The frontal area at the eastern edge of the western equatorial Pacific warm pool in April 2001

The western part of the equatorial Pacific Ocean is characterized, on average, by a relatively warm, fresh, and oligotrophic near‐surface water mass, the warm pool. On its eastern side the warm pool joins cooler, saltier, and richer waters from the equatorial upwelling. Previous works have shown that the boundary between these two water masses is highly variable in zonal location and structure, depending on large‐scale climatic conditions and/or local atmospheric forcing. This paper reports results from an oceanographic cruise carried out in April 2001, dedicated to the study of that boundary. By combining physical and biogeochemical data, the frontal area could unambiguously be found close to 157.5°E, in the far‐western equatorial Pacific, in accordance with La Niña climatic conditions. Some characteristics of the frontal zone were similar to the scarce previous observations: a sharp pCO2 step of about 40 ppm and a variation of NO3 concentration from about 1 μM to complete depletion. Other features were not previously reported, particularly the unexpected lack of a noticeable surface salinity front. Additional data from the Tropical Atmosphere‐Ocean/Triangle Trans‐Ocean Buoy Network (TAO/TRITON) moorings array provide a tentative explanation for that lack of salinity gradient: It appears that at the time of our observations the eastern part of the warm pool presented anomalously high surface salinity, about 0.5 above normal. The local evaporation‐precipitation budget did not explain this high salinity. Rather, it was associated with an anomalous northward extension of the South Pacific high‐salinity tongue in the upper thermocline.