# Atmospheric bistability and abrupt transitions to superrotation: wave-jet resonance and Hadley cell feedbacks

## TL;DR

This paper investigates the mechanisms behind abrupt transitions to superrotation in planetary atmospheres, identifying wave-jet resonance as a robust bistability mechanism and analyzing the conditions for such transitions.

## Contribution

It introduces a simple model to distinguish between Hadley cell and wave-jet resonance feedbacks, demonstrating the robustness of wave-jet resonance in causing bistability.

## Key findings

- Wave-jet resonance leads to robust atmospheric bistability.
- Hadley cell feedback causes more fragile bistability.
- Numerical simulations confirm the wave-jet resonance mechanism.

## Abstract

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.