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

Tracking the processes of the spread of Fukushima-derived ¹³⁷Cs (¹³⁷Cs_F) 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 ¹³⁷Cs_F was separated from the background ¹³⁷Cs by exploiting the constant global fallout ¹³⁷Cs/⁹⁰Sr ratio. To the north of 35°N, ¹³⁷Cs_F peaked in the upper 100 m layer, whereas in and just south of the Kuroshio Extension (KE), ¹³⁷Cs_F exhibited subsurface peaks at depths of 300–500 m. The T/S diagram indicated that the ¹³⁷Cs_F 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 ¹³⁷Cs_F into the ocean interior. In addition, the high activity of regional anticyclonic eddies in the upstream KE resulted in the modification of ¹³⁷Cs_F-rich subtropical mode water (STMW) to L-CMW. Temporal changes in the ¹³⁷Cs_F vertical profiles and inventories revealed that ¹³⁷Cs_F in transitional and subarctic regions has increased since July 2014, implying the existence of additional sources of ¹³⁷Cs_F after July 2014, whereas ¹³⁷Cs_F in and just south of the KE has remained constant since July 2014, indicating that the ¹³⁷Cs_F entrained by STMW has recirculated in the western subtropical gyre. The comparison between surface ¹³⁷Cs_F concentrations in transitional and subarctic regions and those observed in Oyashio waters during 2018 did not support the return of ¹³⁷Cs_F 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 ¹³⁷Cs_F 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 ¹³⁷Cs_F in the North Pacific but also provide important insights into North Pacific water mass circulation and mixing patterns.