[A-CG45] Biogeochemical linkages between the surface ocean and atmosphere

[ACG45-08] Dissolved inorganic radiocarbon as a sensitive tracer for tracking water masses in the upper ocean of the Kuroshio Large Meander Region

*Hui Lan^1,2, Yusuke Yokoyama^1,2,3,4, Shoko Hirabayashi¹, Yosuke Miyairi¹, Siyu Jiang¹, Hiroaki Saito¹, Hiroyasu Hasumi¹, Ichiro Yasuda¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Department of Multi-Disciplinary Sciences, The University of Tokyo, 3.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 4.Research School of Physics, The Australian National University)

Keywords:radiocarbon, dissolved inorganic carbon, Kuroshio large meander, eddy, frontogenesis, water mixing

As a result of the above-ground nuclear bomb tests in the Pacific Ocean in the late 1950s and early 1960s, the level of radiocarbon (¹⁴C) atoms in the atmosphere nearly doubled. This resultant excess ¹⁴C, which is also called the bomb-¹⁴C, is absorbed by the ocean through the air-sea exchange of carbon dioxide (CO₂) as dissolved inorganic carbon (DIC), allowing us to use it as an ideal tracer for studying water mixing processes and biogeochemical cycles. Although the importance of studying dissolved inorganic radiocarbon (Δ¹⁴C-DIC) in ocean circulation research has been recognized for over 50 years, its application in tracking the movement of water masses caused by various physical events remains understudied due to a lack of high-resolution ¹⁴C data.

The Kuroshio is the major northern hemispheric western boundary current that transports an enormous volume of heat and salt northward. It is well known that the Kuroshio in the North Pacific Ocean displays exhibits bimodal behavior to the south of Japan with transitions occurring between the large meander (LM) path and the nonlarge meander (NLM) path. The LM-induced turbulent water mixing processes associated with mesoscale eddies and frontogenesis can pose great influences on local climate, fisheries, marine resources, and ship navigation. Therefore, high-resolution physical and chemical data throughout the upper seawater column need to be obtained to evaluate the impact of the physical events that occurred in the Kuroshio LM region.

In this study, we report the high-resolution Δ¹⁴C values in the upper 200 m seawater collected from the Kuroshio LM region by R/V Hakuho-maru (KH-22-5) to investigate the influence of various physical events triggered by variations of both the shape and position of the Kuroshio. As a result, a water mass with a lower Δ¹⁴C value than surrounding water masses was observed at 100 m depth of station located at the Kuroshio front, which might be caused by the intense near-inertial internal-wave shear that developed during frontogenesis. In addition, the entrainment of coastal water from the Tokara Strait, upwelling and downwelling associated with mesoscale eddies are also considered to be the main factors leading to ¹⁴C variabilities found at the Kuroshio LM. In conclusion, our study reveals that Δ¹⁴C can serve as a sensitive tracer to track water mass movement in the upper ocean, which may advance our understanding of the carbon cycle.