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

中文翻译：

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2011年至2018年黑潮-亲潮交汇区福岛衍生137C垂直剖面的时空变化：对当地水体动力学和盆地尺度环流的影响


追踪福岛衍生铯 (Cs) 的扩散过程有助于更好地了解北太平洋水动态。本研究于2018年5月调查了黑潮-亲潮交汇区Cs和Sr的垂直分布，并利用恒定的全球沉降Cs/Sr比将Cs从背景Cs中分离出来。在北纬35°以北，Cs在上层100 m处达到峰值，而在黑潮延伸带（KE）及其以南，Cs在300-500 m深度处呈现出地下峰值。 T/S图表明，2018年5月，Cs极大值主要分布在较轻的中心模式水（L-CMW）范围内，甚至在KE及其南部也有分布。我们发现反气旋（气旋）涡流可以促进（阻止）Cs侵入海洋内部。此外，KE上游区域反气旋涡的高活动性导致富铯副热带模式水（STMW）转变为L-CMW。 Cs垂直剖面和库存的时间变化表明，过渡区和亚北极地区的Cs自2014年7月以来有所增加，这意味着2014年7月后存在额外的Cs来源，而KE内和南部的Cs自2014年7月以来一直保持不变，表明 STMW 夹带的 Cs 已在西副热带环流中再循环。过渡带和亚北极地区表面 Cs 浓度与 2018 年亲潮水域观测到的 Cs 浓度之间的比较并不支持 Cs 在 2018 年 5 月之前通过西部或整个亚北极环流返回到我们的研究区域。 相比之下，2016-2017年的海面高度分布提供了明确的证据，表明黑潮和KE产生的暖核环和准静止矶口西急流侵入过渡区甚至亚北极地区。因此，我们得出结论，2011-2012年期间俯冲到北太平洋副热带西部的部分Cs自2016年以来已进入过渡带甚至亚北极地区。这些结果不仅增强了我们对Cs的长期扩散和命运的认识不仅在北太平洋，而且还为北太平洋水团循环和混合模式提供了重要见解。