Generation of Equatorial Superrotation and Multiple Jets in Giant Planet Atmospheres

In rapidly rotating planetary atmospheres that are heated from below, equatorial superrotation can occur through convective generation of equatorial Rossby waves. Off-equatorial multiple jets can occur through the baroclinic instability induced by the differential solar heating.

We use simulations with an idealized general circulation model (GCM) to explore systematically the relative roles of baroclinicity, heating from below, and bottom drag. Equatorial superrotation generally occurs when the heating from below is sufficiently strong. However, the threshold heating at which the transition to superrotation occurs increases as the baroclinicity or the bottom drag increases. The greater the baroclinicity, the stronger the angular momentum transport out of low latitudes by baroclinic eddies of extratropical origin. This competes with the angular momentum transport toward the equator by convectively generated Rossby waves and thus can inhibit a transition to superrotation. Equatorial bottom drag damps both the mean zonal flow and convectively generated Rossby waves, weakening the equatorward angular momentum transport as the drag increases; this can also inhibit a transition to superrotation. The strength of superrotating equatorial jets scales approximately with the square of their width. When they are sufficiently strong, their width, in turn, scales with the equatorial Rossby radius and thus depends on the thermal stratification of the equatorial atmosphere.

Away from the equator, the strengthening intrinsic heat flux decreases the tropospheric static stability, which results in baroclinic eddy scales and off-equatorial jet widths that generally decrease with the intrinsic heat flux. The enhanced baroclinicity leads to stronger prograde jets because meridional temperature gradients increase with reduced efficiency of meridional heat transport by baroclinic eddies. Additionally, off-equatorial jets become weaker and narrower as the drag coefficient is increased.

The results have broad implications for planetary atmospheres, particularly for how superrotation can be generated in giant planet atmospheres and in terrestrial atmospheres in warm climates.