Instabilities and Turbulence Observed within Large Internal Solitary Waves in the South China Sea

Instabilities and turbulence were observed within the shoaling internal solitary waves (ISWs) over the continental slope east of Dongsha Atoll in the South China Sea (SCS) using the combination of a shipboard echo sounder, towed turbulence profiler, and moored high-resolution temperature chain. Our observations show clear evidence of ISWs with convective and shear instabilities. It is found that 13 of the 15 observed ISWs, with amplitude ~100–150 m and period 10–15 minutes, reach the condition of convective breaking, i.e., the maximum particle velocities within the ISWs exceed their propagating speeds. The clearest example of convective breaking in an ISW shows warm water near the surface plunging down in the front part of the wave and then rolling up near the trough, with a core of well-mixed water developing at the rear. Kelvin-Helmholtz (KH) billows with amplitude ~10–20 m and period 0.5–2 minutes were found along the front edge and the roll-up of the plunging water. These billows were frequently observed in the other ISWs where the gradient Richardson number was less than ~0.1. Both the convective and shear instabilities significantly enhanced the turbulence within the ISWs, leading to a TKE dissipation rate of O(10^-4 WKg^-1) and a turbulent diffusivity of 0.1 m²s^-1.