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 (137CsF) from background 137Cs. At most stations, 137CsF 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, 137CsF exhibited two subsurface peaks, with another peak occurring at 500 m depth, corresponding to lighter central mode water (L-CMW). Temporal changes in 137CsF vertical profiles showed that 137CsF entrained by STMW has recirculated within the western subtropical gyre, while 137CsF 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 137CsF entering the SCS during 2013-2019 was 0.33 ± 0.10 PBq, equivalent to 1.7-2.2% of total leakage of 137CsF into the ocean. These results enhance our understanding of the protracted spread and fate of 137CsF in the subtropical WNP.
Keywords: (137)Cs; (90)Sr; Fukushima Dai-ichi nuclear power plant accident; Lighter central mode water; Subtropical mode water.
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