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.