  Equatorial superrotation under zonally-symmetric thermal forcing is
investigated in a setup close to that of the classic Held & Suarez (1994)
setup. In contrast to the behaviour in the classic setup, a transition to
equatorial superrotation occurs when the equator-to-pole surface equilibrium
entropy gradient is weakened. Two factors contribute to this transition: 1) the
reduction of breaking Rossby waves from the mid-latitude that decelerate the
equatorial flow and 2) 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 under-resolved and/or over-dissipated.
