Atmospheric superrotation in an idealized GCM: Parameter dependence of the eddy response

Idealized Earth-like general circulation models (GCMs) have been extensively used to study superrotation on so-called “slowly rotating” bodies like Venus and Titan, however they tend to have difficulty producing superrotation if only the rotation rate is reduced to Titan- or Venus-like values. The Rossby number, RO = U/2ΩL, which characterizes the influence of rotation on the circulation, is small for Earth but large for both Venus and Titan. However, the differences in other nondimensional control parameters are often ignored in idealized planetary circulation studies. In this study we use a simplified Earth-like GCM to demonstrate the importance of the other nondimensional parameters in obtaining a superrotating flow, and identify the wave-modes responsible for generating and maintaining superrotation. We show that superrotation only emerges on a planet of slow rotation rate if the atmospheric thermal inertia is simultaneously increased; alternatively, superrotation is obtained if the only planetary radius is reduced. When only the rotation rate is reduced, a nearly axisymmetric circulation with intense Hadley cells is produced that prevents strong and persistent winds over the equator. The mechanism for generating and maintaining superrotation in the model involves a coupling between equatorial and high-latitude waves. However, the generation involves equatorial Kelvin-like waves and maintenance involves equatorial Rossby-like waves.