Statistical state dynamics of weak jets in barotropic beta-plane turbulence

TL;DR

This paper uses the statistical state dynamics framework to analyze the equilibration of jet formation in barotropic turbulence, revealing a new nonlinear instability branch and proposing modifications to existing Ginzburg-Landau models.

Contribution

It identifies a new nonlinear flow-forming instability and compares fully nonlinear SSD dynamics with G-L dynamics, extending understanding of jet equilibrations in turbulence.

Findings

Discovery of a new jet equilibrium branch not connected to G-L branch.

Identification of a nonlinear flow-forming instability below linear criticality.

Proposal of a modified diffusion coefficient to better capture jet evolution.

Abstract

Zonal jets in a barotropic setup emerge out of homogeneous turbulence through a flow-forming instability of the homogeneous turbulent state (`zonostrophic instability') which occurs as the turbulence intensity increases. This has been demonstrated using the statistical state dynamics (SSD) framework with a closure at second order. Furthermore, it was shown that for small supercriticality the flow-forming instability follows Ginzburg-Landau (G-L) dynamics. Here, the SSD framework is used to study the equilibration of this flow-forming instability for small supercriticality. First, we compare the predictions of the weakly nonlinear G-L dynamics to the fully nonlinear SSD dynamics closed at second order for a wide ranges of parameters. A new branch of jet equilibria is revealed that is not contiguously connected with the G-L branch. This new branch at weak supercriticalities involves jets with larger amplitude compared to the ones of the G-L branch. Furthermore, this new branch continues even for subcritical values with respect to the linear flow-forming instability. Thus, a new nonlinear flow-forming instability out of homogeneous turbulence is revealed. Second, we investigate how both the linear flow-forming instability and the novel nonlinear flow-forming instability are equilibrated. We identify the physical processes underlying the jet equilibration as well as the types of eddies that contribute in each process. Third, we propose a modification of the diffusion coefficient of the G-L dynamics that is able to capture the asymmetric evolution for weak jets at scales other than the marginal scale (side-band instabilities) for the linear flow-forming instability.