Pedram Hassanzadeh, from Rice University
"Eddy-jet feedback in annular mode dynamics"
The annular mode is the dominant pattern of extratropical circulation variability in both hemispheres on intraseasonal to interannual time scales and features north-south shift of the midlatitudes jet. The key characteristic of the annular mode is temporal persistence, which is often attributed to an eddy-jet positive feedback internal to troposphere, although the underlying mechanism and magnitude of this feedback have not been well understood. However, some recent studies have challenged this view, showing that the often-used statistical methods cannot distinguish between the internal feedback and an external factor such as stratospheric variability. The key challenge in studying the annular mode dynamics is separating how much of the variability in forcing from eddy momentum is stochastic and how much is due to a feedback/external factor. Here we address this challenge using an approach based on the linear response function of an idealized GCM. This approach allows us to confirm the existence of an internal eddy-jet feedback, quantify its magnitude, and understand the dynamics of the feedback in the idealized atmosphere. Furthermore, using this new understanding, a novel statistical method is proposed and applied to the reanalysis data to quantify the feedback magnitude in the Southern Annular Mode. Future work focused on understanding the contribution of troposphere-stratosphere coupling and the feedback mechanism/magnitude in a hierarchy of GCMs is also discussed.
Host: Ding Ma
