Energy Cascade from Internal Solitary Waves to Turbulence via Near-N Waves in the Northern South China Sea

Abstract

Abstract The high-resolution mooring observations reported here reveal a cascade process from internal solitary waves (ISWs) to turbulent mixing via high-frequency internal waves near the maximum local buoyancy frequency (near-N waves) in the deep water of the northern South China Sea (SCS). Riding on the parent ISW, near-N waves with a peak frequency of 20 cph emerged at the trough of the ISW and extended to the rear face of the ISW. Most of the near-N waves occurred around the thermocline, where the isothermal displacements induced by the near-N waves were largest with an amplitude of 12 m. The energy of near-N waves was 5% of that of the parent ISW, and instability investigations showed that due to the strong shear, Ri in the region of strong near-N waves was less than 1/4, suggesting that the near-N waves were unstable and might dissipate rapidly. Simulations based on the Korteweg–de Vries (KdV)–Burgers equation reproduced the formation of observed near-N waves due to the energy cascade from ISWs. Our observational results demonstrate a new energy cascade route from ISWs to turbulence in the deep water, deepening the understanding of the energy dissipation process of ISWs and their roles in the enhanced mixing in the northern SCS.