Research papersMigration area of the Tsushima Warm Current Branches within the Sea of Japan: Implications from transport of 228Ra

- Surface distribution of 228Ra in Sea of Japan in June-October was examined.
- Surface 228Ra activity (228Ra/226Ra ratio) ranged from 0.8 to 4.3 mBq/L (0.5-2.3).
- Lateral 228Ra profile clarified migration area of the Tsushima Warm Current.
- Fraction of continental shelf water showing lower values in June-July was 8-25%.

We investigated lateral profiles of 228Ra (half-life; 5.75 years) activity and 228Ra/226Ra (1600 years) activity ratio using 241 surface water samples collected in/around the Sea of Japan and the East China Sea (ECS) during June-October of 2009-2014. In the ECS, the 228Ra/226Ra ratio in the surface waters exhibited markedly wide variation (<0.05-3.5) in June, predominantly reflecting the mixing between the 228Ra-rich continental shelf water and the 228Ra-depleted Kuroshio Current water. In July, the surface waters of the central Sea of Japan (135-138°E) became separated into three currents: the Offshore Branch of the Tsushima Warm Current (OBTWC) (228Ra/226Ra =0.7-1.2) at 39-41°N, the Coastal Branch of the TWC (CBTWC) (~0.7) on the southern side, and sub-Arctic Current (~0.7) on the northern side. From the central to northeastern Sea of Japan, the 228Ra/226Ra ratio at the surface (0.8-1.0) was within a range between that of the CBTWC and OBTWC. The fraction of continental shelf water in the CBTWC, OBTWC, and in their combined current was estimated to be 11-16%, ~8%, and 10-11%, respectively.