Annular-mode-like variation in a multi-layer QG model

The annular modes, as the leading modes of variability of the Northern (NAM) and Southern (SAM) Hemisphere tropospheric winds, are characterized by persistent south-north shifts of the midlatitude jets. Previous studies suggest that the persistence of the anomalous zonal winds in the annular modes is extended due to a positive feedback between the wind anomalies and the eddy momentum forcing. This eddy-zonal flow interactions in the annular modes are investigated in this study using a beta-plane multi-layer QG channel model. Our study shows that the positive feedback between the zonal wind anomaly and the baroclinic eddy is sensitive to the strength of surface friction. Only under weak surface friction, through the variation of the lower level critical latitude, a consistent shift of the baroclinic zone with the anomalous zonal wind can be obtained. In the regime of strong surface friction, westerly jet mainly exhibits high frequency variation, which is simply the direct response to the forcing of transient eddies. The transition between the zonal winds' annular-mode-like and non-annular-mode-like variation is found sensitive to the spatial distributions of zonal wind and critical layer/latitude in the lower levels.