Principal Modes of Southern Hemisphere Low-Frequency Variability Obtained from NCEP–NCAR Reanalyses

Abstract

Abstract Analysis of the 300-hPa streamfunction from the 40-year NCEP–NCAR dataset has identified the principal modes of Southern Hemisphere variability on intraseasonal (IS), interannual (IA), and intradecadal (ID) timescales. The variance of streamfunction departures from the annual cycle is zonally symmetric in the IS and IA range with the largest values at midlatitudes. The ID variance is concentrated in the Pacific sector, where it extends to lower latitudes. For the IS band, obtained by applying a 10–50-day bandpass filter to the twice-daily streamfunction fields, three pairs of EOF patterns were obtained. These show eastward-propagating wavenumber-4 and -5 patterns largely confined to middle and higher latitudes, and two interleaved wavenumber-4 patterns expressing intensification of the zonal wind near 30°S. The wavenumber-5 patterns are more prominent in summer (December–February). On the interannual timescale, including all periods beyond 50 days, the leading EOF with 26% of the variance express...