Rossby Number Dependence of Venus/Titan-Type Superrotation and Its Related Intermittency

Venus/Titan-type superrotation driven by stratospheric heating and intermittency seen in the superrotation dynamics are investigated using an idealized general circulation model in a high Rossby number regime (Ro = 7.5 to 23 for the strongest zonal jet) where the superrotation is formed by the meridional circulation and equatorward eddy momentum flux. When the jet core has a Rossby number of ∼23 on a slowly rotating or small planet, fast planetary-scale Rossby waves are transiently amplified by both barotropic and baroclinic energy conversion and intermittently produce equatorward eddy momentum fluxes. At high latitudes, poleward eddy momentum transport also occurs when the poleward heat flux of baroclinic eddies is strong. On such a slowly rotating or small-sized planet, equatorial superrotation is developed efficiently by weak, intermittent equatorward momentum flux in the presence of polar indirect circulation and the speed of the zonal flow is roughly constant over the low- and mid-latitudes. In contrast, on a relatively fast rotating or large-sized planet when the Rossby number is ∼7.5 for the jet core, although the zonal jets and equatorward eddy momentum fluxes intensify, equatorial superrotation is not developed efficiently (i.e., the superrotation intensity and its equatorial efficiency are small). Strong equatorward eddy momentum fluxes are produced continuously by slow barotropic Rossby waves on the equatorward flanks of the jets developed by the strong meridional circulation. The poleward heat fluxes of baroclinic waves are negligible because it is much smaller than the heat flux of the zonal-mean meridional circulation.