夏季长江河口密度层化发育的潮汐应变新机制

Density stratification and turbulent mixing have significant influence on various estuarine processes, such as saltwater intrusion, plume extension, suspended particle settling and nutrient mass transport. Simpson introduced the concept of tidal straining to describe the tidal asymmetry of stratification and mixing in estuaries. However, further studies have suggested that the growth of estuarine stratification is not fully consistent with the prediction of the classic straining theory. Also, the classic straining theory has neglected some important mixing mechanisms. Observation data obtained through turbulent microstructure profiler and bottom boundary layer tripod will be analyzed. Realistic numerical simulations as well as idealized models will be implemented by a high-resolution numerical ocean model. The first step of our analysis is to quantify and investigate the formation of density stratification and turbulent mixing in the Changjiang Estuary in summer. The second step is to dissect the influences of velocity shear and turbulent mixing on the straining mechanism. A new theoretical framework will be derived to describe quantitatively the roles of straining, advection and mixing in controlling stratification, based on the vertical salinity variance method developed in previous studies. One valuable outcome of this research will be the ability to sort out the strongly coupled interaction between stratification, shear and mixing. Another important implication is to extend the classic tidal straining theory which was based on the conservation and superposition of energy to the real estuarine environment, and to enrich and broaden the classic tidal straining theory.

河口的密度层化与湍流混合对盐水入侵、羽状流扩展、悬浮颗粒沉降以及营养物质输移等动力与生态过程有重要影响。Simpson引入潮汐应变机制解释密度层化与湍流混合的潮汐不对称现象，然而研究发现密度层化发育与Simpson经典理论预测并不完全一致，此外经典应变理论对湍流混合的考虑也不够全面。本项目将依据在长江河口的湍流微结构与底边界层座底三脚架等系列观测资料，结合高分辨率河口水动力数值模型，运用前期工作发展的盐度方差方法，阐述夏季长江河口密度层化与湍流混合的演变特征，解析流速剪切与湍流混合对应变机制的影响，发展新的潮汐应变理论描述应变与混合过程对层化发育演变的影响，深入探究应变与层化、剪切及混合之间的内在关系，将基于能量平衡线性叠加的经典潮汐应变理论推广到更加真实的河口动力环境，丰富和发展经典的潮汐应变理论。

项目研究发现盐度混合、盐度扩散通量以及跨等盐度线的交换流三者密切相关。跨等盐度交换流与传统的河口交换流在垂向分布上相似，均为表层向海、底层向陆。跨等盐度线交换流的入流与出流均随盐度增强而增强，不过二者在长时间平均之后的净值等于径流量。卷挟流速在等盐度面上的分布往往是零散稀疏的，这说明上游淡水在跨越等盐度面离开河口时的空间分布是不均匀的。该项研究为河口混合研究提供了新的研究视角和方法，丰富了相关研究理论。