Quantifying the Dynamics of the Annular Modes

The annular modes in both hemispheres are dominant modes of variability in the extratropical circulation. Temporal persistence is a key feature of annular modes in the observed and modeled atmospheres and it has been suggested that a positive feedback between anomalous zonal flow and eddy fluxes is responsible for this persistence. The existence, amplitude, and the underlying physics of the eddy-jet feedback has been a subject of debate and extensive research in the past two decades. A major source of uncertainty is the difficulty in accurately separating the jet-dependent and jet-independent changes of eddy fluxes, which results from the lack of a complete understanding of the midlatitude turbulence. We have used the recently-calculated linear response function of an idealized atmospheric GCM, the GFDL Dry Dynamical Core, to accurately calculate the response of the eddy heat and momentum fluxes to the zonal wind and temperature patterns of the annular mode, which quantifies the jet-dependent component of eddy flux variability. We have then used these results and calculated the contribution of the eddy momentum flux, surface friction, and secondary circulation to the zonal momentum budget and the eddy heat flux, thermal radiation, and secondary circulation to the heat budget of the annular modes at low frequency. We have calculated the budgets for the full zonal wind pattern of the annular mode as well as just for the barotropic component of the wind. The results confirm the existence of a positive eddy-jet feedback due to the following mechanism:

1. The surface friction reduces the enhanced wind anomaly associated with the annular mode but at the same time increases the midlatitude baroclinicity in the lower troposphere
2. The increased baroclinicity enhances the eddy heat flux, which reduces the baroclinicty but also increases the convergence of the eddy momentum flux in the upper troposphere
3. The increased eddy momentum flux strengthens the wind anomaly of the annular mode; the barotropic governor effect associated with this strengthening suppresses the eddies but is overcome by the increased baroclinicity