Characteristics of the Southern Hemisphere Winter Storm Track with Filtered and Unfiltered Data

Abstract

The structure and evolution of the fluctuations in synoptic scales in the Southern Hemisphere (SH) during winter are discussed using six years of European Centre for Medium-Range Weather Forecasts analyses. It is shown that patterns from unfiltered meridional wind series in the SH display all the features needed to represent the synoptic-scale waves. Typical periods and wavelengths are similar to those observed in the Northern Hemisphere (4 days, 4000 km), although over the Pacific Ocean they can be as high as 7?8 days and 4700 km, respectively. As in the Northern Hemisphere, tilts are not geographically fixed but change with the stage of the evolution of the wave. The phase speed of the waves agrees with the low-level winds in extensive areas of the middle latitudes and ranges from 12 m s ?1 in the Indian Ocean to 6 m s?1 in the Pacific Ocean. The estimated group velocities achieve maximum values of about 38 m s ?1, also in the Indian Ocean, and agree with the upper-level maximum winds, in accord with reported model results for the leading fringe of the wave packets. The wave packets show a decay of upstream centers as new ones grow downstream, suggesting that down-stream development contributes to the evolution of the synoptic-scale waves in the SH storm track. This process is observed both in the subpolar and subtropical jets, but the sequence of centers developing downstream is more coherent in the latter, probably due to the weaker baroclinicity.