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

在每个大洋盆地中，在主要大洋锋面的赤道一侧，都发现了具有几乎均一性质的弱分层水域。这些所谓的“模水”在海-气耦合系统中起着重要的作用，它们在年际时间尺度上隔离和释放热量和二氧化碳，并通过形成位涡来影响大尺度环流。目前的气候模型很难准确地形成具有正确的温度和盐度属性的模式水域，这是一个令人担忧的问题，因为它们对年际海洋和气候变异性的影响。解释它们形成的机制还不是很清楚，但似乎是由标志着它们向极地范围的海洋锋面的动态决定的。将探索一种新的模式水形成机制，它与锋面有明确的联系，并涉及碳化、亚中尺度横向混合和锋生。Carbeling是指由于海水状态方程中的非线性，将密度相同但温度和盐度不同的两个水团混合，形成一个新的、密度更大的水团的过程。构成这个项目基础的理论表明，模式水的性质受到锋面上的电缆的影响。动力学必然涉及亚中尺度过程，这在最近几年受到了很大的关注，但是在线性状态方程的框架内。次中尺度运动和通过密度变化产生的气泡之间的耦合有可能产生新的、丰富的物理学，这将在这个项目中得到探索。预期的研究结果将有助于制定战略，改进海洋环流和气候模型中模式水域的模拟，并对量化标记环流边界的锋面的营养物质通量具有重要意义。该项目包括对一位有前途的博士后研究员的指导和支持。一名高中科学教师将接受实验室演示的培训，这些演示说明了海洋、大气和气候的物理，可以纳入他们的课程。一个简单的二维模型正确地预测了观测到的模态水所在的等温层，并指出这种机制可能是持续的，因此是重要的模态水的形成。这个项目将使用更完整的理论和两套数值模拟来测试和推广这个简单的模型，这些数值模拟旨在研究从盆地宽度到锋面宽度的尺度上的过程。该理论将考虑部分补偿锋面，并使用半地转方程来捕捉次级环流对锋生的反馈，并量化其对气旋的影响。流域尺度模拟的目的是检验三维双涡旋环流中模水形成的理论预测，并探索横向混合和包络在建立模水T-S关系中的作用。第二组模拟将配置一个温盐线锋面和一个气旋涡度薄片，该涡度是根据在墨西哥湾流中观测到的气流模拟的。将研究发展的亚中尺度切变不稳定性及其引起的温度和盐的等速混合，以确定如何通过涡旋扩散系数来参数化碳化。此外，还将研究卡贝尔效应对亚中尺度不稳定性的潜在动力学反馈。

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.