A modeling study of salinity variability and its effects in the tropical Pacific Ocean during the 1993–1999 period

Abstract

This paper investigates the mechanisms of near‐surface salinity variability in the tropical Pacific during the 1993–1999 period, including the 1997–1998 El Niño. The role of salinity in the oceanic mean state and variability is also investigated. To this end, a general circulation model is forced by European Remote Sensing (ERS) + Tropical Atmosphere–Ocean (TAO) wind stresses and simple parameterizations of the heat and freshwater fluxes interannual variability. The model correctly reproduces interannual variability of sea surface temperature (SST) and sea surface salinity (SSS). At the peak of the 1997–1998 El Niño, the model and observations show that water fresher than 34.8 psu (usually confined to the western Pacific and close to the South American coast) has invaded the whole equatorial Pacific. This is the combined result of an eastward displacement of the western Pacific fresh pool to 150°W by zonal advection and increased precipitation in the eastern part of the basin. Sensitivity experiments allow us to evaluate the impact of salinity in the equatorial band. The 1993–1996 oceanic mean state is only weakly modified (by less than 0.2°C in SST and 0.05 m s−1 in surface currents) in experiments with homogeneous salinity, but salinity has a larger impact on variability (standard deviation of 0.4°C in SST and 0.15 m s−1 in surface currents). During the 1997–1998 El Niño, salinity effects are the strongest in the vicinity of the eastern edge of the warm and fresh pool. In this region, through its effect on both vertical mixing and horizontal pressure gradients, salinity increases SST by up to 0.8°C and surface currents by up to 0.2 m s−1. Based on these results, it seems advisable to include salinity effects in quantitative studies or forecasts of the El Niño phenomenon.