An Eddy-Zonal Flow Feedback Model for Propagating Annular Modes

TL;DR

This paper develops a reduced-order model that captures the interaction between leading EOFs of zonal-mean zonal wind, revealing how cross-EOF feedbacks influence the persistence and propagation of annular modes in the extratropical circulation.

Contribution

It introduces a coupled EOF model that accounts for cross-EOF feedbacks, providing new insights into the dynamics of propagating annular modes.

Findings

Cross-EOF feedbacks control the oscillation frequency of annular modes.

Stronger feedbacks lead to less persistent, more oscillatory modes.

Model application to reanalysis data confirms strong cross-EOF feedbacks in the Southern Hemisphere.

Abstract

The variability of the zonal-mean large-scale extratropical circulation is often studied using individual modes obtained from empirical orthogonal function (EOF) analyses. The prevailing reduced-order model of the leading EOF (EOF1) of zonal-mean zonal wind, called the annular mode, consists of an eddy-mean flow interaction mechanism that results in a positive feedback of EOF1 onto itself. However, a few studies have pointed out that under some circumstances in observations and GCMs, strong couplings exist between EOF1 and EOF2 at some lag times, resulting in decaying-oscillatory, or propagating, annular modes. Here, we introduce a reduced-order model for coupled EOF1 and EOF2 that accounts for potential cross-EOF eddy-zonal flow feedbacks. Using the analytical solution of this model, we derive conditions for the existence of the propagating regime based on the feedback strengths. Using this model, and idealized GCMs and stochastic prototypes, we show that cross-EOF feedbacks play an important role in controlling the persistence of the annular modes by setting the frequency of the oscillation. We find that stronger cross-EOF feedbacks lead to less persistent annular modes. Applying the coupled-EOF model to the Southern Hemisphere reanalysis data shows the existence of strong cross-EOF feedbacks. The results highlight the importance of considering the coupling of EOFs and cross-EOF feedbacks to fully understand the natural and forced variability of the zonal-mean large-scale circulation.