Mathematics Colloquia and Seminars

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Equatorial superrotation, where atmospheric angular momentum exceeds solid-body rotation, is a relatively common feature of Solar System planets.  The phenomenon has received attention from the planetary community for some time (e.g. Gierasch 75; Rossow and Williams ’83; del Genio et al. ’93,’96; Hourdin et al. ’95; Lebonnois et al. ’10).  Idealized models of Earth’s atmosphere reveal the existence of an alternative climatology with persistent equatorial superrotation (Suarez & Duffy ’92; Saravanan ’93; Shell & Held ’03).  In addition, paleoclimate simulations of a “hot house” spontaneously transition to a superrotating climatology (Caballero & Huber ’10).  These phenomena motivate a set of idealized numerical experiments with a general circulation model (GCM) in which a single parameter, the thermal Rossby number (Ro), is varied.  This clean approach allows us to identify a new, unstable wave mode with gravity-wave character at the equator and Rossby-wave character at high latitudes (Mitchell & Vallis ’10).  This mode is essential to the development and maintenance of superrotation.  I will discuss the prospects for observing the mode in the atmospheres of Titan and Venus, make comparisons with tropical modes in Earth's atmosphere, and speculate on the nature of the instability.