Title: Atmospheric thermal tides and planetary spin I. The complex interplay between stratification and rotation,
Abstract: Thermal atmospheric tides can torque telluric planets away from spin-orbit
synchronous rotation, as observed in the case of Venus. They thus participate
to determine the possible climates and general circulations of the atmospheres
of these planets. In this work, we write the equations governing the dynamics
of thermal tides in a local vertically-stratified section of a rotating
planetary atmosphere by taking into account the effects of the complete
Coriolis acceleration on tidal waves. This allows us to derive analytically the
tidal torque and the tidally dissipated energy, which we use to discuss the
possible regimes of tidal dissipation and examine the key role played by
stratification.
In agreement with early studies, we find that the frequency dependence of the
thermal atmospheric tidal torque in the vicinity of synchronization can be
approximated by a Maxwell model. This behaviour corresponds to weakly stably
stratified or convective fluid layers, as observed in ADLM2016a. A strong
stable stratification allows gravity waves to propagate, which makes the tidal
torque become negligible. The transition is continuous between these two
regimes. The traditional approximation appears to be valid in thin atmospheres
and in regimes where the rotation frequency is dominated by the forcing or the
buoyancy frequencies.
Depending on the stability of their atmospheres with respect to convection,
observed exoplanets can be tidally driven toward synchronous or asynchronous
final rotation rates. The domain of applicability of the traditional
approximation is rigorously constrained by calculations.