The Dynamics of Cross-EOF Eddy Feedbacks in Propagating Annular Modes

Abstract:

Various observational and modeling studies suggest that a zonal mean wind-eddy feedback increases the persistence of the annular modes, through a positive feedback of EOF1 onto itself. However, a recent study shows that under some circumstances, EOF1 and EOF2 can interact and exert feedbacks on each other at some lag times, resulting in decaying-oscillatory, or propagating, annular modes. In recent work, we have questioned the importance of a cross-EOF eddy-zonal flow feedback in variability of the annular modes. We did find evidence that the stronger cross-EOF feedbacks lead to the less persistent annular modes as a result of the oscillation.

In this current work, we aim to investigate the underlying dynamics of the cross-EOF feedbacks for propagating annular modes in both reanalysis and an idealized GCM. While the results so far have pointed out that cross-EOF feedbacks are essential components of the propagating annular modes; however, the propagation itself is likely essential for the existence of cross-EOF feedbacks. Using a finite-amplitude wave activity (FAWA) framework, we show that the cross-EOF feedbacks result from the out-of-phase oscillations of EOF1 (north-south jet displacement) and EOF2 (jet pulsation) leading to an orchestrated combination of equatorward propagation of wave activity (a baroclinic process) and nonlinear wave breaking (a barotropic process), which altogether act to reduce the total eddy forcings. The results highlight the importance of cross-EOF eddy feedbacks to fully understand the dynamics of the large-scale extratropical circulation and also the reason behind the too-persistent annular modes in GCMs.