The seasonal to interannual variability of eddy kinetic energy (EKE) in the Northeastern South China Sea (NE-SCS) has been widely studied and it is recognized that they are strongly related to the state of the Kuroshio pathway in the Luzon Strait. While, due to the lack of long-term observations and high-resolution simulations, the decadal change of EKE in NE-SCS remains unexplored. In this study, we show the EKE trend in the past ∼ 30 years in the NE-SCS by using satellite observation and global HYbrid Coordinate Ocean Model reanalysis with the Navy Coupled Ocean Data Assimilation. It is found that due to the weakening of the Kuroshio in the Luzon Strait since 1990 s, the Kuroshio shows an enhanced looping path in the NE-SCS, inducing stronger EKE in this region. Further analysis confirms that the energy transfer by baroclinic instability is dominant for the increasing of EKE, when the Kuroshio intrudes into the NE-SCS and brings more potential energy inside the circulation. The Kuroshio state along the Luzon Strait is the key for modulating the EKE in the NE-SCS. Furthermore, the long-term weakening of Kuroshio current along the Luzon strait during 1993–2020 is determined by the decreasing of subtropical mode water, corresponding to the positive phase of the Atlantic Multidecadal Oscillation. This study provides insight into the interaction between marginal sea (i.e., the SCS) and the open ocean (i.e., the western Pacific Ocean), finally linking to the global climate change.

中文翻译：

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南海东北海涡动能的长期变化


南海东北部 （NE-SCS） 涡流动能 （EKE） 的季节到年际变化已得到广泛研究，并认识到它们与吕宋海峡黑潮路径的状态密切相关。然而，由于缺乏长期观测和高分辨率模拟，NE-SCS 中 EKE 的年代际变化仍未得到探索。在这项研究中，我们通过使用卫星观测和全球 HYbrid 坐标海洋模型再分析以及海军耦合海洋数据同化，展示了过去 ∼ 30 年在 NE-SCS 的 EKE 趋势。研究发现，由于 1990 年代以来吕宋海峡黑潮的减弱，黑潮在 NE-SCS 中显示出增强的环状路径，在该区域诱导了更强的 EKE。进一步分析证实，当黑潮侵入 NE-SCS 并在环流内带来更多势能时，斜压不稳定性的能量传递是 EKE 增加的主要因素。吕宋海峡沿岸的黑潮状态是调节 NE-SCS EKE 的关键。此外，1993—2020 年吕宋海峡沿岸黑潮的长期减弱是由副热带模态水的减少决定的，对应于大西洋年代际振荡的正相位。这项研究提供了对边缘海（即南海）和公海（即西太平洋）之间相互作用的见解，最终与全球气候变化联系起来。