Continuous southwestward spread of Fukushima-derived ¹³⁷Cs in the subtropical western North Pacific and its intrusion flux into the South China Sea

We provide transect profiles of ¹³⁷Cs and ⁹⁰Sr along 146.5°E, 136°E and 21°N in the subtropical western North Pacific (WNP) during May 2018. Exploiting the constant global fallout ¹³⁷Cs/⁹⁰Sr ratio, we separated Fukushima-derived ¹³⁷Cs (¹³⁷Cs_F) from background ¹³⁷Cs. At most stations, ¹³⁷Cs_F 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, ¹³⁷Cs_F exhibited two subsurface peaks, with another peak occurring at 500 m depth, corresponding to lighter central mode water (L-CMW). Temporal changes in ¹³⁷Cs_F vertical profiles showed that ¹³⁷Cs_F entrained by STMW has recirculated within the western subtropical gyre, while ¹³⁷Cs_F 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 ¹³⁷Cs increased since 2013 and peaked in approximately 2018. The estimated amount of ¹³⁷Cs_F entering the SCS during 2013–2019 was 0.33 ± 0.10 PBq, equivalent to 1.7–2.2% of total leakage of ¹³⁷Cs_F into the ocean. These results enhance our understanding of the protracted spread and fate of ¹³⁷Cs_F in the subtropical WNP.