Variations of the Southern Hemisphere storm-tracks on interannual-time scales

The purpose of this study is to explore interannual changes on the mean flow and storm-tracks of the Southern Hemisphere (SH) during July, using ECMWF reanalyses from 1979 to 1993. The 300-hPa meridional-wind perturbation (v’) was chosen in our study as the key variable to represent short-time scale perturbations. v’ is defined as the difference between each value and the time mean of each month. Seasonal cycle is also removed from the series. No further filtering of the series was performed, since there is evidence that meridional-wind variability is concentrated in the higher frequencies, a fact that is more so in the SH.

Composite fields of months associated with ENSO warm events show typical changes in the mean flow over the Pacific Ocean, like an eastward and equatorward shift of both the subtropical jet and the associated storm tracks. On the other hand, no significant changes of the short-time scale variability was found at subpolar latitudes, although an equatorward displacement of the subpolar jet is observed south of New Zealand.

A modulation of the austral winter storm tracks on frequencies lower than ENSO was detected. While from 1985 to 1991, a storm-track strengthening is observed at subpolar latitudes of the Pacific Ocean, before and after that period, a weakening of the synoptic-scale activity is discernible. It can be pointed out that similar low-frequency variations, are also manifest in longer time series of wind from surface stations over the southern tip of South America. Composite fields, based on periods with positive (negative) storm-track anomalies show over the Pacific Ocean a weakening (intensification) of the subpolar jet. Although a consequent reduction (enhancement) of the mean baroclinic conditions is observed, a strengthening (weakness) of the synoptic-scale activity takes place mainly due to changes in the wave structure that produce poleward (equatorward) eddy heat flux anomalies and thus eddy favourable (detrimental) baroclinic conversions.

A more extensive discussion about changes in the interaction between the mean flow and eddies, that occur over the storm-track region and associated with these low-frequency variations will be given at the Conference.