Differences in El Niño Response over the Southern Hemisphere

Abstract

Southern Hemisphere (SH) circulation conditions during austral springs of ENSO events are examined. Based on previous knowledge that SST variations over the subtropical south-central Pacific (SSCP) region are linked to differences among El Nino (EN) events, a stratification of the springs associated with EN events was performed according to SST conditions over the El Nino-3.4 sector and over the SSCP region. The EN events associated with cold conditions in the SSCP (WC) exhibit enhanced convection not only in the intertropical convergence zone over the central equatorial Pacific but also in the South Pacific convergence zone (SPCZ) extended south- eastward into the subtropical regions. The resulting heating forcing intensifies a localized overturning cell, which is associated with an anomalous Rossby wave source in the central South Pacific extratropical region. Neither the Rossby wave source nor the associated wave pattern is evident during EN events associated with warm conditions in the SSCP and inactive SPCZ (WW). The basic features that characterize the differences in the EN response over the South Pacific can also be identified through the analysis of the SPCZ activity over the central South Pacific. The fact that variations in SPCZ activity lead SST anomaly changes in the SSCP would indicate that the differences in the EN response over the SH might be mainly driven by atmospheric changes, which induces extratropical SST anomalies. The differences in the circulation anomalies that characterize both types of EN response over the SH were further explored through the analysis of the activity of the three leading modes of circulation variability. The combined effect of the three leading patterns describes in some extent the intensification (weakening) of the cyclonic circulation anomaly observed over the southeastern Pacific in WC (WW), associated with an active (inactive) SPCZ. In particular, the interdecadal variability observed in the Pacific by many previous studies influences the circulation response to ENSO over the SH, mainly through changes in the activity of the SH annular mode.