中国科学院海洋研究所机构知识库

黑潮近岸分支流（the Nearshore Kuroshio Branch Current, NKBC）作为黑潮经台湾东北入侵我国近海重要的分支流，将低温、高盐、富营养盐的黑潮次表层水源源不断的输送到东海近海区域，对我国东海近海区域的水文、生态以及渔业生产产生重要的影响。因此加强对 NKBC 特征变化的研究对我国国计民生具有重要现实意义。目前针对 NKBC 的研究更多关注在台湾东北起源处的三维结构、动力机制以及在其下游（28°N 以北）的路径特征。关于 NKBC 路径（尤其是中游区域）的变化及机制研究较少，缺乏针对 NKBC 流幅和强度特征的研究，对其在台湾东北具体的起源深度也有待量化。

本文基于 2017 年和 2019 年 4 次东海大面观测、长时序卫星高度计资料、再分析风场数据以及构建的覆盖东海陆架及黑潮流域的高分辨率三维模式，针对“NKBC 特征（包括路径、流幅、强度）及起源的变化和动力机制”这一关键科学问题进行了深入研究，得到以下结论：

（1）通过对比 2017 年 9 月和 2019 年 9 月的温盐大面观测数据，发现 2019年 9 月 NKBC 中游的路径相较 2017 年 9 月明显的东向偏移。通过分析大面观测同期的绝对海表动力高度、地转流场以及海表风场的差异，阐述了局地海表风场影响 NKBC 路径的动力机制： 2019 年 8-9 月东海海表盛行强的东北风（与 2017年 8-9 月进行比较），强东北风通过 Ekman 输运促使水体向岸堆积，且在近岸区域向西南堆积。因此 2019 年 8-9 月，受压力梯度分布的影响，东海近岸海域产生西南向的沿岸地转流和离岸地转流，其中西南向的沿岸地转流会在底部生成离岸的底 Ekman 流。 2019 年 9 月离岸底 Ekman 流和离岸地转流共同抑制了 NKBC中游路径向岸移动。

（2）通过 2019 年 5 月和 2019 年 9 月期间东海大面观测获得的温盐数据，发现在 28°N 以北的东海陆架，黑潮分支会分裂为黑潮近岸分支流和离岸分支流，且 2019 年 9 月的 NKBC 与 2019 年 5 月相比较弱。基于 ROMS 模式结果，我们得到了 2019 年东海底部流场的空间分布情况，具体给出了 NKBC 流幅和强度的变化趋势，并在动力机制方面给予解释。动量平衡分析表明， NKBC 的流幅和强度是由地转流和底 Ekman 流决定的。与 2019 年 5 月相比，向岸底 Ekman 流的减弱和离岸地转流的增加共同导致 NKBC流幅在 2019年 9月减弱至消失。同时，2019 年 9 月沿岸地转流的减弱也削弱了 NKBC 的强度。敏感性实验表明，强烈的东北风可以通过增强（减弱） 离岸地转流（向岸底 Ekman 流）将 NKBC 流幅的西（东）边界进一步向东（西）推移。弱台湾海峡流可以通过减弱底 Ekman流， 使 NKBC 流幅的东边界向西移动；而较弱的黑潮主流也可以通过增加离岸地转流， 使 NKBC 流幅的东边界向西移动。此外，强（弱） 东北风、弱（强）台湾海峡流和强（弱）黑潮可以减弱（增强） NKBC 的强度，其中风是主导因素。

（3） 我们在 ROMS 模式基础上叠加拉格朗日粒子追踪方法，以此来定量研究 NKBC 在台湾岛以东的具体起源深度。 2019 年春季 NKBC 在台湾以东起源的深度在 100 m – 450 m 范围内，平均深度约 260 m。通过敏感性实验发现，强（弱）的黑潮会减小（增大） NKBC 的强度， 而强（弱）的台湾海峡流会增大（减小）NKBC 的强度。 另外，强（弱）的黑潮会减小（增大） NKBC 起源的平均深度，而强（弱）的台湾海峡流也会减小（增大） NKBC 起源的平均深度。

The Nearshore Kuroshio Branch Current (NKBC), as an important branch current of the Kuroshio invading our country’s nearshore area through northeastern Taiwan, continuously transports the low-temperature, high-salt, and nutrient-rich Kuroshio subsurface water to the nearshore area of the East China Sea (ECS). It has an important influence on hydrology, ecology and fishery production in the nearshore area of the ECS. Therefore, it is of great practical significance to strengthen the research on the change and characteristics of NKBC for the people's livelihood and economy in China. At present, the study of the NKBC focuses more on its downstream path characteristics, three-dimensional structure and dynamic mechanism at the origin of northeast Taiwan. There are few kinds of research on the changes and mechanisms of the path of the NKBC, especially in the middle reaches. And the research on the flow amplitude and intensity of the NKBC is needed too. In addition, the specific origin depth of the NKBC in northeast Taiwan needs to be quantified.

