Slowdown of Subsurface Freshening in the Southwest Pacific Ocean Since 1990

Abstract The southwest Pacific Ocean (SWPO), located on the western side of the South Pacific subtropical gyre, has exhibited a notable trend of subsurface freshening over the past several decades. Using observation‐based ocean salinity data sets, we find that SWPO subsurface freshening has slowed in the most recent three decades (1990–2020), to a rate less than one‐fourth of that observed during the prior three decades (1960–1990). To understand the drivers of this subsurface salinity variability, we conduct surface forcing perturbation experiments using a global ocean‐sea ice model, specifically targeting the distinct impacts of changes in surface heat, freshwater, and momentum (wind stress) fluxes. Our study reveals that the subsurface freshening slowdown in the SWPO since the 1990s is largely (75%) attributed to a weakened poleward shift of the westerly winds. This shift induces isopycnal deepening, enhanced poleward Ekman transport, and subsequent subduction of saltier subtropical surface waters into the ocean interior. Surface heat fluxes also contribute to these to a lesser extent (37%) via the spiciness component, as surface cooling at high latitudes shifts isopycnal outcrops equatorward, enabling saltier surface waters to be subducted; these effects are partly offset by freshwater fluxes (−12%). Further analysis suggests that these surface flux changes are at least partly linked to a multidecadal shift in the interdecadal Pacific oscillation around 1990. The mechanisms identified in this study strengthen our understanding of how ocean interior salinity responds to large‐scale climate variability and provide insights for better detecting and quantifying the intensification of the global water cycle.