Abrupt transition to strong superrotation driven by equatorial wave resonance in an idealized GCM

Atmospheric superrotation is seen on other terrestrial planets (Venus, Titan) but not the present Earth, which is distinguished by equatorial easterlies. Nevertheless, superrotation has appeared in numerical simulations of Earth's atmosphere, from two-layer models to multi-level comprehensive GCMs. In many two-layer models the transition from present climate to superrotation is abrupt, and accompanied by hysteresis. This nonlinear behavior was hypothesized to result from a positive feedback involving transient eddies generated in midlatitudes, although it has been suggested that this feedback is an artifact of the two-layer approximation. Here we present simulations with an idealized multi-level GCM forced with an asymmetric equatorial heating, which show abrupt transitions from classical to strongly superrotating states. We use linear shallow water theory to show that this transition is driven by a resonance in which free equatorial wave modes are excited as the superrotating jet velocity approaches their phase speed. The resonance and bifurcation are most prominent in simulations where the meridional temperature gradient has been reduced, but hysteresis behavior is seen only when the gradient is eliminated completely. Small feedbacks associated with midlatitude transients are also seen, but they serve only to modulate the bifurcation.