Seasonal Variations in the Southern Hemisphere Storm Tracks and Jet Streams as Revealed in a Reanalysis Dataset

Abstract

Regional characteristics of the climatological seasonal variations in Southern Hemisphere (SH) storm tracks are examined based on a reanalysis dataset. When differences in vertical structure between the subpolar and subtropical jet streams (SPJ and STJ, respectively) are considered, the regional characteristics can be interpreted reasonably well from a potential vorticity perspective of baroclinic eddy growth and downstream development of a baroclinic wave packet. Eddy activity in the upper and lower tropospheres is strongest in the core region of the SPJ over the eastern South Atlantic and Indian Ocean throughout the year, even in austral winter when the intense STJ forms over the Indian Ocean and South Pacific. Showing its eddy-driven nature, the SPJ accompanies the strong surface westerlies along a well-defined baroclinic zone above an intense oceanic frontal zone. In this core region of the storm track, low-level eddy activity is strongly correlated with the local near-surface baroclinicity, with its late- winter maximum and summer minimum, while upper-level eddy activity also depends on the incoming wave activity from upstream. Over the South Pacific, storm track activity depends critically on the formation of the STJ. In the absence of the intense STJ in summer and autumn, a single well-defined circumpolar storm track forms along the SPJ. During winter and spring, in contrast, wave activity accumulated in the core region is dispersed mainly toward the STJ, along which vigorous baroclinic eddy growth is unlikely to occur. The South Pacific storm track in the upper troposphere thus bifurcates into two branches along the STJ and SPJ, while at lower levels the storm track forms only along the enhanced baroclinic zone along the SPJ. Thus, under the trapping effect of the intense STJ, the storm track activity over the South Pacific is suppressed in winter, despite the enhanced low-level baroclinicity.