Dynamics in high-Rossby-number regimes and the transition to superrotation

Equatorial superrotation, where winds exceed solid-body rotation, is a relatively common feature of Solar System planets, although it is not a (persistent) feature of the present-day climate on Earth. The process by which the atmosphere of a terrestrial body achieves and maintains superrotation has received attention from the planetary community for some time. Idealized models of Earth's atmosphere reveal the existence of an alternative climatology with persistent equatorial superrotation. Paleoclimate simulations of a “hot house” spontaneously transition to a superrotating climatology and proxy data may indirectly indicate superrotation was a feature of the Pliocene. These phenomena motivate a set of idealized numerical experiments with a dry GCM in which a single parameter, the thermal Rossby number (Ro), is varied. This clean approach permits the identification of dynamical mechanisms giving rise to and sustaining superrotation in a terrestrial atmosphere (Mitchell & Vallis '10). In the high-Ro regime, we identify and describe a new, global mode of variability with a gravity-wave character at the equator and a Rossby-wave character at high latitudes. This mode is key to the development and maintenance of superrotation. We will discuss the prospects for observing the mode in the atmospheres of Titan and Venus.