Remotely Forced Decadal Physical and Biogeochemical Variability of North Pacific Subtropical Mode Water Over the Last 40 Years

Half‐century‐long observations at the 137°E repeat hydrographic section across the western North Pacific have been analyzed to demonstrate remotely forced decadal physical and biogeochemical variability of Subtropical Mode Water (STMW) over the last 40 years. During unstable periods of the Kuroshio Extension (KE) that lagged the warm phase of the Pacific Decadal Oscillation by 3–4 years, high regional eddy activity reduced the formation rate and salinity of STMW in its main formation region south of the KE. At the 137°E section south of Japan, decreasing southwestward advection of oxygen‐rich STMW from the formation region resulted in decreases of its cross‐sectional area, dissolved oxygen, pH, and aragonite saturation state and increases of nutrients and dissolved inorganic carbon, among which changes of the carbon system parameters accelerated their long‐term trends. Such changes reversed and acidification slowed down during stable‐KE periods, especially in the current period since 2010 exhibiting a hiatus of acidification. Plain Language Summary Voluminous water with a temperature of 16–18 °C, called Subtropical Mode Water (STMW), is formed south of the Kuroshio Extension through deep winter convection. STMW is then subducted into the ocean interior and spreads southwestward over a wide area, transporting heat, salt, and chemical substances. The long‐term variability of STMW plays an important role in climate and material cycles but is not well understood. By analyzing half‐century‐long shipboard observations south of Japan, we have found clear decadal variability in physical and biogeochemical properties of STMW over the last 40 years. This decadal variability is associated with that of the Kuroshio Extension, which in turn is remotely controlled by winds over the central North Pacific. Through these processes, stronger (weaker) Aleutian Low accelerates (slows down) acidification in the ocean interior near the Okinawa Islands known for its coral reefs with a time lag of several years. Key Points The formation of Subtropical Mode Water has fluctuated decadally in relation to the Kuroshio Extension variability over the last 40 years The resultant decadal variability in the Water's southwestward advection has altered biogeochemistry in the downstream region south of Japan Such multidisciplinary oceanic variability is remotely forced by winds over the central North Pacific with a lag of several years