Dynamical Feedbacks of the Southern Annular Mode in Winter and Summer

The persistence of the southern annular mode (SAM) is studied during austral winter (June–September) and summer (December–March) using observations of the three-dimensional vorticity budget. Analysis of the relative vorticity tendency equation shows that convergence of eddy vorticity flux in the upper troposphere, coupled with a secondary circulation, constitutes a positive eddy feedback that acts to sustain the vorticity anomaly associated with the jet shift against drag. The feedback exhibits a strong seasonality, with summer months revealing a positive feedback through much of the hemisphere and winter months showing a positive feedback over the Indian Ocean but not over the western Pacific. Results suggest that the lack of wintertime feedback over the western Pacific is due to the weakness of the eddy-driven midlatitude jet in that region.