Sea surface salinity (SSS) and temperature (SST) data collected from voluntary observing ships over 25 years (1976--2000) are analyzed in the Southwestern Tropical Pacific (10¿S--24¿S/160¿E--140¿W). This region lies under the South Pacific Convergence Zone (SPCZ), at the southern edge of the western Pacific warm pool between Tahiti and Darwin, the two places whose atmospheric sea level pressure difference is used to define the Southern Oscillation Index (SOI). Complementary data such as precipitation are used to assist in the analysis. The mean and seasonal variations of these parameters are described. An empirical orthogonal function (EOF) analysis of low-pass filtered time series is then performed to extract the interannual variability. All parameters show an interannual signal which correlates well with the SOI. The Southwestern Tropical Pacific Ocean is saltier and colder during El Ni¿o than during La Ni¿a events. In the southwestern part, there is a shortage (excess) in precipitation during El Ni¿o (La Ni¿a) events. The greatest anomalies appeared during the last La Ni¿a, in 1996 and 1999 as regards SST and in 1999 and 2000 as regards SSS. SST and precipitation El Ni¿o--Southern Oscillation (ENSO)-related anomalies are an order of magnitude smaller than seasonal anomalies, while the SSS ENSO-related signal is twice as strong as the seasonal signal. These facts reflect the northeastward (southwestward) shift of the SPCZ during El Ni¿o (La Ni¿a) events. While consistent with precipitation changes, the ENSO-related variability in SSS can also be partly explained by the displacement of the salinity front that separates fresh, warm pool waters from salty subtropical waters. Computation of surface geostrophic current anomalies from Geosat (1987--1988) and TOPEX/Poseidon (1993--2000) indicates that westward current anomalies developed during the 1987/88 and 1997/98 El Ni¿o and are linked to the displacement of the salinity front. The Southwestern Tropical Pacific salinity front moves westward (eastward) in contrast to the equatorial salinity front which moves eastward (westward) during an El Ni¿o (La Ni¿a) event.