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

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 expresses fluctuations in the strength of the subtropical jet. EOFs 2 and 3 contain mixed representations of the high-latitude mode and an ENSO (El Niño?Southern Oscillation) related pattern largely confined to the Pacific. EOFs 4 and 5 depict a wave train extending from Australia across the South Pacific to the east of South America. These appear to be stable throughout the period of analysis and contribute around 52% of the >50-day variance. Variations on ID timescales were isolated by forming 11-month running means of the streamfunction anomalies, and five leading EOFs were found to be significant. The first of these, with 37% of the variance, captured variations forced by ENSO that extend from the equator to higher latitudes in the South Pacific. The second EOF with 18% of the variance accounted for changes in the circulation occurring in the early part of the record, and its amplitude since 1970 has been small. The third was identifiable as the high-latitude mode.