Differences in El Niño response over the Southern Hemisphere.

VERA C., G. SILVESTRI, V. BARROS AND A.F. CARRIL

JOURNAL OF CLIMATE

AMS

Año: 2004 vol. 17 p. 1741 - 1753

0894-8755

Southern Hemisphere (SH) circulation conditions during austral springs of ENSO events are examined. Basedon previous knowledge that SST variations over the subtropical south-central Pacific (SSCP) region are linkedto differences among El Nin˜o (EN) events, a stratification of the springs associated with EN events was performedaccording to SST conditions over the El Nin˜o-3.4 sector and over the SSCP region. The EN events associatedwith cold conditions in the SSCP (WC) exhibit enhanced convection not only in the intertropical convergencezone over the central equatorial Pacific but also in the South Pacific convergence zone (SPCZ) extended southeastwardinto the subtropical regions. The resulting heating forcing intensifies a localized overturning cell, whichis associated with an anomalous Rossby wave source in the central South Pacific extratropical region. Neitherthe Rossby wave source nor the associated wave pattern is evident during EN events associated with warmconditions in the SSCP and inactive SPCZ (WW).The basic features that characterize the differences in the EN response over the South Pacific can also beidentified through the analysis of the SPCZ activity over the central South Pacific. The fact that variations inSPCZ activity lead SST anomaly changes in the SSCP would indicate that the differences in the EN responseover 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 werefurther explored through the analysis of the activity of the three leading modes of circulation variability. Thecombined effect of the three leading patterns describes in some extent the intensification (weakening) of thecyclonic 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 studiesinfluences the circulation response to ENSO over the SH, mainly through changes in the activity of the SHannular mode.