A synoptic-dynamic interpretation of the Southern Annular Mode

Abstract. In this study, we describe the Southern Annular Mode (SAM) as a function of regional extratropical variability in order to understand the synoptic-dynamic features associated with it. The SAM is often considered to be the leading mode of variability in the extratropics and is used to interpret changes and impacts in the large-scale mid-latitude circulation and surface weather in the Southern Hemisphere. This interpretation of the SAM has however been questioned in recent years, given the SAM's lack of annularity and weak correlation to surface weather and synoptic features. From this regional perspective, we once again highlight the non-annularity of the SAM and show explicitly how these asymmetries can result in the misinterpretation and misattribution of regional surface weather impacts. Through analysing a set of weather features including Rossby wave breaking (RWB) zones, cyclones and jets, we show the SAM can be interpreted as a function of the RWB characteristics across the hemisphere, in line with similar perspectives of the North Atlantic Oscillation. In fact, changes in the storm track and jets occur locally and non-annularly across the hemisphere in response to regional RWB, in contrast to the annular view assumed by the SAM. This interpretation provides opportunity for further understanding of present and future Southern Hemispheric extratropical variability and its associated surface weather impacts.