A new climatology for Southern Hemisphere blockings in the winter and the combined effect of ENSO and SAM phases

Abstract

This study presents 53-year climatology of Southern Hemisphere (SH) blockings in the winter using daily 500-hPa geopotential height data from NCEP–NCAR reanalysis. The variability of SH blocking events and their relationships with combined phases of El Ni˜no/southern oscillation (ENSO) and the southern annular mode (SAM) are examined. Conventional indices were revised and a slightly modified index is proposed to detect latitudinal variations of SH blockings. The South Pacific region is examined in detail. There is no statistically significant long-term trend in the SH blockings. During moderate El Ni˜no, the preferred location SH blocking is observed over East Pacific, and we show that the blocking frequency increases during negative SAM phases. During moderate La Ni˜na the SH blockings are significantly suppressed over Central Pacific, with lower blocking frequency during positive SAM phases. These results indicate that the daily variability of SH blocking is strongly modulated by both ENSO and SAM phases.