南海和吕宋海峡的潮致混合参数化及其对南海深层环流的影响机理研究

Turbulent mixing has an important impact on thermodynamic processes (such as seawater mass, circulation and thermal transports, etc.), and internal tide is the primary source of mechanical energy for the turbulent mixing in the deep ocean. The Luzon Strait (LS) and the South China Sea (SCS) are one of the regions in the global ocean where the strongest internal tide generates and propagates; the enhanced mixing due to tidal dissipation in the LS and SCS plays a key role in maintaining the water transport through the LS as well as the water mass properties and thermohaline circulation in the deep SCS. However, due to the limit of observational data and computational resource, there has been a lack of in-depth study in how to reasonably describe (or parameterize) the processes of internal tide dissipation and tidal mixing in regional ocean model for the LS and SCS region and in the mechanism of how tidal mixing influences the SCS deep circulation. Therefore, this project, based on internal tide simulation and baroclinic energy budget analysis, will aim to establish a parameterization scheme which is able to reasonably represent the processes of internal tide dissipation and tidal mixing in regional ocean model for the LS and SCS region by taking into account the effect of both locally and non-locally generated internal tide dissipation, which is expected to improve the model simulation of the LS water transport and the SCS deep circulation. Moreover, the impact of tidal mixing on the LS water transport and the SCS deep circulation as well as the associated mechanisms are investigated through a series of sensitivity experiments with the established tidal mixing parameterization scheme and energetic/dynamic analysis.

湍流混合过程对海洋水团、环流以及热量输运等热动力过程具有重要影响，而內潮是深海湍流混合最主要的机械能来源。吕宋海峡和南海是全球海洋內潮生成和传播最强的海区之一，内潮耗散所导致的湍流混合对维持吕宋海峡水交换、南海深层水团性质和环流结构起到至关重要的作用。然而, 由于受观测资料及计算条件等因素的制约，目前的区域海洋模式对该海区的潮致混合效应尚缺乏准确合理的刻画(即参数化)，而且对于潮致混合如何影响南海深层环流结构的热动力机制目前尚不清楚。因此，本项目拟利用內潮模拟和內潮能量收支分析的方法，综合考虑局地和非局地内潮能量的贡献，建立能合理刻画南海和吕宋海峡的潮致耗散和潮致混合过程的参数化方案，以提高南海区域模式对吕宋海峡水交换和南海深层环流的模拟能力；在所建立的潮致混合参数化基础上，通过一系列敏感性试验和诊断分析，研究潮致混合对吕宋海峡水交换和南海深层环流结构的影响，并进一步揭示其热力和动力机制。

本研究针对南海海盆尺度半封闭的环流特征，基于麻省理工大学开发的海洋环流模型MITgcm，构建了南海三维非静力内潮模型，通过对南海不同季节内潮生成和传播分别进行模拟，得到内潮在南海的分布特征及大小潮季节变化特征。在此基础上，采用内潮能量收支分析方法，计算得到了南海海盆的垂向积分耗散率，构建了三维垂向扩散系数的气候态分布场，分析了该区域的局地和非局地潮致耗散混合特征及其对扩散的不同贡献。在进一步构建南海环流模型的基础上，我们将三维的垂向混合扩散系数放入环流模型中，并设计了一系列数值敏感性试验，考察了潮致混合过程对南海深层环流和水团性质的影响。研究结果发现吕宋口和南海的潮混合能维持等密度面的梯度，因而能维持从吕宋口进入南海的强深层水体输运。进一步的水团性质分析表明，南海深层潮致混合可以增强水平位密梯度，强化海盆尺度的气旋式环流，诱发更活跃的翻转环流，同时生成次海盆尺度的涡旋。上述结果表明吕宋口和南海的潮混合在水团性质和深层环流的调制中扮演了非常重要的角色，因此需要在粗分辨率的环流模式中加入相应的潮致混合参数化方案。本研究基于能量学构建的三维垂向混合扩散系数，合理地反映了南海及吕宋海峡的潮致耗散混合过程及其对南海水团的影响，因此提高了南海区域海洋模式对南海中深层水团性质的模拟能力。作为项目成果产出的一套较潮致混合参数化方案，为提高南海区域海洋环流数值模拟提供了理论基础和实践经验。