Interannual variability of winter storm tracks in the Southern Hemisphere

Mid-latitude disturbances in the Southern Hemisphere (SH) are the most important features of the large-scale circulation and are the major contributors to the meridional fluxes of momentum, heat and moisture. These high–frequency perturbations may be responsible for severe weather conditions over southern South America, particularly during the austral winter months.

The interannual variability of winter storm-tracks over the southeastern Pacific – southern South America – southwestern Atlantic sector associated with warm and cold phases of the El Niño-Southern Oscillation (ENSO) cycle is addressed in this paper with the objective of analyzing the differential behaviour of the synoptic-scale waves that affect the region. This study makes use of 13 years (1982-1994) of daily 00 UTC analysis from National Environmental Prediction Center (NCEP) re-analysis data set on a 2.5° by 2.5° latitude-longitude grid at 17 mandatory levels. The data set has been stratified based on El Niño 3 index to identify warm, cold and neutral conditions, as this is the more sensitive index for the region.

The standard deviation of the meridional component of the wind at 300 hPa and the eady growth rate at lower leves (below 500 hPa) have been calculated for each phase of ENSO cycle in order to characterize the storm-tracks. Significant differences have been found between cold and warm phases of the ENSO cycle concerning the characteristics, intensity and location of the regions of maximum high-frequency variability over the domain of interest. During the warm phase of the ENSO cycle the Atlantic storm-track seems to be more active compared with the cold phase, while the subtropical branch of the Pacific storm-track seems to be weaker for the warm phase.

The structure of the synoptic scale disturbances has also been analyzed in terms of time-lag regression analysis, using the meridional component of the wind at 300 hPa at selected base points as reference time series. Significant differences in the structure, intensity and the downstream development of the wave-packets have been found. The main differences between warm and cold phases of ENSO cycle are related with the trajectories and structure of the synoptic systems. During the cold phase of the ENSO cycle the wave-packets propagate more northeasterly on the lee side of the Andes and, hence, affects lower latitudes over southern South America. During the warm phase the process of downstream development, particularly over the western Atlantic Ocean is stronger and the wave-packets propagate more efficiently eastwards though the Atlantic Ocean.