Abstract/Summary

Equatorial superrotation under zonally symmetric thermal forcing is investigated in a set-up close to that of the classic Held and Suarez set-up. In contrast to the behaviour in the classic set-up, a transition to equatorial superrotation occurs when the equator-to-pole surface equilibrium entropy gradient is weakened. Two factors contribute to this transition: (i) the reduction of breaking Rossby waves from the midlatitudes that decelerate the equatorial flow and (ii) the presence of barotropic instability in the equatorial region, providing stirring to accelerate the equatorial flow. In the latter, Kelvin waves excited by instability near the Equator generate and maintain the superrotation. However, the superrotation is unphysically enhanced if simulations are underresolved and/or overdissipated.