STRATIFIED TURBULENCE PRODUCED BY INTERNAL WAVE BREAKING - 2-DIMENSIONAL NUMERICAL EXPERIMENTS

In an earlier study (Bouruet-Aubertot et al., J. Fluid Mech., 285: 265 -301, 1995) the evolution of a large-scale, high-frequency, standing i nternal gravity wave was investigated by means of two-dimensional nume rical computations. It was shown that breaking eventually occurs, what ever the wave amplitude. The instability mechanisms leading to breakin g were analysed in detail. In the present paper, we study the strongly nonlinear regime resulting from wave breaking. Buoyancy and velocity variance spectra display a wavenumber range scaling as 0.2N(2)k(y)(-3) and 0.1N(2)k(y)(-3), respectively (N being the Brunt-Vaisala frequenc y and ky the vertical wavenumber). The buoyancy variance spectrum has the same power law and level as in a laboratory experiment performed o n the same subject by Benielli and Sommeria (Dyn. Atmos. Oceans, 23: 3 35-343, 1996), and is very close to fine-scale oceanic temperature spe ctra, The buoyancy flux spectrum appears to be counter-gradient at sma ll scales, confirming results from previously published numerical simu lations. We propose a physical interpretation for this small-scale beh aviour. Finally, the spectrum of the buoyancy flux is compared with th e kinetic energy and buoyancy variance spectra, to quantify its role i n energy transfers. This point is essential for deciding between the t heories proposed to explain the k(-3) spectral range, which is also ob served in the stratosphere.