Statistical mechanics of quasi-geostrophic flows on a rotating sphere

TL;DR

This paper analytically demonstrates that the Miller-Robert-Sommeria statistical mechanics theory predicts non-trivial equilibrium states for quasi-geostrophic flows on a rotating sphere, including phase transitions and symmetry breaking, extending previous planar domain results.

Contribution

It extends the statistical mechanics framework to spherical geophysical flows with topography and finite Rossby radius, classifying equilibria and analyzing phase transitions.

Findings

Identification of non-trivial equilibria on a rotating sphere.

Discovery of a second order phase transition with angular momentum conservation.

Phase diagrams and ensemble relations for large-scale geophysical flows.

Abstract

Statistical mechanics provides an elegant explanation to the appearance of coherent structures in two-dimensional inviscid turbulence: while the fine-grained vorticity field, described by the Euler equation, becomes more and more filamented through time, its dynamical evolution is constrained by some global conservation laws (energy, Casimir invariants). As a consequence, the coarse-grained vorticity field can be predicted through standard statistical mechanics arguments (relying on the Hamiltonian structure of the two-dimensional Euler flow), for any given set of the integral constraints. It has been suggested that the theory applies equally well to geophysical turbulence; specifically in the case of the quasi-geostrophic equations, with potential vorticity playing the role of the advected quantity. In this study, we demonstrate analytically that the Miller-Robert-Sommeria theory leads to non-trivial statistical equilibria for quasi-geostrophic flows on a rotating sphere, with or without bottom topography. We first consider flows without bottom topography and with an infinite Rossby deformation radius, with and without conservation of angular momentum. When the conservation of angular momentum is taken into account, we report a case of second order phase transition associated with spontaneous symmetry breaking. In a second step, we treat the general case of a flow with an arbitrary bottom topography and a finite Rossby deformation radius. Previous studies were restricted to flows in a planar domain with fixed or periodic boundary conditions with a beta-effect. In these different cases, we are able to classify the statistical equilibria for the large-scale flow through their sole macroscopic features. We build the phase diagrams of the system and discuss the relations of the various statistical ensembles.