Decadal variability of Subtropical Mode Water subduction and its impact on biogeochemistry
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Temperature and salinity data from Argo profiling floats during 2005–2014 were analyzed to examine the decadal variability of the North Pacific Subtropical Mode Water (STMW) in relation to that of the Kuroshio Extension (KE) system. The formation volume of STMW in the southern recirculation gyre of KE in the cooling season was larger during the stable KE period after 2010 than the unstable KE period of 2006–2009 by 50 %. As a result, the volume and spatial extent of STMW increased (decreased) in the formation region during the stable (unstable) KE period, as well as in the southern, downstream region with a time lag of 1–2 years. The decadal expansion and contraction of STMW were also detected by shipboard observations conducted routinely in the most downstream region near the western boundary, in terms of not only physical, but also biogeochemical parameters. After 2010, enhanced subduction of STMW consistently increased dissolved oxygen, pH, and aragonite saturation state and decreased potential vorticity, apparent oxygen utilization, nitrate, and dissolved inorganic carbon, among which changes of dissolved inorganic carbon, pH, and aragonite saturation state were against their long-term trends. These results indicate a new mechanism consisting of westward sea surface height anomaly propagation, the KE state transition, and the STMW formation and subduction, by which the climate variability affects physical and biogeochemical structures in the ocean’s interior and potentially impacts the surface ocean acidification trend and biological production.
KeywordsNorth Pacific Subtropical Mode Water Subduction Decadal variability Kuroshio Extension Biogeochemistry
The authors are grateful to the captain, crew, and scientists of R/V Chofu-maru, R/V Keifu-maru, and R/V Ryofu-maru of JMA for their efforts in long-term observations. They also thank Kanako Sato and Kana Nakamoto for their assistance in preparing the Argo float data and Yosuke Iida, Atsushi Kojima, Tomoyuki Kitamura, participants at the 2014 HYARC symposium on air–sea interaction, and two anonymous reviewers for helpful comments on the manuscript. E.O. and T.S. acknowledge support by JSPS through Grant 21340133 and 25287118 and MEXT through 22106007. E.O. is also supported by MEXT through 25121502. B.Q. acknowledges support by NASA through Grant NNX13AE51G and NSF through OCE-0926594. D.S. acknowledges support by JSPS through 24241010. M.I. acknowledges support by MRI’s key research fund C3 and MEXT through 24121003.
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