Contrasting the flavors of El Nino-Southern Oscillation using sea surface salinity observations

The recent detection of a central Pacific type of El Nino has added a new dimension to the El Nino-Southern Oscillation climatic puzzle. Sea surface salinity (SSS) observations collected during 1977-2008 in the tropical Pacific are used to contrast the three eastern Pacific (EP) (1982-1983, 1991-1992, 1997-1998) and seven central Pacific (CP) (1977-1978, 1986-1988, 1990-1991, 1992-1995, 2002-2003, 2004-2005, 2006-2007) types of El Nino events, as well as the six EP (1985-1986, 1988-1989, 1995-1996, 1999-2001, 2005-2006, 2007-2008) and two CP (1983-1984, 1998-1999) types of La Nina events. The EP El Nino events result in large (similar to 30 degrees longitude) eastward displacements of the eastern edge of the low-salinity warm pool waters in the equatorial band, a resulting well-marked SSS freshening (similar to-1) near the dateline, and a SSS increase (similar to+1) below the mean position of the South Pacific Convergence Zone (SPCZ). The CP El Nino events are characterized by smaller (50%) eastward displacements of the eastern edge, a similar to 15 degrees longitude westward shift of the equatorial SSS freshening, and a comparatively reduced (similar to 50%) SSS increase in the SPCZ. A qualitative analysis indicates that changes in zonal currents and precipitation can account for the observed contrasted signature in SSS. Eastward current anomalies appear over most of the equatorial band during EP El Nino events. In contrast, there is a tendency for zonal current convergence slightly west of the dateline during CP El Nino events, consistent with the confinement of the warm/fresh pool in the western central equatorial basin, the related quasi-inexistent northeastward migration of the SPCZ, and associated heavy precipitation regime.