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 Cs-137 and Sr-90 along 146.5(degrees)E, 136(degrees)E and 21N in the subtropical western North Pacific (WNP) during May 2018. Exploiting the constant global fallout Cs-137/Sr-90 ratio, we separated Fukushimaderived Cs-137 ((CsF)-Cs-137) from background (13)7Cs. At most stations, (CsF)-Cs-137 exhibited only one subsurface peak at 300 m depth, corresponding to subtropical mode water (STMW); however, at 25-28N along 146.5(degrees)E and 25-26N along 136(degrees)E, (CsF)-Cs-137 exhibited two subsurface peaks, with another peak occurring at 500 m depth, corresponding to lighter central mode water (L-CMW). Temporal changes in (CsF)-Cs-137 vertical profiles showed that (CsF)-Cs-137 entrained by STMW has recirculated within the western subtropical gyre, while (CsF)-Cs-137 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-137 increased since 2013 and peaked in approximately 2018. The estimated amount of (CsF)-Cs-137 entering the SCS during 2013-2019 was 0.33 +/- 0.10 PBq, equivalent to 1.7-2.2% of total leakage of (CsF)-Cs-137 into the ocean. These results enhance our understanding of the protracted spread and fate of (CsF)-Cs-137 in the subtropical WNP.