Ocean Acidification

Abstract

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.

Key Points

• Surface layer CO₂ chemistry in the Kuroshio recirculation region in summer showed decadal variability over the past 25 years
• The variability was correlated with subtropical mode water formation and also remotely connected with the Pacific Decadal Oscillation
• In winter, the variability was influenced partially by subtropical mode water formation and dominantly by vertical convection

Plain Language Summary

To project future increases in ocean CO₂ levels and the resulting ocean acidification in response to rising human industrial CO₂ emissions and their mitigation, it is crucial to understand the natural processes that control variability in ocean CO₂ content. In the subtropics of the western North Pacific, to the south of Kuroshio, we found that the dissolved inorganic carbon (DIC) in the surface layer varies with the thickness of the subtropical mode water (STMW). This water mass, which forms to the south of the Kuroshio Extension in winter, is distributed below the surface layer in the studied region. Its volume changes on a decadal scale in association with the Pacific Decadal Oscillation, exhibiting impacts on surface layer DIC in different manners between summer and winter. In summer, the DIC content increased when the STMW was thicker due to the increased supply of DIC from the subsurface layer to the surface. In winter, the DIC content in the surface layer increased as the winter mixed layer developed deeper while it was also partially influenced by STMW thickness.

Ono H., Toyama K., Enyo K., Iida Y., Sasano D., Oka E. & Ishii M., 2025. Impact of subtropical mode water formation variability on surface layer CO₂ chemistry in the western North Pacific. Journal of Geophysical Research: Oceans 130(5): e2024JC021748. doi: 10.1029/2024JC021748. Article (subscription required).