Oceanic rings and jets as statistical equilibrium states

TL;DR

This paper applies equilibrium statistical mechanics to oceanic flows, explaining the formation and behavior of rings and jets as natural self-organized structures in a quasi-geostrophic model.

Contribution

It demonstrates that oceanic rings and jets can be understood as statistical equilibrium states, predicting their shapes, drift, and ubiquity without relying on specific generation mechanisms.

Findings

Rings are close to a statistical equilibrium, matching observed shapes and drift.

Mid-basin eastward jets are described as marginally unstable equilibrium states.

The theory provides a unified explanation for large-scale oceanic structures.

Abstract

Equilibrium statistical mechanics of two-dimensional flows provides an explanation and a prediction for the self-organization of large scale coherent structures. This theory is applied in this paper to the description of oceanic rings and jets, in the framework of a 1.5 layer quasi-geostrophic model. The theory predicts the spontaneous formation of regions where the potential vorticity is homogenized, with strong and localized jets at their interface. Mesoscale rings are shown to be close to a statistical equilibrium: the theory accounts for their shape, their drift, and their ubiquity in the ocean, independently of the underlying generation mechanism. At basin scale, inertial states presenting mid basin eastward jets (and then different from the classical Fofonoff solution) are described as marginally unstable states. These states are shown to be marginally unstable for the equilibrium statistical theory. In that case, considering a purely inertial limit is a first step toward more comprehensive out of equilibrium studies that would take into account other essential aspects, such as wind forcing.