Impact of Subtropical Mode Water Formation Variability on Surface Layer CO2 Chemistry in the Western North Pacific

Deep understanding of contemporary trends, variability and their controlling mechanisms of ocean CO₂ chemistry are crucial in projecting the ocean CO₂ uptake and acidification. In surface layers of the 137°E high-frequency repeat hydrographic section in the Kuroshio recirculation (KR) region of the western North Pacific, salinity-normalized total dissolved inorganic carbon (nDIC) in summer exhibited a significant interannual to decadal variability ranging ±15 μmol kg⁻¹ for the years 1994–2018. It was positively correlated with the thickness of the underlying subtropical mode water (STMW) (±160 m) due to the shallowing/deepening of the upper STMW and negatively with the Pacific Decadal Oscillation (PDO) index when a 3-year lag was applied. The nDIC variability caused large variability in the aragonite saturation index (±0.2), but the impact of nDIC variability on CO₂ partial pressure (pCO₂^sea) and pH were smaller than that of temperature variability that concurrently acted in the opposite direction. In contrast, the variability of nDIC in the winter mixed layer was primarily controlled by the deepening of mixed layer but was also affected by the thickness of the STMW due to the entrainment of upper STMW in which nDIC was lower/higher when the STMW was thicker/thinner. These observations indicate that the changes in the wind forcing in the central North Pacific remotely influence the CO₂ chemistry in surface layers of the KR region through the changes in the formation and advection of the STMW and have different impacts between summer when surface layer is stratified and winter when mixed layer deepens.