Salinity effect-induced ENSO amplitude modulation in association with the interdecadal Pacific Oscillation

A 110-year ensemble simulation of an ocean general circulation model (OGCM) was analyzed to identify the modulation of salinity interdecadal variability on El Niño-Southern Oscillation (ENSO) amplitude in the tropical Pacific during 1901–2010. The simulating results show that sea surface salinity (SSS) variation in the region exhibits notable and coherent interdecadal variability signal, which is closely associated with the Interdecadal Pacific Oscillation (IPO). As salinity increases or reduces, the SSS modulations on ENSO amplitude during its warm/cold events vary asymmetrically with positive/negative IPO phases. Physically, salinity interdecadal variability can enhance or reduce ENSO-related conditions in upper-ocean stratification, contributing noticeably to ENSO variability. Salinity anomalies associated with the mixed layer depth and barrier layer thickness can modulate ENSO amplitude during positive and negative IPO phases, resulting in the asymmetry of sea surface temperature (SST) anomaly in the tropical Pacific. During positive IPO phases, SSS interdecadal variability contributes positively to El Niño amplitude but negatively to La Niña amplitude by enhancing or reducing SSS interannual variability, and vice versa during negative IPO phases. Quantitatively, the results indicate that the modulation of the ENSO amplitude by the SSS interdecadal variability is −15%–28% during negative IPO phases and −30%–20% during positive IPO phases, respectively. Evidently, the SSS interdecadal variability associated with IPO and its modulation on ENSO amplitude in the tropical Pacific are among factors essentially contributing ENSO diversity.