Spatiotemporal variations in vertical profiles of Fukushima-derived 137Cs in the Kuroshio-Oyashio confluence region from 2011 to 2018: Implications for local water mass dynamics and basin-scale circulations

Abstract

Tracking the processes of the spread of Fukushima-derived 137Cs (137CsF) contributes to a better understanding of North Pacific water dynamics. In this study, the vertical distributions of 137Cs and 90Sr in the Kuroshio-Oyashio confluence region were investigated in May 2018, and 137CsF was separated from the background 137Cs by exploiting the constant global fallout 137Cs/90Sr ratio. To the north of 35°N, 137CsF peaked in the upper 100 m layer, whereas in and just south of the Kuroshio Extension (KE), 137CsF exhibited subsurface peaks at depths of 300–500 m. The T/S diagram indicated that the 137CsF maxima were distributed mainly within the range of lighter central mode water (L-CMW) during May 2018, even in and just south of the KE. We found that anticyclonic (cyclonic) eddies can promote (prevent) the intrusion of 137CsF into the ocean interior. In addition, the high activity of regional anticyclonic eddies in the upstream KE resulted in the modification of 137CsF-rich subtropical mode water (STMW) to L-CMW. Temporal changes in the 137CsF vertical profiles and inventories revealed that 137CsF in transitional and subarctic regions has increased since July 2014, implying the existence of additional sources of 137CsF after July 2014, whereas 137CsF in and just south of the KE has remained constant since July 2014, indicating that the 137CsF entrained by STMW has recirculated in the western subtropical gyre. The comparison between surface 137CsF concentrations in transitional and subarctic regions and those observed in Oyashio waters during 2018 did not support the return of 137CsF to our study area via the western or whole subarctic gyre by May 2018. In contrast, the sea surface height distributions from 2016 to 2017 provide clear evidence that the warm-core rings and quasistationary Isoguchi western jet generated from the Kuroshio Current and KE intruded into the transitional region and even into the subarctic region. Therefore, we concluded that a portion of the 137CsF that subducted into the subtropical western North Pacific during 2011–2012 have entered the transition zone and even the subarctic region since 2016. These results not only enhance our understanding of the protracted spread and fate of 137CsF in the North Pacific but also provide important insights into North Pacific water mass circulation and mixing patterns.