北极地区夏季“海冰减退-海浪增强”反馈机制及其影响的模型研究

The Arctic region is undergoing a rapidly changing period, which is characterized by decreasing of the summer sea ice extend and the thinning of the multi-year ice. In this period, the ice floes in the Arctic become fragile to the atmosphere and marine forcing. This project is to derive a parameterization scheme based on an innovative wave-induced ice flexure failure theory, and, combining with the strain failure theory, an integrated ice breakup mechanism is to be established. On the other hand, this project is to analyze the wave attenuation process under varied ice and wave situation, adopting corresponding wave attenuation theory, and form a wave attenuation scheme with clear and complete physical process. Based on the wave-ice interaction scheme above, a FVCOM ice-wave coupling model is to be established and validated with buoy observed wave data and ice breakup cases in the MIZ, which have been shared by the research collaborators. This model will be applied to simulate the dynamic evolution process of wave and ice in the Arctic region, and study the phenomenon in the rapidly changing Arctic: the influences on wave growing, propagation and dissipation of changing of ice extend and floe size; the influences of increasing wave energy on floe size, ice thermodynamic property and the feedback effect on ice rapidly retreating. The “ice decreasing-wave increasing” feedback mechanism and its influence on summer ice retreat is to be deduced in this project. This project is essential to understand and forecast the trend of Arctic summer ice retreating in rapidly changing period, and is the model forecasting basis of the research of the global climate change.

北极夏季海冰持续减退释放的自由海面促进了海表波浪的发展，由此增强的海冰-海浪动力学相互作用在北极夏季海冰热力学过程中变得尤为重要。本课题拟在推导海冰弯矩屈服理论的基础上，建立海浪作用下海冰破碎机制；根据海冰状况和海浪特征，应用物理机制相符的海浪耗散理论，建立物理过程清晰的海冰作用下海浪耗散机制。进而构建海冰-海浪动力学耦合机制，开发基于FVCOM的海冰-海浪耦合模型，利用海冰边缘区波浪数据和海冰破碎案例对模型进行验证。应用模型模拟夏季北极海浪、海冰动态演变过程，重点探讨自由海面面积、海冰尺寸变化对波浪能成长、传播和耗散的影响，并科学评估波浪能增长对海冰尺寸、热力学特性的影响，及其对夏季海冰减退的反馈作用。本课题有利于揭示 “海冰减退-海浪增强”反馈机制及其对北极夏季海冰减退的影响，对认识快速变化期北极海冰减退过程和预测其变化趋势起到关键性作用，为探索全球气候变化等重要问题提供模型预报基础。

北极地区夏季的快速海冰融化过程使无冰海域的面积持续增大。这为北极地区夏季海表波浪的生成和成长提供了良好的风区条件。当海浪传播进入海冰覆盖海域内，海冰会消耗海浪的能量。同时，海浪在海冰区内的传播有能力诱发海冰块体的破碎，从而改变海冰块体的尺寸，进而提高夏季北极地区的海冰减退速度。.本项目基于FVCOM模型的海冰和海浪模块，优化设计并建立了一个充分耦合海冰-海浪动力学相互作用的耦合模型。其中海冰-海浪的动力学相互作用机制由紧密联系的两部分组成：海冰对海浪能成长和耗散的影响机制以及海浪引起海冰破碎机制。在海冰破碎机制中，应变屈服和应力屈服都被充分考虑，其中应力屈服理论是基于海浪引起的弯矩对海冰内部应力的影响合理推导得到的。本项目建立了一个理想模型用于重现SIPEX-2航次过程中的海浪场和海冰破碎现象，模拟结果显示，耦合了应力和应变屈服的海冰破碎模型成功后报了该航次观测到的海冰破碎事件，而单一的应变屈服模型未能成功后报。通过计算不同海浪周期和海冰厚度对应的海冰破碎临界波高可以发现，应变屈服是在高频端控制海浪引起的海冰破碎，而应力屈服控制低频端。忽视上述任何一种海冰破碎理论都有可能在模型结果中高估海冰的水平尺寸，从而低估夏季海冰的侧向融化速率，进而增加夏季极地海冰面积的模拟误差。.本项目还建立了一个适用于全球尺度FVCOM非结构网格的球坐标海浪模型，用于分析海冰对北极地区海浪能生成和传播的影响。本项目提出一个新的海冰引起海浪能耗散源汇项，成功消除了海浪能空间传播过程中原有源汇项的数值耗散问题。本文通过将模型计算的有效波高和谱峰周期与海浪浮标资料和Jason-2卫星遥感资料比较，验证了模型精度。根据应变屈服模型和模型计算结果，本文估计海冰破碎时间普遍发生在海冰密集度低于0.4的海区。本项目成果对北极夏季“海冰减退-海浪增强”正反馈机制提供了依据，特别是在北冰洋的太平洋一侧，该反馈机制尤为重要。