Equatorial superrotation through linear instabilities in large Rossby number atmospheres

We focus on several types of linear instability that may associate the spin-up, as well as the maintenance of superrotation on large Rossby number (Ro) planets like Titan. By examining instabilities in a plausible range of wind profiles, we will present mechanisms responsible for the convergence of momentum over the equator, which leads to superrotation. Persistent superrotation, which refers to a global atmospheric circulation with a westerly zonal jet over the equator, has been seen on large Ro planets such as Titan and Venus. Some small Ro planets such as Earth, however, exhibit transient superrotation, whose formation may have a different mechanism from that for the large Ro planets. Numerical simulations (Mitchell and Vallis 2010) show that by increasing Ro, a dry atmosphere can pass from Earth-like to superrotating, with weak Rayleigh friction and Newtonian cooling. By performing the linear instability analysis, we can have a better understanding of individual modes that possibly participate the formation of superrotation, their relative importance and possible parameter regime for the occurrence of superrotaion.