Spatio-temporal variability of the equatorial front in the eastern tropical Pacific from remote sensing salinity data (2010–2015)

In the eastern Pacific, the Equatorial Front (EF) separates the relatively cold and salty Equatorial Surface Water to the South from the relatively warm and fresh Surface Tropical Water to the North. Although previous studies have succinctly described the main EF characteristics, little is known about its spatial pattern and temporal variability beyond seasonal scale. In this study, we used 5 years of remote-sensing sea surface salinity (SSS) data, acquired by SMOS in a synoptic and continuous manner, to describe the spatio-temporal variability of the haline EF (HEF). After validating the SSS satellite dataset with concomitant in situ data, we developed an algorithm based on maximum SSS gradients to automatically identify the HEF position on every SMOS SSS map. This algorithm applied to the 4-day resolution SMOS SSS product, allows us to investigate the spatio-temporal variability of the HEF between 2010 and 2015. Seasonally, west of the Galapagos Islands, the HEF is centered at ~2°N and shows relatively weak meridional displacements. In contrast, east of the Galapagos Archipelago, the HEF varies on average between ~4°S in spring and ~2°N in autumn. At intraseasonal scales, the HEF shows meridional fluctuations of ±1.25° around its average position. These intraseasonal fluctuations are partly related to Kelvin Waves, which was more noticeable since 2014. Over the 5-year studied period, the HEF position showed a northward trend of 0.3–0.4 latitudinal degrees per year between 2010 and 2015. However, the relatively short time series does not permit to investigate whether this low-frequency trend is due to long-term variability or interdecadal fluctuations.