How do tidal waves interact with convective vortices in rapidly-rotating planets and stars?

Abstract

The dissipation of tidal inertial waves in planetary and stellar convective regions is one of the key mechanisms that drive the evolution of star-planet/planet-moon systems. In this context, the interaction between tidal inertial waves and turbulent convective flows must be modelled in a realistic and robust way. In the state-of-the-art simulations, the friction applied by convection on tidal waves is modelled most of the time by an effective eddy-viscosity. This approach may be valid when the characteristic length scales of convective eddies are smaller than those of tidal waves. However, it becomes highly questionable in the case where tidal waves interact with potentially stable large-scale vortices, as those observed at the pole of Jupiter and Saturn. They are potentially triggered by convection in rapidly-rotating bodies in which the Coriolis acceleration forms the flow in columnar vortical structures along the direction of the rotation axis. In this paper, we investigate the complex interactions between a tidal inertial wave and a columnar convective vortex. We use a quasi-geostrophic semi-analytical model of a convective columnar vortex. We perform linear stability analysis to identify the unstable regime and conduct linear numerical simulations for the interactions between the convective vortex and an incoming tidal inertial wave. We verify that in the unstable regime, an incoming tidal inertial wave triggers the most unstable mode of the vortex leading to turbulent dissipation. For stable vortices, the wave-vortex interaction leads to the momentum mixing while it creates a low-velocity region around the vortex core and a new wave-like perturbation in the form of a progressive wave radiating in the far field. The emission of this secondary wave is the strongest when the wavelength of the incoming wave is close to the characteristic size of the vortex.