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

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.

Automated summary

Through analyzing the Ra-228 activities and Ra-228/Ra-226 activity ratios in surface waters of the southwestern Sea of Japan from 2015 to 2020, it was found that the Ra-228/Ra-226 ratios showed significant seasonal variation, with higher ratios in the months of July-September. Furthermore, the fractions of the Kuroshio Current from the Pacific Ocean, as determined by Ra-228 activities and salinity, exhibited fluctuations throughout the seasons, influenced by local water circulations.