Index Cycles in the Southern Hemisphere during the Global Weather Experiment

Abstract

Analysis of the zonal wind variations during the Southern Hemisphere winter of 1979 has shown the existence of an irregular cycle with an average period of 27 days. Opposing variations in the 500 hPa zonal wind are found in bands centered on 48.75° and 67.5°S and an index based on their difference has been used to depict the changes in circulation patterns. At higher latitudes the changes at the 500 hPa level and at mean sea level resemble the changes in circulation between the mean winter and equinoctial seasons. In particular the wavenumber 3 which is closely associated with the semiannual oscillation duplicates its movement between winter and the equinoctial seasons. During low index situations blocking is favored near and to the east of New Zealand while in high index situations the South Pacific High is well developed. A preliminary comparison of the zonal index variations with those in other years suggests that they were larger and more regular than usual during the winter of 1979.