海洋环流与波动重点实验室知识库

台湾东北附近海域是黑潮与东海陆架水体进行水交换最活跃和最关键的海区，同时也是东海陆架上多支黑潮分支流的起源地。台湾东北黑潮跨陆坡入侵东海陆架的黑潮水体对东海的海洋环流结构、营养盐输入、海洋生物资源以及近海赤潮灾害等海洋环境的诸多方面均有十分重要的影响。因此，研究台湾东北附近海域的流场结构和变化规律对理解和预测东海海洋环境的变化具有重要的科学意义和实际应用价值。伴随黑潮入侵东海陆架的显著过程，台湾东北附近海域形成了许多独特的海洋现象，这些伴生海洋现象的生消及其多尺度时空变化使得台湾东北附近海域的流场结构十分复杂多变。本论文针对台湾东北黑潮入侵东海陆架的流场结构及其变化规律的难点，集中开展了对台湾东北陆坡逆流这一关键海洋现象的深入研究，主要探讨了陆坡逆流的多尺度时空变化特征，陆坡逆流与台湾东北黑潮跨陆坡入侵陆架的关系及相关动力机制等科学问题，取得以下主要研究成果。

1. 本文利用台湾东北陆坡逆流的历史流速观测数据、近期的ADCP连续观测数据，并结合卫星高度计数据和HYCOM数据等多种海洋数据资料分析总结了台湾东北陆坡逆流的时空变化特征，数据分析结果表明台湾东北陆坡逆流为局地次表层的流速结构。2017年5月– 2018年5月期间，观测站点处记录的逆流核心深度约为200–400 m，沿陆坡方向平均流速剖面的西南向最大流速为19.0 cm/s，全矢量平均流速剖面的西南向最大流速为24.2 cm/s。此外，台湾东北陆坡逆流还存在显著的季节、季节内和短期变化特征。
2. 基于对台湾东北局地流场结构，特别是陆坡逆流时空特征的认识和总结，本文明确提出台湾东北陆坡逆流在棉花峡谷附近存在显著西向入侵东海陆架的现象。利用HYCOM数据资料粗略估算的陆坡逆流在棉花峡谷通道的次表层西向入侵东海陆架的水体输运量约为0.52 Sv。本文还利用区域海洋数值模式ROMS对台湾东北陆坡逆流及其在棉花峡谷附近显著西向入侵陆架的过程进行了数值模拟，并引入地形β螺旋理论合理地解释了这一现象发生的原因。
3. 揭示并阐述了台湾东北陆坡逆流的垂直迁移与台湾东北黑潮跨陆坡入侵东海陆架变化之间的线性关系及两者紧密关联的动力机制。陆坡逆流增强（减弱）并抬升（下沉）则黑潮在台湾东北跨西部陆坡入侵陆架的强度减弱（增强），而跨东部陆坡入侵陆架的强度增强（减弱）。台湾东北陆坡逆流的垂直迁移主要反映了台湾东北陆坡附近水体层结的起伏变化，而陆坡附近水体层结的变化则决定了台湾东北陆坡附近跨陆坡方向位势涡度梯度强弱的变化，进而导致台湾东北黑潮跨陆坡入侵东海陆架的强度发生相应变化。
4. 本文基于前人的研究结果并结合台湾东北陆坡逆流和黑潮入侵东海陆架的时空变化特征给出了台湾东北附近海域黑潮跨陆坡入侵东海陆架的三条主要路径示意图。该示意图充分考虑了台湾东北次表层全年存在的陆坡逆流，通过台湾东北陆坡逆流可以将台湾东北三维流场结构及其多尺度变化特征联系起来，为理解台湾东北流场的时空变化特征提供新的视角和观点。

本论文针对台湾东北次表层陆坡逆流的基础性研究工作为理解台湾东北局地三维流场结构、台湾东北黑潮入侵东海陆架的方式及其变化规律提供了新的视角和观点，有助于将来进一步了解东海陆架上黑潮分支流在其源地的产生过程、并理解它们发生变化的原因。相关研究结果也可为未来台湾东北黑潮入侵东海陆架流态变异的研究以及东海海洋环境变化的可预报性及相关交叉学科的研究提供有价值的参考。

