research [@Gerber2008a; @Gerber2008b; @Nie2014].

![image](F02.pdf){width="\textwidth"}

![image](F03.pdf){width="50.00000%"}

Eddy-jet feedback strength
==========================

The LRF will first be used to calculate the “ground truth" of the eddy-jet feedback strength associated with the leading EOF of $\langle [u] \rangle$ (i.e., the annular mode), as well as the second EOF, in the idealized GCM. Three different statistical methods, namely, fitting cross-correlation functions (LH01), lag regression (S13) and regression using low-pass filtered data (introduced in the present study), will be used to estimate the eddy feedback strength of the annular mode in the idealized GCM, and evaluated against the result from the LRF. Then we will apply the statistical methods to investigate the eddy feedback associated with the annular mode in the reanalysis data.

Linear response function
------------------------

With a zonally symmetric time-invariant forcing, the deviations of mean state in EXP from that in CTL (Figures 7ab) are nearly identical to the pattern of the annular mode (Figures 2bc)[, with a pattern correlation of 0.995]{}. Note that the changes in the mean state from CTL to EXP are caused by the imposed external forcing and are long term averages so that the eddies are in statistical equilibrium with the mean state. The changes of eddy fluxes 