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. Plain Language Summary: Superrotation has been observed in planetary atmospheres and its dynamics have been investigated in previous studies. However, Venus/Titan‐type superrotation driven by stratospheric heating is not fully understood. The Venus/Titan‐type superrotation and intermittency of the dynamical process are elucidated using an idealized general circulation model with large Rossby numbers (Ro = 7.5 to 23) defined by the ratio of inertial force to Coriolis force. This work confirms that (i) the results are equivalent for the same Rossby number regardless of whether due to planetary rotation or size, and (ii) the superrotation mechanism caused by the meridional circulation and equatorward eddy momentum flux is a fundamental process commonly seen in the high Ro regime. Based on the sensitivity experiments with the planetary rotation or size altered, we discuss how the superrotation is maintained and compile its dynamics using the Rossby number. The superrotation is developed efficiently by weak, intermittent equatorward eddy momentum flux on a small‐sized or slowly rotating planet with a high Rossby number. In contrast, although zonal jets and continuous eddy momentum fluxes intensify on a relatively large‐sized or fast‐rotating planet with a relatively low Rossby number, the equatorial superrotation is not efficiently developed. Key Points: Rossby number dependence of Venus/Titan‐type superrotation and its related intermittency is elucidated using an idealized general circulation modelSuperrotation is developed efficiently by weak, intermittent equatorward eddy momentum flux for high Rossby numberEquatorial superrotation is not efficiently developed for low Rossby number, though zonal jets intensify at high latitudes [ABSTRACT FROM AUTHOR]