Zonal displacement of the western equatorial Pacific “fresh pool”

Abstract

The distribution of salt in the tropical Pacific Ocean and its variability are potentially important in better understanding the El Niño Southern Oscillation (ENSO) ocean‐atmosphere coupled system. Complementary data sets (including sea surface salinity (SSS) derived from a ship‐of‐opportunity network, Geosat sea level and derived zonal surface currents, and outgoing longwave radiation‐derived precipitation, evaporation, and cruise measurements along the 165°E longitude) are used to describe and understand SSS changes in the western equatorial Pacific over several ENSO cycles. It is first shown that the 1974–1989 zonal displacements of the eastern edge of the western equatorial Pacific “fresh pool” (SSS < 35), marked by a salinity front and closely related to the eastern edge of the warm pool, were dominated by interannual variations that are highly correlated with the Southern Oscillation Index (SOI). Owing to the availability of basin‐wide Geosat surface current data, it is then shown that the eastward extension of the eastern edge of the fresh pool during the 1986–1987 El Niño, as well as the westward retreat during the 1988–1989 La Niña, were chiefly the result of zonal salt advection in the equatorial band. Although the equatorial upwelling extended toward the western Pacific during La Niña, its effectiveness in changing the near‐surface salinity (and temperature) is found questionable. Bearing in mind the uncertainty in determining open‐ocean precipitation, evaporation, and mixed‐layer depth, it appears that the net freshwater flux probably played a minor role in the zonal displacements of the eastern edge of the fresh pool, as compared to zonal salt advection. The roles of first baroclinic Kelvin and first meridional mode Rossby waves are discussed considering that they account in a large part for zonal advection.