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

摘要

To understand complicated current systems, we examined the Ra-228 activities and Ra-228/Ra-226 activity ratios in 290 surface waters in the southwestern Sea of Japan from 2015 to 2020. The Ra-228/Ra-226 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 (f(K)), based on the Ra-228 activities and salinity, in the western part of the study area were lowest in July after the steep decrease in the f(K) 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.