Superrotation in Terrestrial Atmospheres

Although a number of terrestrial planetary atmospheres exhibit equatorial superrotation, such as Venus and Titan, the causes of superrotation in these atmospheres are not well understood. It is known that, in order to converge momentum into the equatorial region, there must be wave activity generation near the equator and dissipation away from the equator. We have performed a systematic study using an atmospheric General Circulation Model (GCM) to assess how the generation of superrotation depends on three parameters: the rotation rate of the planet, the pole to equator surface temperature gradient, and the convective lapse rate. We expected these parameters to be important in creating Rossby waves sources near the equator (by convection) and away from the equator (by baroclinic instability). The different wave activity sources compete in their effect on equatorial eddy momentum flux convergence/divergence. The results show that, for certain ranges of these parameters, equatorial Rossby wave sources dominate and cause the atmosphere to superrotate. Based on scaling arguments for the eddy momentum flux convergence in midlatitudes and for the wave activity generation in the equatorial region, we were able to define a number that diagnoses whether or not a given terrestrial atmosphere will have equatorial superrotation (Sr ≥ 1) or subrotation (Sr < 1).