Continuous southwestward spread of Fukushima-derived Cs in the subtropical western North Pacific and its intrusion flux into the South China Sea.

We provide transect profiles of Cs and Sr along 146.5°E, 136°E and 21°N in the subtropical western North Pacific (WNP) during May 2018. Exploiting the constant global fallout Cs/Sr ratio, we separated Fukushima-derived Cs (Cs) from background Cs. At most stations, Cs exhibited only one subsurface peak at 300 m depth, corresponding to subtropical mode water (STMW); however, at 25-28°N along 146.5°E and 25-26°N along 136°E, Cs exhibited two subsurface peaks, with another peak occurring at 500 m depth, corresponding to lighter central mode water (L-CMW). Temporal changes in Cs vertical profiles showed that Cs entrained by STMW has recirculated within the western subtropical gyre, while Cs entrained by L-CMW has turned southwestward and arrived the western basin in 2018. In the Luzon Strait, the entrance to the South China Sea (SCS), subsurface Cs increased since 2013 and peaked in approximately 2018. The estimated amount of Cs entering the SCS during 2013-2019 was 0.33 ± 0.10 PBq, equivalent to 1.7-2.2% of total leakage of Cs into the ocean. These results enhance our understanding of the protracted spread and fate of Cs in the subtropical WNP.