Implications of the Observed 25 Day Periodicity in the Extratropical Circulation for Weather and Climate

Recent research suggests that large-scale variability in the Southern Hemispheric circulation exhibits robust periodicity on timescales of approximately 25 days. The periodicity is associated with a distinct pattern of large-scale variability, referred to as the Southern Baroclinic Annular Mode (SBAM). The SBAM is characterized by quasi-zonally symmetric variations in eddy kinetic energy, but is also associated with periodicity in eddy fluxes of heat and large-scale precipitation. Although periodicity associated with the SBAM is apparent in hemispheric averages of eddy quantities and precipitation, the implications for weather and climate remain unclear. This talk will explore the dynamics of the periodicity of the SBAM and its projection onto individual storms and regional weather via observational analyses, output from climate models, and a simple wave-mean flow interaction model. It is argued that on regional scales, the SBAM periodicity arises from the combined effects of two-way interactions between the extratropical baroclinicity and the wave fluxes of heat, as well as differential propagation rates of the baroclinicity and the heat fluxes. Implications for medium-range forecasting are also briefly discussed.