Since August 2017, the large meander (LM) path of the Kuroshio south of Japan has continued to date for more than five years, with a local Kuroshio recirculation gyre off Shikoku (RGOS). The formation and temporal evolution of Subtropical Mode Water (STMW) in the isolated RGOS were examined by using Argo profiling float and shipboard observation data. Since the late winter of 2018, which was the first winter of the current LM event, multiple STMW layers with different densities overlapping with each other have been observed for more than three years inside the RGOS. The deeper STMW layer of 18.0 °C, which was formed in the late winter of 2017 before the current LM event began, has survived at least until September 2021 while its thickness and horizontal extent has decreased gradually. We may have succeeded in discovering the longest survived STMW ever observed due to a suitable environment for STMW conservation inside the RGOS. On the other hand, the shallower STMW layer(s) of 19.0–19.8 °C, which was originally formed over the deeper STMW layer in the late winter of 2018, has been repeatedly renewed or dissipated in the following years. The oxygen utilization rate at the core of the deeper STMW at a depth of 500 dbar was estimated to be 4.6 μmol kg⁻¹ yr⁻¹, which is believed to be explained solely by oxygen consumption due to remineralization.

自 2017 年 8 月以来，日本南部黑潮的大曲流 (LM) 路径已持续五年多，四国附近有当地的黑潮再循环环流 (RGOS)。利用 Argo 剖面浮标和船上观测数据研究了孤立 RGOS 中副热带模式水 (STMW) 的形成和时间演化。自2018年冬末，即当前LM事件的第一个冬季以来，在RGOS内部已经观察到多个不同密度的相互重叠的STMW层长达三年多。更深的 STMW 层（18.0 °C）形成于 2017 年冬末，当前 LM 事件开始之前，至少存活到 2021 年 9 月，但其厚度和水平范围逐渐减小。由于 RGOS 内部有适合 STMW 保存的环境，我们可能成功地发现了迄今为止观察到的存活时间最长的 STMW。另一方面，19.0-19.8 °C 的较浅 STMW 层最初于 2018 年冬末在较深层 STMW 层上形成，在随后的几年中不断更新或消散。在500 dbar深度的较深STMW核心处的氧利用率估计为4.6 μmol kg ^-1  yr ^-1，这被认为仅由再矿化引起的氧消耗来解释。