zonal mean zonal wind associated with the annular mode is characterized by an equivalent barotropic dipole, which is, as expected, in thermal wind balance with the zonal mean temperature anomaly. Variations in the zonal index represent [north-south]{} vacillations of the eddy-driven jet [e.g., @Hartmann1998].

![image](g01.pdf){width="\textwidth"}

For model outputs, both hemispheres are analyzed, but the Northern Hemisphere is flipped and plotted as the Southern Hemisphere, as the model is symmetric about the equator. The climatology in the simulations with the same model configuration has been well documented [e.g., @Held1994]. In brief, a confined midlatitude jet centering around 40$^\circ$S, 10$^\circ$ equatorward to the eddy-driven jet in the reanalysis data, is produced in the CTL (Figure 2a). The zonal index is again calculated as the leading PC of $\langle [\mathbf{u}]\rangle$. The leading EOF of $\langle [\mathbf{u}]\rangle$ explains 50$\%$ of the total variance in the model, while the second EOF explains 18$\%$. Despite the idealized nature of the GCM, [the tropospheric dipolar]{} pattern of zonal wind of the annular mode produced in the model compares reasonably well with the Southern annular mode in the reanalysis data (Figures 2bc).

![image](F01.pdf){width="\textwidth"}

Simple model of feedback
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[In their seminal work]{}, LH01 introduced a simple model of 