Internal wave breaking: P.S.I. vs self–acceleration

In continuously stratified fluid, vertically propagating internal gravity waves of moderately large amplitude can become unstable and possibly break due to a variety of mechanisms including (with some overlap) modulational instability, parametric subharmonic instability (PSI), self-acceleration, overturning and convective instability. In PSI, energy from primary waves is transferred, for example, to waves with half frequency. This mechanism has been studied in simulations and analytic theory for plane periodic primary waves and has been studied in experiments for low modes in a box filled with uniformly stratified fluid. Conversely, self-acceleration refers to a mechanism whereby a wavepacket induces a mean flow (analogous to the Stokes drift of surface waves) which itself advects the waves until they become convectively unstable. Although self-acceleration cannot operate for plane waves or for modes in a box, simulations show self-acceleration dominates over PSI if the wavepacket has sufficiently small vertical extent.