Turbulent mixing by breaking gravity waves

The characteristics of turbulence caused by three-dimensional breaking of i nternal gravity waves beneath a critical level are investigated by means of high-resolution numerical simulations. The flow evolves in three stages. I n the first one the flow is two-dimensional: internal gravity waves propaga te vertically upwards and create a convectively unstable region beneath the critical level. Convective instability leads to turbulent breakdown in the second stage. The developing three-dimensional mixed region is organized i nto shear-driven overturning rolls in the plane of wave propagation and int o counter-rotating streamwise vortices in the spanwise plane. The productio n of turbulent kinetic energy by shear is maximum. In the last stage, shear production and mechanical dissipation of turbulent kinetic energy balance. The evolution of the flow depends on topographic parameters (wavelength and amplitude), on shear and stratification as well as on viscosity. Here, onl y the implications of the viscosity for the instability structure and evolu tion in terms of the Reynolds number are considered. Smaller viscosity lead s to earlier onset of convective instability and overturning waves. However , viscosity retards the onset of smaller-scale three-dimensional instabilit ies and leads to a reduced momentum transfer to the mean flow below the cri tical level. Hence, the formation of secondary overturning rolls is sustain ed by lower viscosity. The budgets of total kinetic and potential energies are calculated. Althoug h the domain-averaged turbulent kinetic energy is less than 1% of the total kinetic energy, it is strong enough to form a patchy and intermittent turb ulent mixed layer below the critical level.