Planetary ageostrophic instability leads to superrotation

Dry GCM simulations with hemispherically and zonally symmetric forcing spontaneously transition to superrotation provided (a) the Hadley cell is not too strong and (b) baroclinic instability is suppressed. The former condition is met with sufficiently weak thermal forcing and the latter condition is met in high-Rossby-number (Ro) regimes. The mechanism giving rise to spontaneous superrotation involves a new, global instability that couples an equatorial Kelvin wave to mid- or high-latitude Rossby waves in the presence of a substantial meridional shear; we refer to this as ‘planetary ageostrophic instability'. We demonstrate planetary ageostrophic instability using a new, global primitive equation eigenvalue solver, and further show that intermediate values of a ‘bulk Froude number', Fr, a measure of the meridional Doppler shift relative to equatorial gravity wave phase speeds, are required for the instability. The values of Ro and Fr in the atmospheres of Earth, Mars and Titan are consistent with the existence of superrotation only on Titan.