波浪混合对长江冲淡水扩展的影响研究

The dispersal of Changjiang River plume plays a substantial role on the hydrodynamics, biogeochemistry, productivity and dynamical stability of the East China Sea. A better understanding of its spreading and seasonality is therefore essential for prediction of environment consequences. Surface wave mixing, induced by the breaking waves and non-breaking waves, is the key mixing source of the upper ocean. Its effects on the upper ocean dynamics have been demonstrated by laboratory experiments and in-situ observations. However, its roles on the dispersal of Changjiang River plume are still unclear. Revealing the effects of surface wave induced mixing on the dispersal of Changjiang River plume and the underlying dynamical processes is a fundamental scientific question that warrants a timely investigation...We plan to use both observations and wave-current coupled modeling system to investigate the impact of surface-wave-induced mixing on the dynamics of the Changjiang River plume. The Coupled-Ocean-Atmosphere-Wave-Sediment-Transport (COAWST) modeling system will be adopted. We hypothesize that the surface-wave-induced mixing deepens the plume thickness, dampens its offshore propagation speed and increases the plume-induced mass transport. The key regions with maximum wave effects are the far-field and front regions, where the plume is thinner, the water is deeper and tidal mixing is unable to intrude into the surface layer. To verify these ideas, we will first investigate how the inclusion of surface-wave -induced mixing improves the simulation of the Changjiang River plume, by comparing between model setup with and without the wave-mixing effects and also in-situ observations. Since the Changjiang River plume has distinct modes during summer and winter, the seasonal dependence of wave effects will be examined. Effects of surface-wave -induced mixing on the cross-shelf transport in summer and the along-shelf transport in winter will be evaluated. Second, we will examine how wave mixing alters the plume dynamics according to different plume regions: the near-field, the far-field and the plume front regions. The mixing efficiency will be quantitatively calculated by analyzing the vertical salt flux, and the work done by surface-wave-induced mixing. The underlying mechanisms will also be investigated. The final issue to pursue is how the variability of wind forcing affects the surface wave mixing through energy input and wave-current interaction, and eventually influences the plume structure. The findings from this project will provide scientific basis to fully understand the dynamics of the Changjiang River plume and help to establish the operational observing and forecasting system of the Changjiang River plume.

长江冲淡水的扩展变化对东海的动力调整和环境演变有重要影响；波浪混合作为上层海洋的重要混合源，对长江冲淡水扩展的影响和作用机理至今还不清楚，是目前亟需解决的重要科学问题。本研究拟使用浪-流耦合模式，并结合观测资料，研究波浪混合，包括波浪破碎混合和波浪运动混合，对长江冲淡水水平扩展形态、垂直层结结构及其时空演变的影响；阐明对应长江冲淡水扩展的不同海域，包括近河口区、远河口区和羽流锋面区，波浪混合的不同作用机理和波浪的混合效率；研究风场变化导致的波浪混合变化对长江冲淡水结构的影响，理解影响长江冲淡水扩展的关键混合机制，定量评估波浪混合在长江冲淡水扩展演变中所扮演的角色。本项目研究对深入理解长江冲淡水动力学，准确评估长江冲淡水引起的环境效应，提高对东海海洋环境变化的预测能力有重要意义。

长江冲淡水的扩展变化对东海的动力调整和环境演变有重要影响，波浪作为海气界面的重要动力过程，对长江冲淡水的影响和作用机理尚不明确。本项目针对这一科学问题，旨在阐明波浪对长江冲淡水结构和输运的影响，揭示波浪影响长江冲淡水的关键机制和关键区域。项目建立了适用于长江口-东中国海的ROMS水动力模式，深入验证了模型的模拟能力，对2004-2015年东中国海水动力环境演变进行了后报模拟，给出了长江冲淡水周旬尺度和年际尺度的变化特征。项目构建了该区域的浪-潮-流耦合模式，并结合观测资料分析，研究了多个浪-流耦合过程对长江冲淡水扩展的影响，分析了波浪影响下长江冲淡水扩展的季节变化特征和机理，讨论了不同风情对浪-流相互作用和长江冲淡水扩展的影响；并开展了长江冲淡水影响海表面温度和缺氧区等物理、生态环境的机理研究。研究发现，长江口及邻近海域风浪和涌浪共存，二者都很重要。波浪对长江冲淡水的扩展影响显著：夏季，波浪致动量输入减弱了长江冲淡水向南的扩展，起主要作用，波浪致底应力变化则限制了长江冲淡水跨陆架方向的扩展，起次要作用，波浪破碎混合的影响最小；冬季，浪-流耦合效应阻滞了长江冲淡水的扩展速度，其中，波浪致底应力变化起主要作用，波浪致动量输入次之，波浪破碎混合的影响最小。与密西西比-查法拉亚冲淡水的对比研究发现，不同冲淡水系统中浪-流耦合过程的作用机制和重要性不同，潮汐强度、地形坡度和冲淡水类型的差异造成了不同冲淡水系统间浪-流耦合效应的差异。本项目较为全面的研究了浪-流耦合过程对长江冲淡水扩展的影响，证实并阐明了浪-流耦合效应在长江冲淡水扩展中的作用，并通过和密西西比-查法拉亚冲淡水的对比，理解了不同冲淡水系统中波浪的作用和机制差异。本研究促进了冲淡水动力学理论体系的完善，对提高东海海洋环境的模拟和预报能力也有积极贡献。