An eddy-zonal flow feedback model for propagating annular modes

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 (Lorenz and Hartmann 2001). 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, i.e., EOF1 (EOF2) changing the eddy forcing of EOF2 (EOF1) at the quasi-steady limit. Using the analytical solution of this model, we derive conditions for the existence of the propagating regime (the decaying-oscillatory solution) 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. Extending the model to three interacting EOFs shows negligible feedbacks from EOF3, suggesting that a two-EOF model is sufficient to represent the current Southern Hemisphere extratropical circulation variability. 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 and to evaluate how well GCMs simulate the annular modes.

Results are currently under review: Lubis, S.W. and Hassanzadeh, P., 2020. An Eddy-Zonal Flow Feedback Model for Propagating Annular Modes. arXiv preprint arXiv:2007.08589.

Lubis, S.W. and Hassanzadeh, P., 2020. An Eddy-Zonal Flow Feedback Model for Propagating Annular Modes. arXiv preprint arXiv:2007.08589.