Evolution and breaking of a propagating internal wave in stratified ocean

The processes of evolution and breaking of a propagating internal wave are directly numerically simulated using the pseudo-spectral method. The mechanism of PSI (parametric subharmonic instability) involved in the evolution is testified clearly, which dominates gradually in nonlinear resonant interactions. As a consequence, the energy cascades to a second plant wave packet with low frequencies and higher wavenumbers with respect to the primary wave. With the growth of this wave packet, wave breaking occurs and causes strong nonlinear regime, i.e. stratified turbulence. The strong mixing and intermittent of the turbulence can be learned from the evolution of the total energy and kurtosis of vorticity vs. time. Some statistic properties of the stratified turbulence are analyzed, including the spectra of KE (kinetic energy) and APE (effective potential energy) along vertical wavenumber k(y). The results show that these spectra display a wavenumber range scaling as 0.1 N-4 k(y)(-3) and 0.2 N-4 k(y)(-3), respectively (N is the Brunt-Vaisala frequency), which is called buoyancy sub-range. The Cox number of diapycnal diffusivity is also calculated, and it shows a good consistency with the observations and deductions in ocean interiors during the stage of the stratified turbulence maintaining a fairly active level.