Future Reduction of North Pacific Subtropical Mode Water: Its Impacts on Physical and Biogeochemical Conditions in the North Pacific Subtropical Gyre

Abstract

North Pacific Subtropical Mode Water (STMW) is one of the largest ventilated water masses in the world's oceans, with a thickness of a few hundred meters, occupying a broad region over the North Pacific subtropical gyre. Formed in the deep winter mixed layer south of the Kuroshio Extension, STMW transports heat, oxygen, and dissolved carbon into the subsurface ocean and plays an important role in regulating physical and biogeochemical conditions. Compiling historical observations since 1965, we show that STMW thickness has already decreased by approximately 14%. Using high-resolution future ocean projections, we further show that this decline will continue throughout the 21st century as winter mixed layers in the STMW formation region become progressively shallower; STMW thickness decreases by approximately 46% under RCP8.5% and 16% under RCP2.6 by the end of the century, accompanied by substantial reductions in STMW subduction. Despite enhanced oceanic CO₂ uptake, the amount of dissolved inorganic carbon (DIC) transported into the subsurface ocean decreases because reduced STMW subduction outweighs the increase in DIC concentrations. STMW warms by approximately 2.7°C under RCP8.5 and 1.2°C under RCP2.6, roughly twice the warming elsewhere in the North Pacific, contributing to a substantial decline in subsurface dissolved oxygen through reduced oxygen solubility. The associated subsurface warming reduces subsurface density, suppresses the strengthening of upper-ocean stratification, and thereby mitigates the increase in seasonal sea surface temperature variability. These results suggest that future changes in STMW will substantially alter physical and biogeochemical conditions over the North Pacific subtropical gyre.