A new mechanism for Mode water formation at a thermohaline ocean front

温盐海洋前沿模式水形成的新机制

• 批准号: 1459677
• 负责人: Leif Thomas
• 金额: $ 39.05万
• 依托单位: Stanford University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-09-01 至 2018-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1459677&HistoricalAwards=false
• 关键词: new; mechanism; Mode; water; formation

In every ocean basin, on the equatorward side of major ocean fronts, layers of weakly stratified waters with nearly homogeneous properties are found. These so-called "mode waters" play an important role in the coupled ocean-atmosphere system by sequestering and releasing heat and carbon dioxide on interannual timescales and by affecting the large-scale circulation through shaping the potential vorticity field. Current climate models have difficulties accurately forming mode waters with the correct temperature and salinity properties, which is a concern given their influence on interannual ocean and climate variability. The mechanisms explaining their formation are not well understood but appear to be shaped by the dynamics of the ocean fronts that mark their poleward extent. A new mode water formation mechanism that has a clear connection to fronts and involves cabbeling, submesoscale lateral mixing, and frontogenesis will be explored. Cabbeling refers to the process by which two water masses of equal density but different temperature and salinity are mixed to create a new, denser water mass, as a result of nonlinearities in the equation of state for seawater. The theory that forms the basis of this project suggests that the properties of mode waters are influenced by cabbeling at fronts. The dynamics necessarily involves submesoscale processes, which have received much attention in recent years, but in the framework of a linear equation of state. The coupling between submesoscale motions and cabbeling through changes in density has the potential for new, rich physics that will be explored in this project. The expected findings should help inform strategies to improve the simulation of mode waters in ocean circulation and climate models and should be important for quantifying the flux of nutrients at the fronts that mark gyre boundaries. The project includes mentoring and support of a promising postdoctoral researcher. A high school science teacher will be given training on laboratory demonstrations that illustrate the physics of the ocean, atmosphere, and climate that can be incorporated in their courses. A simple two-dimension model for the water mass transformation due to cabbeling at a thermohaline front forced by frontogenetic strain and equilibrated by lateral mixing correctly predicts the isopycnal layers where mode waters are observed to reside and suggests that the mechanism could be responsible for persistent, hence significant, mode water formation. This project will test and extend the simple model using a more complete theory and two sets of numerical simulations designed to study the process on scales spanning the width of a basin to the width of a front. The theory will consider partially compensated fronts and use the semi-geostrophic equations to capture the feedback of secondary circulations on frontogenesis and quantify its effect on cabbeling. The objectives of the basin-scale simulations are to test the theoretical prediction for mode water formation in a three-dimensional, double-gyre circulation and to explore the role of lateral mixing and cabbeling in setting the T-S relation of mode waters. The second set of simulations will be configured with a thermohaline front collocated with a sheet of cyclonic vorticity modeled after flow observed in the Gulf Stream. The submesoscale shear instabilities that develop and the isopycnal mixing of temperature and salt they induce will be studied to determine how cabbeling should be parameterized via an eddy diffusivity. In addition, the potential dynamical feedbacks of cabbeling on the submesoscale instabilities will be investigated.

在每一个海盆中，在主要海洋锋的赤道一侧，都发现了具有几乎均匀性质的弱分层沃茨层，这些所谓的“模态沃茨”在海气耦合系统中起着重要作用，它们在年际时间尺度上吸收和释放热量和二氧化碳，并通过形成位涡场影响大尺度环流。目前的气候模式很难准确地形成具有正确温度和盐度特性的模式沃茨，考虑到它们对年际海洋和气候变化的影响，这是一个令人关切的问题。解释其形成的机制尚不清楚，但似乎是由海洋锋的动态形成的，标志着其向极地的范围。一个新的模式水形成机制，有一个明确的连接，锋，并涉及cabbeling，次中尺度横向混合，锋生将进行探讨。Cabbeling是指两个相同密度但温度和盐度不同的水团混合以产生一个新的，密度更大的水团的过程，这是由于海水状态方程的非线性。构成本项目基础的理论表明，模式沃茨的性质受到锋面的缆线化的影响。动力学必然涉及亚中尺度过程，这在近年来受到了很大的关注，但在一个线性状态方程的框架。次中尺度运动和通过密度变化的cabbeling之间的耦合有可能产生新的，丰富的物理学，将在这个项目中进行探索。预期的研究结果应有助于为改进海洋环流和气候模型中模式沃茨模拟的战略提供信息，并对量化标志着环流边界的锋面的营养通量具有重要意义。该项目包括指导和支持一个有前途的博士后研究人员。一名高中科学教师将接受实验室演示的培训，这些演示说明了可以纳入课程的海洋，大气和气候的物理学。一个简单的二维模型的水团转换由于cabbeling在热盐锋锋迫使锋生应变和平衡的横向混合正确预测的等密度层模式沃茨观察到居住，并建议该机制可能是负责持久的，因此显着的，模式水的形成。该项目将使用更完整的理论和两套数值模拟来测试和扩展简单模型，旨在研究从盆地宽度到前缘宽度的尺度上的过程。该理论将考虑部分补偿锋，并使用半地转方程来捕获次级环流对锋生的反馈，并量化其对cabbeling的影响。流域尺度模拟的目的是检验三维双环流模式水形成的理论预测，并探讨横向混合和cabbeling在设定模式沃茨的T-S关系中的作用。第二组模拟将配置一个温盐锋搭配一个表的气旋涡模拟后，在墨西哥湾流中观察到的流动。将研究亚中尺度剪切不稳定性的发展和等密度混合的温度和盐，他们诱导，以确定如何cabbeling应通过涡流扩散参数化。此外，还将研究cabbeling对次中尺度不稳定性的潜在动力反馈。