The observations collected during a series of three oceanographic cruises dedicated to the study of the frontal zone at the eastern edge of the equatorial Pacific warm pool are used to study the role of salinity in the static stratification of the ocean upper layers. By combining physical and biogeochemical data, the position of the frontal zone at the surface could be accurately determined between 157.5 degrees E and 173.4 degrees E. A multiparameter approach is required due to the uncertainty that may result when one parameter is only considered. In the subsurface the thermohaline structures around the frontal zone reveal the predominant role of the ocean salinity stratification within the isothermal layers. However, the actual methods to determine the salinity barrier layer thickness cannot separate in detail the different dynamical regimes near the frontal zone and another variable to identify the role of the salinity in the static vertical stratification is then proposed and tested on several data sets. This method takes into account the thermal and the saline dependencies in the Brunt-Vaisala frequency in order to isolate the specific role of the salinity stratification in the layers above the main thermocline. The results show that the equatorial western Pacific is characterized by an increasing east-west gradient in salinity stratification whereas the stronger values are found westward of the eastern edge of the warm pool. This diagnostic then provides the basis for an examination of the dynamical mechanisms involved in the formation of the salinity stratification. It also confirms the importance of the ocean salinity in the variability of the equatorial Pacific warm pool. The potentially important implications for the El Nino-Southern Oscillation variability are discussed.