The Transition to Superrotation in Terrestrial Atmospheres

TL;DR

This paper demonstrates that adjusting the thermal Rossby number in axisymmetric global atmospheric simulations can induce a transition from Earth-like to superrotating atmospheres, resembling Venus or Titan, by suppressing Rossby waves.

Contribution

It reveals how a single non-dimensional parameter controls the transition to superrotation in simplified atmospheric models, highlighting the roles of different wave modes and instabilities.

Findings

Superrotation occurs when the deformation radius exceeds the planetary radius.

A global baroclinic wave converges eastward momentum to the equator during spinup.

Superrotation is maintained by a global barotropic mode in steady state.

Abstract

We show that by changing a single non-dimensional number, the thermal Rossby number, global atmospheric simulations with only axisymmetric forcing pass from an Earth-like atmosphere to a superrotating atmosphere that more resembles the atmospheres of Venus or Titan. The transition to superrotation occurs under conditions in which equatorward-propagating Rossby waves generated by baroclinic instability at intermediate and high latitudes are suppressed, which will occur when the deformation radius exceeds the planetary radius. At large thermal Rossby numbers following an initial, nearly axisymmetric phase, a global baroclinic wave of zonal wavenumber one generated by mixed barotropic-baroclinic instability dominates the eddy flux of zonal momentum. The global wave converges eastward zonal momentum to the equator and deposits westward momentum at intermediate latitudes during spinup and before superrotation emerges, and the baroclinic instability ceases once superrotation is established. A global barotropic mode of zonal wavenumber one generated by a mix of high- and low-latitude barotropic instability is responsible for maintaining superrotation in the statistically steady state. At intermediate thermal Rossby numbers, momentum flux by the global baroclinic mode is subdominant relative to smaller baroclinic modes, and thus strong superrotation does not develop.