Abstract
ABSTRACTThis 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 Niño/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 Niño, 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 Niña 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.
Highlights
Blocking events often result from a strong meridional deformation of mid-latitude flows causing the main jet stream to be divided into two branches
We provide a comprehensive analysis of the main statistical properties and preferred locations of blockings stratified according to categories of the El Nino/southern oscillation (ENSO) cycle and we investigate the combined effects of ENSO and southern annular mode (SAM) phases on blocking frequency
We show that Southern Hemisphere (SH) blocking events are more persistent at high latitudes, consistent with other studies
Summary
Blocking events often result from a strong meridional deformation of mid-latitude flows causing the main jet stream to be divided into two branches. A few previous studies have examined the relationships between SH blocking and El Nino/southern oscillation (ENSO) (Rutllant and Fuenzalida, 1991; Renwick, 1998; Marques and Rao, 2000; Wiedenmann et al, 2002). They found a relatively higher frequency of SH blocking events over Southeast Pacific during El Nino years.
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