Multistability and rare spontaneous transitions in barotropic $\beta$-plane turbulence

TL;DR

This paper reveals that turbulent planetary atmospheres can have multiple metastable states with rare transitions, studied through large deviation theory and instantons in a barotropic beta-plane turbulence model.

Contribution

It introduces a statistical mechanics framework using rare event algorithms to analyze metastability and spontaneous transitions in barotropic turbulence, uncovering complex dynamics and multiple attractors.

Findings

Jets spontaneously nucleate from westward jets.

Mean transition time follows an Arrhenius law with exponential decrease as dissipation lessens.

Rich dynamics with multiple instanton pathways governed by permutation symmetries.

Abstract

We demonstrate that turbulent zonal jets, analogous to Jovian ones, which are quasi-stationary, are actually metastable. After extremely long times, they randomly switch to new configurations with a different number of jets. The genericity of this phenomenon suggests that most quasi-stationary turbulent planetary atmospheres might have many climates and attractors for fixed values of the external forcing parameters. A key message is that this situation will usually not be detected by simply running the numerical models, because of the extremely long mean transition time to change from one climate to another. In order to study such phenomena, we need to use specific tools: rare event algorithms and large deviation theory. With these tools, we make a full statistical mechanics study of a classical barotropic beta-plane quasigeostrophic model. It exhibits robust bimodality with abrupt transitions. We show that new jets spontaneously nucleate from westward jets. The numerically computed mean transition time is consistent with an Arrhenius law showing an exponential decrease of the probability as the Ekman dissipation decreases. This phenomenology is controlled by rare noise-driven paths called {\it instantons}. Moreover, we compute the saddles of the corresponding effective dynamics. For the dynamics of states with three alternating jets, we uncover an unexpectedly rich dynamics governed by the symmetric group ${\cal S}_3$ of permutations, with two distinct families of instantons, which is a surprise for a system where everything seemed stationary in the hundreds of previous simulations of this model. We discuss the future generalization of our approach to more realistic models.