Wave propagation and tidal dissipation in giant planets containing regions of stable stratification

Tidal interactions impact the long-term evolution of orbits and spins of planet-satellite systems. Observations of these orbits and spins for giant planets in our solar system suggest efficient tidal dissipation. We also know that tidal dissipation is strongly influenced by the internal structure of a planet. This, coupled with evidence for stable stratification or semi-convective layers within giant planets, motivates considering the effect of stratification on tidal interactions.

In this thesis we analyse how stable stratification within giant planets can alter tidal dissipation rates through excitation and subsequent dissipation of internal and inertial waves. We combine a mixture of analytical calculations with numerical results in a global spherical Boussinesq model, both with and without rotation. We analyse the free modes and transmission of waves through layered staircase structures, as well as studying the dissipation rates of tidally forced systems where stably stratified layers form.

We find that a staircase density structure can alter the free modes and the transmission of waves through such a medium. We find our results tend towards the behaviour of a continuously stratified medium as the number of steps in the staircase increases, and that the transmission of short wavelength waves through a staircase is only efficient when they are resonant with the free modes.

Enhanced tidal dissipation arises when the tidal forcing frequency is close to a resonance of the system. By varying parameters, we find overarching trends in dissipation rates. In particular, that an extended core can enhance the inertial wave response, but such a response is not strongly altered by the properties of the core itself. Therefore, we find that as well as introducing additional internal gravity and gravito-inertial wave resonances into the system, stable stratification can enhance the inertial wave response, all of which can contribute to the resultant tidal dissipation.