The thermohaline variability of the western tropical Pacific at an El Ni¿o-Southern Oscillation (ENSO) timescale is investigated for the 1995--1998 ENSO period along three transects, relying mostly on thermosalinograph and expandable bathythermograph data distributed along three cross-equatorial shipping lines. The water masses crossing these transects are studied chronologically by using estimates of wind forcing and precipitation. Zonal advection tends to shift eastward (westward) the fresh pool of the western Pacific and the zonal salinity front at its eastern edge during an El Ni¿o (a La Ni¿a) event. Because of increased wind stress at the beginning of an El Ni¿o episode, the fresh pool can be destroyed by upwelling of saltier water and increased evaporation. It is, however, reconstructed further on its way to the east, by more favorable wind and precipitation conditions. This erosion/reconstruction process is then discussed in relation to ENSO development. ¿ 2001 American Geophysical Union