SSS changes in the tropical Pacific during ENSO events

Numerous past studies indicate that notable changes in sea-surface salinity (SSS) in the western Pacific warm pool and in the South Pacific Convergence Zone occur at the interannual time scale, in relation to ENSO events (1).

This is illustrated below for the western Pacific warm pool in Figure 1 showing the interannual anomalies in SSS averaged over a key area versus the well-known Southern Oscillation Index (SOI) which record the occurrence of El Niño (SOI<0) and La Niña (SOI>0) events. It clearly shows negative SSS anomalies during El Niño events, mostly reflecting the eastward displacements of the low-salinity western Pacific warm pool, and vice versa during La Niña events (2). The correlation coefficient between the two low-pass filtered monthly time series is R=0.88 over 1971-2009, with the SOI leading by one month, therefore stressing that the SSS time series in that specific region can be used as a relevant climatic index for ENSO. More details about the use of SSS time series to monitor and analyse ENSO-related changes can be found in (3).

Figure 1. (top) Time series of the interannual SSS anomalies (SSSA, blue curve, left vertical scale) and Southern Oscillation Index (SOI, red curve, right vertical scale), and (bottom) normalized SSS errors (N-SSSError). The SSS anomalies and errors are averaged in the western equatorial Pacific, between 150°E-170°W and 2°S-2°N. The SSSA values are relative to the 1971-2000 mean monthly climatology. The SSSA, SOI and N-SSSError values are smoothed using a 25-months Hanning filter for clarity. Note that the relatively weaker agreement between the SSSA and the SOI before the 1970’s corresponds to the higher values of the normalized SSS errors due to a poor SSS data density in the 1950’s and 1960’s. The SSS values and the corresponding errors are extracted from our SSS gridded product (4) and can be downloaded from here. Adapted from (5).

1 e.g., Delcroix and Hénin, J. Geophys. Res., 1991; Gouriou and Delcroix, J. Geophys. Res., 2002.
2 Delcroix and Picaut, J. Geophys ; Res., 1998; Picaut et al., J. Geophys. Res., 2001.
3 Singh et al., J. Geophys. Res., 2011.
4 Delcroix et al., Deep Sea. Res., 2011.
5 Singh and Delcroix, Geophys. Res. Let., 2011.