  Strong eastward jets at the equator have been observed in many planetary
atmospheres and simulated in numerical models of varying complexity. However,
the nature of the transition from a conventional state of the general
circulation, with easterlies or weak westerlies in the tropics, to such a
superrotating state remains unclear. Is it abrupt or continuous? This question
may have far-reaching consequences, as it may provide a mechanism for abrupt
climate change in a planetary atmosphere, both through the loss of stability of
the conventional circulation and through potential noise-induced transitions in
the bistability range. We study two feedbacks which may lead to bistability
between a conventional and a superrotating state: the Hadley cell feedback and
a wave-jet resonance feedback. We delineate the regime of applicability of
these two mechanisms in a simple model of zonal acceleration budget at the
equator. Then, we show using numerical simulations of the axisymmetric
primitive equations that the wave-jet resonance feedback indeed leads to robust
bistability, while the bistability governed by the Hadley cell feedback,
although observed in our numerical simulations, is much more fragile in a
multilevel model.