Seas off northeastern Taiwan is the major source region of the Kuroshio waters which feeds the Kuroshio branch currents on the East China Sea (ECS) shelf.A step further, the Kuroshio on-shelf intrusion waters and Kuroshio branch currents have great influences on the circulation, nutrient supplement, ecological environment and marine economy in the ECS.Unique localphenomena were found coexist with the Kuroshio intrusion here. The flow structures and the variations of these phenomena as well as the rareof continuousin-situ observations make it tough and challenging for oceanographers to understand the detailed flow structureand their mechanisms off northeastern Taiwan. This thesis aims to reveal the Kuroshio intrusion patterns and variations off northeastern Taiwan, and focus our attention on the spatiotemporal features of the along-slope counter-flow off northeastern Taiwan, the relation between the counter-flow and the Kuroshio on-shelf intrusion, and the relevant mechanisms. The innovative results are summarized as follows:

1. The spatiotemporal features of the along-slope counter-flow off northeastern Taiwan was revealed using historical in situ observations, recent velocity data retrieved from ADCP, in conjunction with satellite altimetry data and HYCOM analysis data. The results revealed that the along-slope counter-flow flows in the subsurface layer with core depth of 200 m to 400 m at the in-situ site. The maximum of the mean along-slope velocity profile of the counter-flow was 19.0 cm/s and the maximum of the mean velocity profile of the counter-flow was 24.2 cm/s during the time from May 2017 to May 2018. The along-slope counter-flow also shows significant seasonal, intra-seasonal and short-term variations.
2. We explicitly proposed that the along-slope counter-flow significantly intrudes onto the ECS shelf near the MHC while flowing southwestward along the continental slope.The counter-flow intrusion process near the MHC carries the Kuroshio subsurface water below the water depth of 150 m onto the ECS shelf, and a rough estimation of the counter-flow on-shelf volume transport near the MHC derived from the HYCOM data is 0.52 Sv. The along-slope counter-flow and the significant counter-flow intrusion was also reproduced in our ROMS model outputs and the topography β spiral theory was applied to explain the counter-flow intrusion near the MHC.
3. The linear relationship between the vertical migration of the along-slope counter-flow and Kuroshio on-shelf intrusion off northeastern Taiwan was brought to light. The along-slope counter-flow was enhanced (weakened) and uplifted (lowered) while the Kuroshio on-shelf intrusion through the western slope off northeastern Taiwan was deceased (increased), and the Kuroshio on-shelf intrusion through the eastern slope off northeastern Taiwan was increased (decreased). The vertical migration of the along-slope counter-flow well reflects the stratification variations of the water column near the slope off northeastern Taiwan, and the variation of the water stratification change the cross-shelf potential vorticity distribution which determines the Kuroshio intrusion intensity variations off northeastern Taiwan.
4. At last, considering historical studies, based on the spatiotemporal patterns of the Kuroshio on-shelf intrusion and the along-slope counter-flow off northeastern Taiwan, this thesis provides sketch maps of three major Kurohsio on-shelf intrusion path off northeastern Taiwan. The sketch maps fully considered the quasi-steady along-slope counter-flow in the subsurface layer, revealed connection behind the multi-scale variations of the flow structures off northeastern Taiwan, and provide new perspectives on the spatiotemporal variations of the flow field and Kuroshio on-shelf intrusion off northeastern Taiwan.

This thesis makes basic contributions to the understanding of the flow structures and their variations off northeastern Taiwan. A step further, a better understanding of local flow structure in the source region will provide the details of formation and variation of the Kuroshio branch currents on the East China Sea shelf. The results may also help oceanographers to make breakthroughs in the multi-scale spatiotemporal variations of the Kuroshio on-shelf intrusion off northeastern Taiwan and forecasting of the marine environment and marine economy northeast of Taiwan, eastern Zhejiang and in the East China Sea in the future.