Ring Assistance of Tracer Transport Across the Kuroshio Extension: Observation and Simulation

Tracking the spread of radionuclides following the accident at the Fukushima-Daiichi Nuclear Power Plant (FDNPP) in 2011 and since the start of controlled discharges of purified water in 2023 is important for assessing radiation risks and for understanding water mass transport in the confluence zone of the Kuroshio Extension (KE) and Oyashio currents. It is widely accepted that the powerful KE acts as a nearly impermeable barrier to cross-jet transport (CJT) of tracers. However, FDNPP-derived radiocesium was observed south of the KE front, raising the question of how such transport can occur. A 13-year particle-tracking simulation showed that virtual tracers released near the FDNPP site could cross the KE by moving southward along transport pathways, either due to shifts in the position of KE meanders or via Kuroshio rings. In both scenarios, tracers may end up on the southern side of the KE. CJT occurs when groups of tracers coherently cross the jet over a short period, taking advantage of transient transport corridors. In 2024, virtual FDNPP tracers were identified inside two cyclonic rings south of the KE. Observations from a cruise in June 2024 confirmed that these rings contained core water of subarctic origin. This finding was further supported by altimetry-based and reanalysis data. Both rings were features with diameters of 200–250 km and geostrophic currents reaching up to 160 cm/s. The ring generated by the first meander facilitated rapid CJT within a few days, while the ring formed at the second meander resulted in a longer transport pathway. The study effectively combines long-term numerical particle tracking and the real-time capture of transient mesoscale features with targeted shipboard observations.