Amplified vertical salinity contrasts in the northwestern tropical Pacific under ocean warming

Abstract

Analyses of vertically layered structures in ocean salinity present a recently amplified vertical contrast in the northwestern tropical Pacific, which has been attributed to the reversal of the long-term linear trend in salinity within the upper ocean from 1960 to 2023. Based on data obtained from both observations (Argo and WOD) and reanalysis (EN4), salinity trends shifted from freshening (-0.04 psu/40 yr) to salinification (+0.04 psu/60 yr) in the near-surface (above 24.6σθ), while subsurface (below 25σθ) freshening further strengthened from -0.03 psu/40 yr to -0.1 psu/60 yr after 2000. The near-surface salinification can be partly explained by atmospheric forcing related to global warming. The anomalous cyclonic wind-induced Ekman suction and wind-driven horizontal salt transport were favorable for increased salinity in upper layers. Nevertheless, the oceanic dynamic forces governed the vertical salinity structure. Under a warming climate, heat influx and warm water accumulation due to diabatic effects play a deterministic role in isopycnal deepening. The changes in salinity evoked by isopycnal changes were investigated from two perspectives: entrainment at the bottom of 24.6σθ and heaving variabilities for the upper layer above 24.6σθ, were primary factors in near-surface salinification. However, the relative significance of heaving variabilities decreased with depth and the major controlling factor became contingent on spiciness variabilities. It is suggested that, owing to a northward migration of the outcrop line, subduction along the path of the geostrophic streamline from the ventilation region, where freshened spiciness anomalies can be injected into subduction surfaces, tended to dominate the significant freshening trend at subsurface isopycnals.