Asymmetry of Salinity Variability in the Tropical Pacific during Interdecadal Pacific Oscillation Phases

It has been recognized that salinity variability in the tropical Pacific is closely related to the Interdecadal Pacific Oscillation (IPO). Here, we use model simulations from 1900 to 2017 to illustrate obvious asymmetries of salinity variability in the tropical Pacific during positive and negative IPO phases. The amplitude of salinity variability in the tropical Pacific during positive IPO phases is larger than that during negative IPO phases, with a more westward shift of a large Sea Surface Salinity (SSS) anomaly along the equator. Salinity budget analyses show that the asymmetry of salinity variability during positive and negative IPO phases is dominated by the difference in the surface forcing associated with the freshwater flux [FWF, precipitation (P) minus evaporation (E)], with a contribution of 40%-50% near the dateline on the equator. Moreover, the relationships between the salinity variability and its budget terms also show differences in their lead-lag correlations during positive and negative IPO phases. These differences in salinity variability during different IPO phases produce asymmetric effects on seawater density which can reduce or enhance upper-ocean stratification. Therefore, the salinity effects may modulate the intensity of El Nino-Southern Oscillation (ENSO), resulting in an enhanced (reduced) El Nino but a reduced (enhanced) La Nina during positive (negative) IPO phases by 1.6 degrees C psu(-1) (1.3 degrees C psu(-1)), respectively. It is suggested that the asymmetry of salinity variability may be related to the recent change in ENSO amplitude associated with the IPO, which can help elucidate ENSO diversity.

Automated summary

Model simulations from 1900 to 2017 reveal asymmetric salinity variability in the tropical Pacific during positive and negative Interdecadal Pacific Oscillation (IPO) phases. Positive IPO phases exhibit larger salinity variability with a westward shift of Sea Surface Salinity (SSS) anomaly along the equator. Surface forcing associated with freshwater flux is the dominant factor contributing to the asymmetry of salinity variability during IPO phases. These differences in salinity variability have an impact on seawater density and can modulate the intensity of El Nino-Southern Oscillation (ENSO).