Kuroshio fractions in the southwestern sea of Japan; implications from radium isotopes

To understand complicated current systems, we examined the 228Ra activities and 228Ra/226Ra activity ratios in 290 surface waters in the southwestern Sea of Japan from 2015 to 2020. The 228Ra/226Ra ratios exhibited large seasonal variation, showing higher ratios in July–September (1.5–2.5) than in other months. The fractions of the Kuroshio Current from the Pacific Ocean (fK), based on the 228Ra activities and salinity, in the western part of the study area were lowest in July after the steep decrease in the fK from May to July (0.95–0.6). This is due to the water mixing ratio of the East China Sea continental shelf side waters to the Kuroshio Current, reflecting the seasonal change and local water circulations. •Fine-resolution 228Ra distributions were examined in southwestern Sea of Japan.•228Ra at the surface showed large seasonal variation reflecting Kuroshio fractions.•Kuroshio fractions were 0.6–0.95, showing a steep decrease from May to July.•Kuroshio fractions were tightly coupled to radiocesium concentrations.