Storm Tracks in the Southern Hemisphere

Abstract

From 1979 to 1989, global European Centre for Medium Range Weather Forecasts (ECMWF) analyses of many meteorological variables important in baroclinic storms have been bandpass filtered to highlight fluctuations on 2- to 8-day time scales in order to illuminate the relationships among the variables in baroclinic storm-track regions. Because of the greater zonal symmetry in the Southern Hemisphere (SH) versus the Northern Hemisphere (NH), it is possible to examine these relationships over a zonal sector without being dominated by local effects associated with jet stream entrance and exit regions of the storm tracks. The bandpassed variance of geopotential height is used to define the storm track, and its meridional profile has a pronounced midlatitude maximum in the SH. The locations of the jet stream and the variance maxima of meridional and zonal velocity components, vorticity, vertical motion, specific humidity, and temperature all bear a distinctive relationship to the storm track, but the latitudes of their maxima are displaced from the center of the storm track. Many covariance quantities such as the transient eddy temperature, moisture, vorticity, and vertical and horizontal momentum fluxes also exhibit a strong storm-track signature. The observed relationships among the eddy quantities can generally be understood in terms of geostrophic theory and perturbation analysis applied to baroclinic systems. Storm-track activity is remarkably persistent throughout the year in both location and intensity in the SH. The storm track is farthest poleward in the transition seasons as part of a semiannual cycle, but remains near 50°S year round, and is strongest in the southern Indian Ocean and weakest in the South Pacific. There is a strong relationship between the storm track and the major tropospheric polar jet stream and its associated lower-level baroclinicity throughout the year, and the distribution of storm-track activity can be accounted for by baroclinic theory. In contrast to the NH, strongest meridional temperature gradients in middle latitudes are found in the summer half-year in the SH. Whereas the NH storm-track activity is much weaker in summer and shifts poleward, the SH activity is as strong as in winter and, if anything, shifts slightly equatorward. The zonal symmetry is greater in summer and meridional profiles are sharper, implying less variability in the storm track both within and between seasons. In winter, high-frequency storm-track activity extends over a broader range of latitudes and continues to be mainly associated with the polar jet stream. The question of the impact of the storm-track eddies on the mean flow is examined using zonal mean and a localized Eliassen?Palm flux. Baroclinic effects from the poleward heat flux dominate in winter and the eddies act to decelerate the upper tropospheric westerlies. Barotropic effects in the upper troposphere, mostly from the meridional momentum flux convergence, help to maintain the mean westerly distribution by accelerating the main polar jet and maintaining the mean split in the flow near New Zealand in both summer and winter and even dominate the baroclinic component in the storm track in summer. Diabatic heating by the eddies may also help maintain the mean flow.