  We study how stably stratified or semi-convective layers alter tidal
dissipation rates associated with the generation of inertial, gravito-inertial,
interfacial and surface gravity waves in rotating giant planets. We explore
scenarios in which stable (non-convective) layers contribute to the high rates
of tidal dissipation observed for Jupiter and Saturn in our solar system. Our
model is an idealised spherical Boussinesq system incorporating Coriolis forces
to study effects of stable stratification and semi-convective layers on tidal
dissipation. Our detailed numerical calculations consider realistic tidal
forcing and compute the resulting viscous and thermal dissipation rates. The
presence of an extended stably stratified fluid core significantly enhances
tidal wave excitation of both inertial waves (due to rotation) in the
convective envelope and gravito-inertial waves in the dilute core. We show that
a sufficiently strongly stratified fluid core enhances inertial wave
dissipation in a convective envelope much like a solid core does. We
demonstrate that efficient tidal dissipation rates (and associated tidal
quality factors $Q'$) -- sufficient to explain the observed migration rates of
Saturn's moons -- are predicted at the frequencies of the orbiting moons due to
the excitation of inertial or gravito-inertial waves in our models with stable
layers (without requiring resonance-locking). Stable layers could also be
important for tidal evolution of hot and warm Jupiters, and hot Neptunes,
providing efficient tidal circularisation rates. Future work should study more
sophisticated planetary models that also account for magnetism and differential
rotation, as well as the interaction of inertial waves with turbulent
convection.
