This paper investigates the variability of sea surface salinity (SSS) in the western equatorial Pacific fresh pool. For this purpose, we processed data collected from thermosalinographs embarked on merchant ships. Two main cross-equatorial shipping lines that are representative of the oceanic conditions in the western tropical Pacific were selected: the Japan-Tarawa-Fiji line that crosses the equator near 173 ¿E (eastern track) and the New-Caledonia-Japan line that crosses the equator near 156 ¿E (western track). We show that there is a strong SSS variability in the region at monthly as well as interannual timescales. This high variability is attributed to the successive passages of a zonal salinity front, trapped in the ⟨5 ¿N--5 ¿S⟩ equatorial band and migrating in phase with the southern oscillation index. We also found the eastern track to be more variable in SSS because it is more exposed to these SSS front incursions. We carried out a detailed study of the mechanisms responsible for this variability; it revealed that the rainfall input acts as a source of freshwater responsible for the existence of a contrasted distribution of SSS (mainly high-salinity waters in the central Pacific and low-salinity waters in the western Pacific). However, the main mechanism responsible for the SSS variability is zonal advection that makes the two distinct masses of water converge, resulting in a salinity front which shifts back and forth in the equatorial band. ¿ 1998 American Geophysical Union