OBSERVATIONS OF SEA-SURFACE SALINITY IN THE WESTERN PACIFIC FRESH POOL - LARGE-SCALE CHANGES IN 1992-1995

This paper investigates the variability of sea surface salinity (SSS) in the western equatorial Pacific fresh pool. For this purpose, we pro cessed data collected from thermosalinographs embarked on merchant shi ps. Two main cross-equatorial shipping lines that are representative o f the oceanic conditions in the western tropical Pacific were selected : the Japan-Tarawa-Fiji line that crosses the equator near 173 degrees E (eastern track) and the New-Caledonia-Japan line that crosses the e quator near 156 degrees E (western track). We show that there is a str ong SSS variability in the region at monthly as well as interannual ti mescales. This high variability is attributed to the successive passag es of a zonal salinity front, trapped in the [5 degrees N-5 degrees S] equatorial band and migrating in phase with the southern oscillation index. We also found the eastern track to be more variable in SSS beca use 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 respo nsible for the existence of a contrasted distribution of SSS (mainly h igh-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.