to slow down low-level westerlies, it also enhances baroclinicity, which leads to stronger eddy generation. When the eddies propagate away, in the upper troposphere, from the latitudes where they are generated, the associated anomalies of eddy momentum flux reinforce the original zonal wind anomaly. As another example, @Gerber2006 argued that anomalous baroclinicity is not necessarily required for a positive eddy-jet feedback, as the mean flow anomaly can change the position of the critical latitudes for wave breaking and influence the eddy momentum flux convergence.

Quantifying the strength of eddy-jet feedback is important for understanding both internal variability and response to external forcing. One common issue with the current GCMs is that the simulated annular mode is too persistent compared to observations [@Gerber2008a], which not only indicates biases of jet variability, but also suggests overestimation of changes in the extratropical circulation in response to anthropogenic forcing in the models. According to the fluctuation-dissipation theorem [@Leith1975], the magnitude of the forced response is positively related to the timescale of the unforced variability, a relationship that has been confirmed qualitatively in some atmospheric models [e.g., @Ring2008; @Chen2009].

Based on the assumption that the mean-state-independent eddy forcing does not have long-term memory, LH01 and 