Our observations of internal solitary-like waves (ISWs) on the northeastern shelf of the South China Sea suggest the presence of shear instability and overturns in the rear of the wave in the pycnocline. The observations indicate that shear instabilities occur when i) , where  is the horizontal length of the unstable region with  and  is the wavelength at half amplitude; ii) , where  is the area of the unstable region inside the wave (above the isopycnal of 1022 ) and  is the total area inside the wave. Most of the waves have a minimum  well below 0.1 in the pycnocline. In addition, over 80% of our observed trailing high-frequency internal waves (HIWs) are unstable. This reveals the important role of fission in ISW dissipation in the pycnocline. In other words, ISW fission generates dynamically unstable HIWs. The dissipation in the bottom boundary layer (BBL) is estimated form bottom drag. It indicates that the contribution of ISWs and trailing HIWs to dissipation in the BBL depends on the local coupling between wave-induced velocity and low frequency currents.

        The observations reveal a case of turbulence interaction: mixing due to previous turbulence creates weakly stratified layers in the pycnocline. This can modulate turbulence generation through shear instabilities in the wave. This case appears both in the wave trains and under/above the trough of ISWs. Numerical simulations using fully nonlinear DJL model are performed. It is revealed that a weakly stratified layer could trigger shear instabilities in the wave in the presence of background current, which modulates wave-induced shear. Neither in the absence of background current nor the background shear alone could trigger shear instabilities in the weakly stratified layer.