Role of medium-scale waves on the Southern Annular Mode

Abstract

[1] We examined the role of short-period waves on the Southern Annular Mode (SAM) through analysis of 6-hourly reanalysis data of the European Centre for Medium-Range Weather Forecasts (ECMWF ERA-Interim). We focused on medium-scale waves, those with periods shorter than about 6 days. Although medium-scale waves generally have much smaller amplitudes than synoptic waves, they have a much higher correlation with the SAM index. As a result, about one third of wave-driven meridional circulation and zonal wind acceleration associated with the SAM is generated by medium-scale waves. Their contribution is large in spite of their climatologically smaller amplitude. Moreover, two thirds of SAM-related meridional circulation and zonal wind acceleration comes from waves of periods shorter than about 3 days. Our analysis demonstrates a positive feedback between westerly winds around 60°S and amplification of synoptic and medium-scale waves, which sustains hemispheric SAM variability. For the feedback process between zonal wind and waves, baroclinic conversion is a key process to both synoptic waves and medium-scale waves. Our analysis also shows that wave forcings calculated from daily data explain only half of the total forcings and that the contribution from short-period waves is very significant around 50°S.