浙闽沿岸流在东山外海分离机制研究

Satellite and in-situ data show part of the China Coastal Current deflects offshore off Dongshan (Southern Taiwan Strait) in winter, which makes coastal water spread off-shore to the shelf and greatly weakens the along-shore volume transport of coastal current. There have not been any studies of the phenomenal reports and mechanism investigation of this off-shore flow. This proposal is going to study its spatial and temporal variations, dynamics mechanism, and occurrence conditions. Our preliminary results show that the northeasterly wind induces a reversed along-shore sea level gradient anomalies over the bank area to the north and south of Dongshan through a change of along-shore coastal current. The along-shore sea level anomalies, as a surface feature of the varied flow dynamics, has a deep link with the off-shore flow and adjacent flow dynamics adjustment to the varied winds and topography. Starting from this idea, this research plan to use a set of equations to study the dynamics of this off-shore flow. The historic time series data of HF radar, buoy, gauge station, remote sensing and in-situ will be collected and analyzed. Combining with the three-dimensional ocean model experiments and dynamical theory analysis, the offshore flow dynamics and mechanism will be well studied. This study can be a special case for the off-shore current in a shallow bank with two reversed flows under the influence of varied winds. The results of this study can provide physical and dynamic explanations for the biogeochemical along-shore transport from the East China Sea to the South China Sea and cross-shelf transport from coastal area to the shelf region.

遥感和观测数据显示，冬季浙闽沿岸流在东山外海形成离岸流，使浙闽沿岸水东扩到台湾浅滩北侧并极大地削弱浙闽沿岸水往南的输运量。对这支离岸流的研究还末见报道，其离岸的时空规律、机制以及产生的环境动力条件是本项目的研究重点。观测表明在东山外海东北季风的变化可以造成南北反向的沿岸水位梯度异常及沿岸流南北向的调整，这些异常及调整是邻近海域水文动力变化的表征，其与离岸流及附近流系的演变有着密切的关联。本课题研究以此为切入点，研究该离岸流的动力机制。课题拟采用东山外海的地波雷达、浮标，潮位站所监测的长时间序列资料，结合遥感数据分析、补充现场航次调查，通过三维海洋模型实验及诊断，动力理论分析对该离岸流的规律及机理进行剖析和研究。该研究是物理海洋在多流系交汇、浅滩陆坡等复杂地形条件下近岸动力学的典型案例，成果将为东海向南海及跨陆架生物地球化学迁移过程提供物理背景及动力依据。

遥感和观测数据显示冬季浙闽沿岸流在东山外海形成离岸流，使浙闽沿岸水东扩到台湾浅滩北侧并极大地削弱浙闽沿岸水往南的输运量。其离岸的时空规律、机制以及产生的环境动力条件是本项目的研究重点。本课题按计划书设计的研究方案和技术路线，完成了申请书中所包含的研究内容。课题塔载台湾海峡共享航次利用延平2号完成了台湾海峡2019年春大面航次、锋面航次的观测；塔载台湾海峡共享航次利用海监203进行2021年夏季大面、羽流专题调查。在台湾海峡及沿岸布放了漂流浮子，对沿岸流的流态进行跟踪。在东山外海布放了水文浮标进行了长达两年的长期观测。完成了大小网格嵌套台湾海峡三维浪潮流及生态系统耦合模型，模型同化海表温度及地波雷达数据。课题着重研究了闽浙沿岸流在东山外海分离的机制，分析浙闽沿岸流在分离时导致中层冷水产生过程及机理，分析沿岸流锋面处亚中度过程及其对生态系统的影响。. 课题分析并揭示东山外海冬季有三种流态存在，在强东北季风时沿岸水跨过浅滩往南，东北风弱时南海暖流进入台湾海峡往北，而当风速在中值时沿岸流会在浅滩西侧离岸、调头并汇入南海暖流往北。理论分析表明浅滩西北部的沿岸流离岸的机理是在风速中值时，北边来的沿岸流在EKMAN输送作用下贴岸往南，而北上的南海暖流往北，形成了负的切变涡度。该涡度将诱导一个离岸的流，同时浅滩西侧的小坡度强化了离岸的输送；从而在此处形成了另一个具有较强的跨海峡输送区。模型的敏感性实验也证明了该机理的合理性。. 研究深入了对台湾海峡海洋动力过程的理解，并拓展了物理生态耦合过程的研究。在课题中采用结构网格和非结构网格的嵌套方式建立的台湾海峡高精度同化模型已在福建省海洋预报台实现业务化运行，可进行浪潮流及生态系统的耦合模拟。