Storm tracks and annular modes

In this paper, it is demonstrated by using a weakly nonlinear model that low-frequency annular modes with a dipole meridional structure can be excited by the eddy fluxes from synoptic-scale eddies. If the storm track organized by synoptic- scale eddies is zonally confined in a narrow localized region as observed in the Northern Hemisphere (NH), the eddy-driven dipolar pattern will exhibit both a relatively short zonal scale and a zonal asymmetry during its life cycle, which is attributed to the strong downstream energy dispersion of Rossby waves. Such a zonal asymmetry is found to be sensitive to the relative position between the preexisting storm track and dipole mode. However, if the preexisting storm track is zonally confined in a rather wide localized region, as observed in the Southern Hemisphere (SH), the Rossby wave dispersion almost disappears. In this case, the eddy-driven dipole mode with a relatively large zonal scale exhibits a zonal symmetry, which is almost insensitive to its position relative to the preexisting storm track. This sheds light on why eddy-driven dipole modes in the SH are more likely to be annular modes.