for the vertical transports and redistributions of momentum, energy, nutrients, and dissolved gases. Given that spectral features exhibit significant variations under diverse dynamics, we examine three-dimensional structures of internal wave spectra and their associated features in the South China Sea, where internal waves coexist with various multiscale processes. Our findings reveal two deep-sea regions abundant with near-inertial internal waves. We then proceed to propose a formula for deriving a parameter crucial for this parameterization to be used in mapping global ocean mixing.