The Role of High-frequency Eddy Forcing in the Maintenance and Transition of the Southern Hemisphere Annular Mode

Abstract

The role of high-frequency eddy forcing in the Southern Hemisphere Annular Mode (SAM) is diagnosed with the use of the 21-yr (1979-1999) NCEP/NCAR daily reanalysis dataset. The SAM is described as a meridional vacillation of the polar jet. The present study focuses on a zonally localized sector, 30°W-20°E, where both the polar jet and high-frequency eddies are dominant. In the maintenance of the extreme phases of the SAM, two positive feedback processes play important roles. The storm tracks move with the polar jet between the extreme positions of the polar jet. The displacement of the storm tracks sustains their anomalous positions. Together with this process, the alternation of the horizontal eddy structure also plays a role in the maintenance of the extreme phases. The southeast-northwest tilt of eddy is emphasized when the polar jet resides at higher latitudes, as has been mentioned in earlier studies. The southeast-northwest oriented eddies transport momentum to higher latitudes, keeping the jet in the higher latitudes. To both the poleward and equatorward jet transitions, momentum flux anomalies due to highfrequency eddies contribute. Characteristics of the eddies associated with these momentum flux anomalies are examined. For the poleward transitional phase, flux anomalies are induced by both increased eddy kinetic energy and an emphasized southeast-northwest tilt of the eddy. For the equatorward transitional phase, changes in the horizontal eddy structure are found to be more important.