Persistent Positive Anomalies in the Southern Hemisphere Circulation

Abstract

Long time series of reanalyses, from NCEP–NCAR and from ECMWF, are used to investigate the occurrence of persistent positive anomalies (PPAs) in the 500-hPa geopotential height field over the Southern Hemisphere extratropics during 1958–2001. Defining persistent anomalies as those of at least 100 m in magnitude lasting for at least 5 days, it is found that the region of most frequent occurrence is over the South Pacific. A cluster analysis of monthly PPA counts shows two distinct patterns, one a zonal wavenumber-1 (ZW1) pattern centered over the southeast Pacific near 60°S and the other a zonal wavenumber-3 (ZW3) pattern with centers near New Zealand and over the southern Atlantic and Indian Oceans. Results were insensitive to the choice of dataset, and to the removal of a linear trend from the daily height fields. The southeast Pacific PPA region is strongly modulated by ENSO, while the ZW3 pattern appears only weakly related to ENSO variability. A strong upward trend is apparent in occurrence of the ZW3 cluster, related to a matching trend in the variance of the height fields, particularly those from ECMWF. Such trends are at least in part a consequence of changes in the observing system, particularly the introduction of satellite soundings in the late 1970s.