Structure and evolution of the Annular Modes in an aquaplanet GCM

The structure and evolution of the Annular Modes within a full-physics atmospheric model coupled to an "aquaplanet" mixed-layer ocean model are investigated. The annular modes are defined as the leading EOF of the 10 day low-pass filtered winter zonal-mean surface pressure. The structure of this EOF, which is zonally symmetric by definition, resembles the structure of the observed Northern and Southern Annular Modes, which are also known to be strongly zonally symmetric. However, preliminary results indicate that the strength of the daily projection onto the EOF is dominated by zonally localized events. Thus, it appears that the annular modes in this model are primarily a statistical representation of zonally asymmetric features. The dynamics of annular mode events are discussed.