The effects of nonlinearities on tidal flows in the convective envelopes of rotating stars and planets in exoplanetary systems

TL;DR

This study uses 3D nonlinear hydrodynamical simulations to explore how nonlinear effects influence tidal inertial waves, energy redistribution, and dissipation in the convective envelopes of rotating stars and planets, revealing significant deviations from linear predictions.

Contribution

It introduces a realistic tidal forcing framework and demonstrates the impact of nonlinearities, such as zonal flows and instabilities, on tidal dissipation rates and wave dynamics in stellar and planetary convective zones.

Findings

Strong zonal flows modify tidal dissipation rates.

Nonlinear effects explain discrepancies with linear theory.

Corotation resonances and parametric instabilities significantly influence tidal response.

Abstract

In close exoplanetary systems, tidal interactions drive orbital and spin evolution of planets and stars over long timescales. Tidally-forced inertial waves (restored by the Coriolis acceleration) in the convective envelopes of low-mass stars and giant gaseous planets contribute greatly to the tidal dissipation when they are excited and subsequently damped (e.g. through viscous friction), especially early in the life of a system. These waves are known to be subject to nonlinear effects, including triggering differential rotation in the form of zonal flows. In this study, we use a realistic tidal body forcing to excite inertial waves through the residual action of the equilibrium tide in the momentum equation for the waves. By performing 3D nonlinear hydrodynamical simulations in adiabatic and incompressible convective shells, we investigate how the addition of nonlinear terms affects the tidal flow properties, and the energy and angular momentum redistribution. In particular, we identify and justify the removal of terms responsible for unphysical angular momentum evolution observed in a previous numerical study. Within our new set-up, we observe the establishment of strong cylindrically-sheared zonal flows, which modify the tidal dissipation rates from prior linear theoretical predictions. We demonstrate that the effects of this differential rotation on the waves neatly explains the discrepancies between linear and nonlinear dissipation rates in many of our simulations. We also highlight the major role of both corotation resonances and parametric instabilities of inertial waves, which are observed for sufficiently high tidal forcing amplitudes or low viscosities, in affecting the tidal flow response.