Evolution of climate depends upon the intensity of the thermohaline circulation itself controlled by deep ocean mixing.
Internal waves develop in stratified media (e.g. oceans) where the fluid density varies
linearly with depth due to salinity and temperature.
Generated at the bottom of the ocean, they propagate upwards and destabilize, which is through to cause deep ocean mixing.
We study the stability of an internal wave beam through direct numerical simulations and find that the instability differs with the mean advection speed. The pictures above are snapshots of the density variation in a vertical plane for two simulations. When the mean speed is zero (right), the wave develops small scale instability whereas for large mean speed (left), the instability is large scale.
We show that this selection depends on the group velocity of the perturbation, large scale and small scale being part of two differents wave packets. This correspond to an extension of the classical absolute and convective instability selection principe.