长江河口和苏北海域之间的水体交换机制研究

The Changjiang Estuary (CE) and Subei Coastal Water (SCW) are close in locations, between which there exist active exchanges of water mass, sediments, and nutrients. In recent years, the severe macro-algal blooming events in SCW raised a fundamental question: where is the origin of nutrient-rich low-salinity water in SCW? It is thus essential to answer whether and how the Changjiang diluted water can extend to SCW. SCW locates north to CE, thus the possible extension of Changjiang diluted water to there is in a direction opposite to the effect of Coriolis forcing. Such a phenomenon is very unique and is very valuable to investigate. However, the dynamics controlling the water mass exchange between these two regions is poorly understood, and the observational data are also lacking. To this end, this study will use the combined methods of field survey and numerical simulation to investigate the characteristics, variations, and mechanisms of the water mass transport, freshwater transport, the transport route, and the associated residence time. Their responses to the river discharge, tide, and monsoon will also be investigated. The field survey includes a flux measurement across a transection and cruising measurement covering the shallow region of SCW.

长江河口和苏北海域相互毗邻，两者之间水体、沉积物和营养盐等物质交换频繁。近几年苏北水域频发绿潮等生态灾害事件，为了追溯该区域低盐水和营养盐的来源，迫切需要回答长江冲淡水是否及如何扩展至苏北水域的重要科学问题。苏北海域位于长江口北侧，长江冲淡水往此方向的扩展和一般起主导作用的科氏力效应相反，对此进行研究可以丰富冲淡水扩展这一河口海岸动力学关键议题的知识体系。因此，研究这两个水域之间的水体交换规律和机理将具有重要的理论和应用价值，但以往的研究尚未系统深入，观测数据也较为缺乏。针对这一问题，本项目在已有的研究基础上，在长江口和苏北海域交界的区域设置通量观测断面，辅以大面走航观测，结合三维数值模拟等手段，研究长江口和苏北海域之间的水通量、淡水通量、水体输送路径和存留时间等，以及它们对径流、潮汐和季风等动力因素的响应机制。

本项目的研究目标是通过现场观测、数值模拟和理论分析相结合的研究方法，揭示长江口和苏北海域之间的水体交换规律和动力机制，为苏北海域海洋生物、化学和地质等研究提供理论支持。在项目的资助下，于长江河口和苏北海域进行了现场观测，收集了关键的基础数据，并进行了大量的数值模拟进行现象和机制的研究。关键成果是发现冬季盛行季风下苏北海域近岸的水体输送方向是逆风北向，从而证实在全年时间尺度上长江口物质可扩展至苏北海域。这一发现更新了长江口和苏北海域之间源汇关系以及中国近海近岸环流的传统认识。研究对此逆风环流进行了动力机制的研究，提出了潮汐应力和潮致有效底摩擦的新机制，丰富了近海动力学的理论体系。项目还量化了长江和苏北地方河流对苏北低盐水形成的贡献比例，发现前者起主导作用。这些成果对我国近海海洋学的研究起到了一定的推动作用。