The leading EOF of zonal-mean variability (the annular mode) represents the north/south movement of the mid-latitude jet. Analysis of the momentum budget demonstrates that EOF1 dominates over other EOFs because of a positive feedback between the EOF1 zonal-mean wind anomalies and the eddy momentum fluxes. The mechanism of the feedback is as follows: above normal baroclinic wave activity is generated in the region of enhanced westerlies. The propagation of waves from their source latitude is associated with (1) eddy momentum fluxes which enhance the westerly wind anomalies and (2) a mean meridional circulation which helps restore the baroclinicity of the wave source region.

At first sight, this positive feedback mechanism would also appear to work for EOF2 which represents the strengthening/sharpening versus the weakening/broadening of the jet. Indeed, above normal baroclinic wave activity is also generated in the enhanced westerlies for EOF2. This anomalous wave source, however, is offset by the effect of upper level PV gradients on the propagation of wave activity. A stronger/sharper (weaker/broader) jet is a stronger (weaker) waveguide which inhibits (enhances) the meridional propagation of wave activity.

In addition, we find that the response of the eddies to the rapid onset of annular mode anomalies is not monotonic. Instead, a transient period of negative eddy forcing is followed, after several days, by a period of long-term reinforcement by the eddies. Our results suggest that a baroclinic mid-latitude jet, that organizes its own wave momentum forcing, becomes quite sharp. Given a uniform distribution of wave activity, a sharp jet will tend to attract wave activity resulting in a negative momentum forcing. The positive response to the change in the annular mode follows only after baroclinic instability has had time to establish positive wave activity anomalies in the region of westerly anomalies.