珠江河口弱混合层发育演变与动力机制

The interactions between the river plume extension and the benthic salinity intrusion can form a three-layer stratified shear flow, and a weakly-mixing layer is developed in the mid-depth of estuary. The weakly-mixing layer of estuary has a significant influence on the momentum exchange and material vertical diffusion, which is an important small-scale dynamic structure for controlling the biogeochemical cycle and its ecological effects. At present, there is little report on the phenomenon of the weakly-mixing layer of estuary at home and abroad, and its evolution and dynamical mechanism are still unclear. Based on the advanced observation techniques and methods such as the turbulent microstructure profiler, the halocline observation system and the bottom-mounted tripod, this project will conduct the field observations in Lingdingyang Estuarine Bay of Pear River Estuary. The measurements of turbulence and stratification at the characteristic stations are used to clarify the dynamical structure and spatiotemporal distributions of the weakly-mixing layer of estuary. Through analyzing the stratified shear flow in the river plume, the turbulent mixing in the bottom boundary layer, and the momentum and energy balance in the weakly-mixing layer, to reveal the meso-micro scale processes and dynamical mechanism of the evolution of the weakly-mixing layer. The turbulent vertical eddy diffusion rate is estimated by the full-water turbulence observation data to discuss the influence of the evolution of the weakly-mixing layer on the vertical transport of the material. This will provide a new dynamical explanation for the formation and maintenance of hypoxia in the estuary.

河口表层羽状流扩展与底层高盐水入侵可以在河口水体形成三层分层剪切流结构，中部发育一个明显的弱混合层。河口弱混合层对水体的动量交换和物质垂向扩散有重要影响，是控制河口生地化循环及其生态效应的重要小尺度动力结构。目前对于河口弱混合层这一现象国内外鲜有报道，其发育演变规律与动力机制尚不清楚。本项目拟以珠江伶仃洋河口为例，通过湍流微结构、盐跃层观测系统、座底三脚架观测系统等先进观测技术和手段，在河口弱混合层发育的特征站位进行湍流与层化现场观测，阐明弱混合层的动力结构和时空分布特征。通过河口羽状流内的分层剪切流、底边界层湍流混合及弱混合层的动量平衡和能量平衡理论分析，揭示河口弱混合层发育演变的中小尺度过程与动力机制。基于全水深湍流观测数据估算湍流垂向涡动扩散率，探讨河口弱混合层对物质垂向输移的影响，为河口缺氧区的形成与维持提供新的动力解释。

河口表层羽状流扩展与底层高盐水入侵可以在河口水体形成三层分层剪切流结构，中部发育一个明显的弱混合层。河口弱混合层对水体的动量交换和物质垂向扩散有重要影响，是控制河口生地化循环及其生态效应的重要小尺度动力结构。目前对于河口弱混合层这一现象国内外鲜有报道，其发育演变规律与动力机制尚不清楚。本项目通过湍流微结构、潜标观测系统、座底三脚架观测系统等先进观测技术和手段，于2020年7月在河口弱混合层发育的特征站位进行湍流与层化现场观测，阐明了弱混合层的动力结构和时空分布特征。当上层羽状流层处于次临界状态时，水体不活跃而趋于稳定，层化抑制湍流发育。羽状流内强烈的湍流混合仅发生在近表层而无法进一步侵蚀内部层化水体，使得上层羽状流层与底部混合层相分离，在两层之间形成弱混合层。水体中层弱混合层阻碍了表底层水体之间的垂向交换，导致表层水体中的高浓度溶解氧无法向底层输送。弱混合层内的平均湍流涡动扩散系数为2.5×10-6 m2 s-1，可以得到缺氧发生的临界弱混合层厚度为3 m。河口湍流混合是河口生物地球化学循环过程中的基础前沿科学问题，系统研究湍流混合对于河口缺氧现象的影响，对于深入理解缺氧区的动力结构和过程，以及缺氧的物理-生地化耦合模拟的参数化等具有重要的意义，为相关的海洋防灾减灾提供参考依据。