CHARACTERISTICS OF THE SOUTHERN-HEMISPHERE WINTER STORM TRACK WITH FILTERED AND UNFILTERED DATA

The structure and evolution of the fluctuations in synoptic scales in the Southern Hemisphere (SH) during winter are discussed using six yea rs of European Centre for Medium-Range Weather Forecasts analyses. It is shown that patterns from unfiltered meridional wind series in the S H display all the features needed to represent the synoptic-scale wave s. Typical periods and wavelengths are similar to those observed in th e Northern Hemisphere (4 days, 4000 km), although over the Pacific Oce an they can be as high as 7-8 days and 4700 km, respectively. As in th e Northern Hemisphere, tilts are not geographically fixed but change w ith the stage of the evolution of the wave. The phase speed of the wav es agrees with the low-level winds in extensive areas of the middle la titudes 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 val ues 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 t he leading fringe of the wave packets. The wave packets show a decay o f upstream centers as new ones grow downstream, suggesting that downst ream development contributes to the evolution of the synoptic scale wa ves in the SH storm track. This process is observed both in the subpol ar and subtropical jets, but the sequence of centers developing downst ream is more coherent in the latter, probably due to the weaker barocl inicity.