Séminaire

Primitive-equation numerical simulations and analysis of reduced models suggest that stimulated generation—the transfer of energy from balanced flows to existing internal waves—is a leading contender for an ocean mesoscale energy sink. Here we study stimulated generation using an asymptotic model that couples barotropic quasi-geostrophic flow and near-inertial waves with the exp(imz) structure on the f-plane. A detailed description of the conservation laws of this vertical plane-wave model illuminates the mechanism of stimulated generation associated with vertical vorticity and lateral strain. In particular, there are two sources of wave potential energy, and corresponding sinks of balanced kinetic energy: (1) the refractive convergence of the wave action density into anticyclones (and divergence from cyclones) and (2) enhancement of wave-field gradients by geostrophic straining.

We quantify the energy conversion and describe the phenomenology of stimulated generation using numerical solutions of decaying ocean macroturbulence modified by near-inertial waves. The initial conditions are a uniform inertial oscillation and a two-dimensional turbulent field emergent from random initial conditions. In all solutions, stimulated generation co-exists with a transfer of balanced kinetic energy to large scales, which is associated with vortex merger. And geostrophic straining accounts for most of the generation of wave potential energy, which represents a sink of 10-20% of the initial balanced kinetic energy. But refraction is fundamental because it creates the initial eddy- scale lateral gradients in the near-inertial field that are then enhanced by advection. In these quasi-inviscid solutions, wave dispersion is the only mechanism that upsets stimulated generation: with barotropic balanced flow, lateral straining enhances the wave group velocity; the waves accelerate and thus rapidly escape from the straining regions. Because of this wave escape, the wave field does not suffer a direct cascade to dissipative scales.

bruno.deremble@ens.fr