First High-Resolution Observations of Coexisting Dissipative Oscillations and Kelvin–Helmholtz Billows in Internal Solitary Waves

Abstract

Shoaling internal solitary waves (ISWs) can rapidly transfer substantial energy to turbulence, yet the small-scale processes governing this transition are rarely observed directly. This study presents high-resolution shipboard-mooring measurements on the continental slope of the South China Sea. Numerous oscillations were observed at the trailing edge of the ISWs, with amplitudes up to 15 m, wavelengths up to 150 m, and depth-varying frequencies approaching the local buoyancy frequency. These oscillations are generated by enhanced dissipation during ISW shoaling, consistent with dynamics described by the KdV–Burgers equation. Distinct Kelvin–Helmholtz (KH) billows were resolved at both the trough and trailing edge of the ISWs. KH billows at the trough produce temperature inversions of 1°C, whereas those at the trailing edge are more energetic, with horizontal and vertical scales of up to 140 and 20 m, respectively, temperature inversions of up to 3°C, and turbulent kinetic energy dissipation and mixing rates reaching ${10}^{-2.5}W{\text{kg}}^{-1}$ and ${10}^{0.5}{m}^2{s}^{-1}$, respectively. Dissipative oscillations at the ISW trailing edge trigger shear instability through water-layer compression, a new mechanism coexisting with the classical ISW-core shear instability. These results reveal a dual-instability framework within shoaling ISWs, clarify the role of dissipative oscillations in promoting turbulence and mixing, and provide observational constraints for improving parameterizations of wave-driven mixing in the ocean.