Observations, Simulations, and Dynamics of Jet Stream Variability and Annular Modes

The characteristics of the dominant pattern of extratropical variability (the so-called annular modes) are examined in the context of the theory that eddy-driven jets are self-maintaining. It is shown that there is genuine hemispheric symmetry in the variation of the zonal wind in the Southern Hemisphere but not the Northern Hemisphere. The annular mode is shown to be baroclinic in nature; it is associated with changes in the baroclinic eddy source latitude, and the latitude of the eddy source region is organized by the mean flow. This behavior is expected if there is a baroclinic feedback that encourages the maximum baroclinic instability to be coincident with the maximum zonal wind speed, and discourages the meridional vacillation of the eddydriven jet stream. It is shown that the strength of the thermally indirect circulation that gives rise to the baroclinic feedback appears to influence the time scale of the annular mode. When the thermally indirect circulation is stronger the annular mode has a longere-folding time in a simplified GCM. Preliminary results indicate that the same dynamics are important in the real atmosphere.