Spontaneous generation of inertia-gravity waves in a rotating stratified flow

Inertia-gravity waves exist ubiquitously throughout the stratified parts of the atmosphere and ocean. We report on a laboratory study of the generation of inertia-gravity waves using a rotating annulus. A sophisticated visualization technique allows the flow to be observed at very high resolution in space and time. We find that localized inertia-gravity waves are spontaneously emitted by all baroclinically-unstable flows. This observation is in concordance with the hypothesis of Ford, McIntyre & Norton (2000) that, with few if any exceptions, all unsteady vortical flows will tend to emit freely propagating inertia-gravity waves, implying the non-existence of a strict slow manifold. Furthermore, the spatial locations of the inertia-gravity waves are well predicted by the source term derived by Ford (1994).

We use a quasi-geostrophic numerical model to simulate the large-scale flow, and we investigate the impacts of inertia-gravity waves by including a simple stochastic parameterization. The impact of the parameterization on the large-scale dynamics is generally small, but in a flow which is baroclinically-unstable to multiple zonal wavenumbers with approximately equal growth rates, it can strongly affect wavenumber selection and can trigger spontaneous wavenumber transitions. These effects are unlikely to be captured by the deterministic gravity wave drag parameterizations used in general circulation models.