Estimating the effects of ENSO upon the observed freshening trends of the western tropical Pacific Ocean

A significant surface freshening trend and an eastward expansion of fresh surface waters have been documented in the western tropical Pacific, consistent with the expected effects of climate change. The highest El Nino Southern Oscillation (ENSO) variability in Sea Surface Salinity (SSS) has been also documented in that region, with different quantitative signatures for the Eastern and Central Pacific ENSO events (EP and CP ENSO, respectively). This study hence analyses to what extent have the EP and CP ENSO events contributed to the long-term freshening trends, relying on 1955-2008 in situ SSS data and on EP and CP ENSO main features. We show the influence of EP ENSO events to be negligible, while CP El Nino events contribute to enhance the long-term freshening trend (up to 30%) in the far western equatorial Pacific and moderately reduce that freshening (up to 10%) in the South Pacific Convergence Zone (SPCZ). Our results thus suggest that the observed eastward expansion of the surface covered by low-salinity waters in the western half of the tropical Pacific is mostly due to climate change rather than to the documented possible increased occurrence and intensity of CP El Nino events. The sensitivity of the trend estimates to the different periodicity of the SSS data records is also discussed. Citation: Singh, A., and T. Delcroix (2011), Estimating the effects of ENSO upon the observed freshening trends of the western tropical Pacific Ocean, Geophys. Res. Lett., 38, L21607, doi: 10.1029/2011GL049636.