Unique current connecting Southern and Indian Oceans identified from radium distributions

We examined the spatial variations in (226)Ra and (228)Ra (activities) concentrations from the surface to a depth of 830 m in the Indian and Southern Oceans from December 2019 to January 2020. (226)Ra concentrations at the surface increased sharply from 30° S to 60° S along a ~ 55° E transect (1.4–2.9 mBq/L), exhibiting small vertical variations, while (228)Ra decreased southward and became depleted in the Southern Ocean. These distributions indicated the ocean-scale northward lateral transport of (226)Ra-rich and (228)Ra-depleted currents originating from the Antarctic Circumpolar Current (ACC). (226)Ra concentrations indicated that the fractions of the ACC at depths of 0–800 m decreased from 0.95 to 0.14 between 60° S and 30° S. The ACC fractions in the subantarctic western Indian Ocean were higher than those previously reported in the eastern Indian region, indicating preferential transport of the ACC. The fractions obtained were approximately equivalent to those in the western Indian Ocean in the 1970s. This could be attributed to the minimal southward shift of the polar front due to global warming over the last 50 years.