Salinity interdecadal variability in the western equatorial Pacific and its effects during 1950–2018

Abstract

Ocean reanalysis products are used to examine salinity variability and its relationships with temperature in the western equatorial Pacific during 1950–2018. An ensemble empirical mode decomposition (EEMD) method is adopted to separate salinity and temperature signals at different time scales; a focus is placed on interdecadal component in this study. Pronounced interdecadal variations in salinity are seen in the region, which exhibits persistent and transitional phases in association with temperature. A surface freshening is accompanied by a surface warming during the 1980s and 1990s, but saltening and cooling in the 2000s, with interdecadal shifts occurring around in the late 1970s, late 1990s, and during 2016–2018, respectively. Determined by anomaly signs of temperature and salinity, their combined effects can be density-compensated or density-uncompensated, correspondingly acting to produce density variability that is suppressed or enhanced, respectively. The temperature and salinity effects are phase- and depth-dependent. In the subsurface layers at 200 m, where salinity and temperature anomalies tend to be nearly of the same sign during interdecadal evolution, their effects are mostly density-compensated. The situation is more complicated in the surface layer, where variations in sea surface salinity (SSS) and sea surface temperature (SST) exhibit different signs during interdecadal evolution. SST and SSS tend to be of opposite sign during the persistent phases with their effects being density-uncompensated; but they can be of the same sign during the transitional periods and the corresponding changes in SST and SSS undergo density-compensated relationships. Examples are given for the relationships among these fields which exhibit phase differences in sign transitions in the late 1990s; salinity effects are seen to cause a delay in phase transition of density anomalies. Furthermore, the relative contributions to interdecadal variabilities of density and stratification are quantified. The consequences of interdecadal salinity variability are also discussed in terms of their effects on local SST.