Decadal-scale variability of the North Pacific subtropical mode water (STMW) and its influence on the pycnocline are examined by analyzing Japan Meteorological Agency (JMA) repeat hydrographic observations along the 137°E meridian from 1972 to 2019, with a particular focus on the summer season when the seasonal upper pycnocline develops above the STMW. The STMW appears between 20° and 32°N at 137°E, with the thickness varying on decadal timescales of approximately 9–15 years. Argo float observations suggest that the observed change in the STMW thickness originates in the wintertime mixed layer south of the Kuroshio Extension in the preceding year. The STMW has a substantial impact on the pycnocline. The presence of thick STMW shoals the upper pycnocline, occasionally concurrent with the deepening of the lower main pycnocline. The change is robust in the upper pycnocline, where the heaving of isopycnal surfaces occurs with density anomalies up near the surface. The subtropical front (STF) at subsurface depths, which is associated with a northward shoaling of the upper pycnocline and is maintained by the STMW in the climatology, also changes on decadal timescales. A thick STMW increases the northward shoaling of the upper pycnocline and intensifies the STF. On decadal timescales, the STF variations are accounted for by the STMW-induced change in the upper pycnocline slope. The change in the STF due to mode waters is consistent with previous findings from numerical models.

中文翻译：

---

北太平洋副热带模式水的年代际变化及其对沿 137°E 观测到的气旋的影响

通过分析日本气象厅 (JMA) 1972 年至 2019 年沿 137°E 子午线的重复水文观测，研究了北太平洋副热带模式水 (STMW) 的年代际变化及其对偏斜的影响，其中特别关注的是夏季，当季节性上层斜斜发展在 STMW 之上时。STMW 在 137°E 出现在 20°和 32°N 之间，厚度在大约 9-15 年的十年时间尺度上变化。Argo 浮点观测表明，观测到的 STMW 厚度变化起源于上一年黑潮延伸以南的冬季混合层。STMW 对pycnocline 有重大影响。厚的 STMW 的存在使上部的火山灰斜面形成浅滩，偶尔与下部主要的火山灰斜线的加深同时发生。这种变化在上密克林中很明显，在那里等密度表面的隆起发生在表面附近的密度异常。地下深处的副热带锋 (STF) 与上密苏里向北的浅滩有关，并在气候学中由 STMW 维持，也在年代际时间尺度上发生变化。厚的 STMW 增加了上层密斜的向北浅滩并加强了 STF。在年代际时间尺度上，STF 的变化是由 STMW 引起的上部pycnocline 斜率的变化引起的。由于模式水而导致的 STF 变化与先前的数值模型发现一致。这与上后旋斜向北的浅滩有关，并在气候学中由 STMW 维持，也在年代际时间尺度上发生变化。厚的 STMW 增加了上层密斜的向北浅滩并加强了 STF。在年代际时间尺度上，STF 的变化是由 STMW 引起的上部pycnocline 斜率的变化引起的。由于模式水导致的 STF 变化与先前从数值模型中得到的结果一致。这与上后旋斜向北的浅滩有关，并在气候学中由 STMW 维持，也在年代际时间尺度上发生变化。厚的 STMW 增加了上层密斜的向北浅滩并加强了 STF。在年代际时间尺度上，STF 的变化是由 STMW 引起的上部pycnocline 斜率的变化引起的。由于模式水导致的 STF 变化与先前从数值模型中得到的结果一致。