Planetary ageostrophic instability leads to superrotation

We demonstrate the existence of a global-scale, linear instability in the atmospheres of slowly rotating and/or small planets that spontaneously emerges and produces momentum convergence at the equator, thus supporting the development of planetary superrotation. We identify the instability as being barotropic, ageostrophic in nature, coupling an equatorial Kelvin wave with midlatitude or high-latitude Rossby waves. This coupling requires a frequency matching of the Doppler-shifted wave components and moderate spatial overlap between them, which are determined by two nondimensional parameters, the Rossby and Froude numbers. By diagnosing these parameters, we find that this instability is an essential and necessary process to obtain superrotation in dry atmospheric, general circulation models with axisymmetric forcing. The Rossby and Froude numbers for Solar System bodies are consistent with the presence or absence of superrotation, which suggests that they provide useful diagnostics for predicting the emergence of superrotation in the atmospheres of terrestrial planets.