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

• Published in: Astronomy and astrophysics (Berlin) 2023-05, Vol.673, p.A6
• Main Authors: Dandoy, V., Park, J., Augustson, K., Astoul, A., Mathis, S.
• Format: Article
• Language: English
• Subjects: Acceleration; Angular momentum; Asymptotic methods; Context; Convection; Convective flow; Coriolis force; Dissipation; Eddy viscosity; Far fields; Fluid flow; Mathematical models; Perturbation; Planetary evolution; Planetary rotation; Planets; Robustness (mathematics); Rotating bodies; Simulation; Stars; Stellar rotation; Turbulent flow; Viscosity; Vortices; Wavelengths
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202243586

Context. 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 and planet–moon systems. This dissipation is particularly efficient for young low-mass stars and gaseous giant planets, which are rapid rotators. 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 commonly modeled as an effective eddy viscosity. This approach may be valid when the characteristic length scales of convective eddies are smaller than those of the tidal waves. However, it becomes highly questionable in the case where tidal waves interact with potentially stable large-scale vortices such as those observed at the poles of Jupiter and Saturn. The large-scale vortices 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. Aims. We investigate the complex interactions between a tidal inertial wave and a columnar convective vortex. Methods. We used a quasi-geostrophic semi-analytical model of a convective columnar vortex, which is validated by numerical simulations. First, we carried out linear stability analysis using both numerical and asymptotic Wentzel–Kramers–Brillouin–Jeffreys (WKBJ) methods. We then conducted linear numerical simulations of the interactions between a convective columnar vortex and an incoming tidal inertial wave. Results. The vortex we consider is found to be centrifugally stable in the range –Ω p ≤ Ω 0 ≤ 3.62Ω p and unstable outside this range, where Ω 0 is the local rotation rate of the vortex at its center and Ω p is the global planetary (stellar) rotation rate. From the linear stability analysis, we find that this vortex is prone to centrifugal instability with perturbations with azimuthal wavenumbers m = {0,1, 2}, which potentially correspond to eccentricity, obliquity, and asynchronous tides, respectively. The modes with m > 2 are found to be neutral or stable. The WKBJ analysis provides analytic expressions of the dispersion relations for neutral and unstable modes when the axial (vertical) wavenumber is sufficiently large. We verify that in the unstable regime, an incoming tidal inertial wave triggers the growth of the most unstable mode of the vorte