Submesoscale instabilities near the sea-floor and their effects on the ocean circulation and mixing

海底边界层的混合过程被认为对大气环流有很强的控制作用。例如，与地形上的破裂内波相关的湍流混合的增强被认为在闭合纬向翻转环流的深分支方面起着核心作用。同样地，当大尺度水流沿沿着倾斜地形流动时，底边界层中的湍流混合会因底摩擦而减速。重要的是，这些过程在BBL中产生的温度和盐度场在垂直方向上混合良好，但在水平方向上没有混合，类似于海面附近的锋面。这种相似性表明，许多类型的亚中尺度（0.1 - 10公里的水平尺度）不稳定性，迄今为止几乎只在上层海洋锋的背景下进行了研究，也可能在BBL中活跃。在表层海洋中，亚中尺度过程在抑制表层混合层，从而抵消湍流混合，以及污染物、溶解气体和营养物等示踪物的扩散和输送中起着重要作用。因此，了解亚中尺度过程可能在BBL中活跃的条件，以及它们对环流和混合的影响，对了解深海和沿海海洋的物理和地球化学性质具有潜在的深远影响。该项目将支持一位有前途的早期职业科学家，他构思并撰写了该提案的大部分内容，新的科学成果将被纳入当地高中学生的外展讲座和实验室演示。该项目将使用理论和理想化数值模拟相结合的方法来研究斜坡上的水流摩擦自旋下降过程中产生的BBL中的亚中尺度不稳定性的动力学。初步计算表明，倾斜地形上的BBL可以支持一个亚中尺度斜压不稳定模式，BBL对应于表面混合层不稳定。这些不稳定性是快速增长的，并且对地形坡度相对不敏感，这表明它们可能是海洋BBL的共同特征。初始的数值模拟还表明，这些斜压不稳定性可能会导致垂直浮力通量，在某些条件下，可能会主导其他更广泛研究的垂直浮力通量的来源，如在地形的内波的破碎。拟议的研究将扩大这些初步结果，使用线性稳定性分析和理想化的数值模拟，提供一个完整的探索BBL亚中尺度不稳定性的参数依赖性，重点是湍流通量的浮力和位涡。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Mixing processes in the ocean bottom boundary layer (BBL) are thought to exert a strong control on the general circulation. For example, the enhancement of turbulent mixing associated with breaking internal waves over topography has been suggested to play a central role in closing the deep branch of the meridional overturning circulation. Likewise, turbulent mixing in the BBL can result when large-scale currents flowing along sloping topography are decelerated by bottom friction. Importantly, these processes generate temperature and salinity fields in the BBL that are well mixed in the vertical but not in the horizontal, similar to fronts near the sea surface. This similarity suggests that many types of submesoscale (horizontal scales of 0.1 - 10 kilometers) instabilities, which to-date have been studied almost exclusively in the context of upper-ocean fronts, may also be active in the BBL. In the surface ocean, submesoscale processes are known to play an important role in restratifying the surface mixed layer, thereby counteracting turbulent mixing, and in the dispersal and transport of tracers such as pollutants, dissolved gases, and nutrients. Understanding the conditions under which submesoscale processes may be active in the BBL, as well as their effect on the circulation and mixing, therefore has potentially far-reaching implications for understanding the physics and biogeochemical properties of both deep and coastal oceans. This project will support a promising early career scientist who conceived and wrote the bulk of this proposal and new scientific results will be incorporated into outreach lectures and laboratory demonstrations for local high-school students.This project will use a combination of theory and idealized numerical modeling to study the dynamics of submesoscale instabilities in the BBL generated during the frictional spin-down of a current on a slope. Preliminary calculations suggest that BBLs over sloping topography can support a submesoscale baroclinic instability mode, a BBL counterpart to surface mixed-layer instabilities. These instabilities are fast-growing, and relatively insensitive to the topographic slope, suggesting they may be a common feature of the ocean BBL. Initial numerical simulations also suggest that these baroclinic instabilities can lead to vertical buoyancy fluxes which, under some conditions, may dominate other more widely-studied sources of vertical buoyancy fluxes, such as the breaking of internal waves at topography. The proposed research will expand upon these initial results, using linear stability analysis and idealized numerical modeling, to provide a complete exploration of the parameter dependence of BBL submesoscale instabilities, with a focus on turbulent fluxes of buoyancy and potential vorticity. Boundary layer dynamics for flow over idealized variations in topography, including channel flows, will also be considered.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.