Seasonal Variations in the Southern Hemisphere Storm Tracks and Associated Wave Propagation

Abstract

Seasonal variations in the Southern Hemisphere (SH) storm track characteristics and associated wave propagation are studied using 19 years of NCEP-NCAR gridded data. It is found that the SH storm track is strongest in the austral autumn season and weakest in spring. The characteristics of wave packets are studied by computing 1-point lag correlation maps with unfiltered meridional wind at 300 hPa. It is found that the eastward group velocities of waves are much higher than the phase velocities in the transition seasons. This shows that the downstream development occurs throughout the year and is a basic feature of upper-tropospheric waves in the midlatitudes of the SH. This extends previous studies made for winter and summer seasons. Based on the indices that show wave coherence and correlation maps, it is found that the pathways in the transition seasons split into two branches east of Australia, in a way similar to what happens in the zonal wind distribution.

Changes in synoptic weather patterns in the polar regions in the twentieth and twenty-first centuries, part 1: Arctic

“…An alternative approach has been the application of synoptic timescale filters to pressure or height data and analyzing the variance of the filtered data (Trenberth, 1991;Nakamura and Shimpo, 2004). Other authors have argued for the use of unfiltered data (Berbery and Vera, 1996;Rao et al, 2002), but in any case the relationship of these variances to cyclone and anticyclone trajectories remains problematic (Wallace et al, 1988). A more general method for analyzing the circulation as a whole (as opposed to only cyclones, or cyclone and anticyclone centers) is the use of empirical orthogonal function (EOF) analysis (e.g.…”

Interannual variations of storm tracks in the Southern Hemisphere and their connections with the Antarctic oscillation

“…These baroclinic waves are concentrated into the storm tracks. In the SH the principal storm track lies between 45 and 55°S in all seasons, although in winter an additional branch of the storm track is seen at 20-30°S between longitudes 150 and 90°W (Rao et al, 2002). In this article, interannual variations of storm tracks are studied for the period January 1974 through December 2000.…”

“…RESULTS Figure 1 shows the mean and standard deviation of v e and rainfall. Figure 1(a) shows that the midlatitude storm track (as given by v e ≥ 18 m s −1 ) is most intense in MAM (Rao et al, 2002). In austral winter (June, July and August; JJA) there is an additional storm track in the subtropics.…”

“…Some recent studies have examined storm tracks in the SH (Berbery and Vera, 1996;Chang, 2000;Rao et al, 2002). In the SH, the principal storm track at 300 hPa lies between 45 and 55°S in all seasons, although in winter an additional branch of the storm track is seen at 20-30°S between longitudes 150 and 90°W (Rao et al, 2002).…”

“…However, he examined only summer and winter seasons. Rao et al (2002) extended this study by analysing all four seasons. They found that the SH storm tracks, as given by the variance of unfiltered meridional wind, are strongest in austral autumn (March, April and May; MAM) and weakest in austral spring (September, October and November).…”