Based on the four large-scale observations of the ECS in 2017 and 2019, based on the high-resolution three-dimensional model of the ECS and the adjacent sea area, combined with the satellite altimeter data and wind field reanalysis data, this paper makes a deep study on the changes and dynamic mechanism of the path, flow amplitude, intensity and origin of the NKBC, and obtains the following conclusions:

(1) Based on CTD observation data of the East China Sea in September 2017 and September 2019， this study found that the middle paths of the NKBC in September 2019 is more eastward than that in September 2017. By comparing the differences of sea surface height, geostrophic current, and wind during the observation period of the two years, we explained the dynamic mechanism of local sea surface wind field influence NKBC path. The southwestward wind which prevails in the ECS from August to September in 2019 is stronger, compared with that in 2017. The stronger southwestward wind in 2019 pushed the water to accumulate towards more shoreward and southwestward in the nearshore area. Therefore, from August to September 2019, affected by the distribution of pressure gradients, alongshore geostrophic current and offshore geostrophic currents occurred in the coastal waters of the ECS. The stronger offshore geostrophic current and the offshore bottom Ekman current which is induced by the southwestward alongshore geostrophic current jointly restrained the inshore intrusion of the NKBC.

(2) Temperature and salinity data, obtained by two snapshot surveys during 19– 20 May 2019 and 12–25 September 2019 across the ECS shelf, revealed that the Kuroshio intrusion to the north of 28°N comprised the Nearshore Kuroshio Branch Current and the Offshore Kuroshio Branch Current at the bottom of the ECS during spring 2019 and that the NKBC was weak during autumn 2019. Based on the results of the ROMS model, we have obtained the spatial distribution of the flow field at the bottom of the ECS in 2019, specifically given the changing trend of the amplitude and intensity of the NKBC, and explained the dynamic mechanism. Analyses of the momentum balances indicated that the amplitude and the intensity of the NKBC are determined by the combination of the geostrophic flow and bottom Ekman current. In comparison with May 2019, a weakened shoreward bottom Ekman current and increased offshoreward geostrophic flow caused the amplitude of the NKBC to disappear in September 2019. Meanwhile, a diminished northeastward alongshore geostrophic flow in September 2019 also weakened the intensity of the NKBC. Sensitivity experiments indicated that a strong southwestward wind can push the western (eastern) boundary of the NKBC further offshoreward (shoreward) by increasing (decreasing) the offshore geostrophic flow (bottom Ekman current). A weak Taiwan Strait current (TWC) can move the eastern boundary of the NKBC shoreward by decreasing the onshore bottom Ekman current. A weak Kuroshio Current (KC) can move the eastern boundary of the NKBC shoreward by increasing the offshoreward geostrophic flow. Furthermore, a strong (weak) southwestward wind, weak (strong) TWC and strong (weak) KC can diminish (enhance) the intensity of the NKBC. Of the three factors, the wind plays a major role in influencing the NKBC.

(3) We superimpose the Lagrangian particle tracking method on the basis of the ROMS model to quantitatively study the specific origin depth of NKBC east of Taiwan Island. In spring 2019, the NKBC flows east of Taiwan at a depth ranging from 100 m to 450 m, with an average depth of about 260 meters. Through sensitive experiments on the intensity of the Taiwan Strait current and the intensity of the Kuroshio east of Taiwan, this article further concludes that the strong (weak) Kuroshio speed in the east of Taiwan will reduce (increase) the intensity of the NKBC. The strong (weak) Taiwan Strait current will increase (decrease) the intensity of the NKBC. At the same time, the strong (weak) Kuroshio speed in the east of Taiwan will decrease (increase) the average depth of the origin of the NKBC. The strong (weak) Taiwan Strait current will also reduce (increase) the average depth of the origin of the NKBC.