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

海底边界层中的混合过程被认为对大气环流有很强的控制作用。例如，与地形上破裂的内波相关的湍流混合的增强被认为在关闭经向翻转环流的深层分支方面起着中心作用。同样，当沿倾斜地形流动的大尺度水流被底部摩擦减速时，边界层中的湍流混合也会产生。重要的是，这些过程在边界层产生了温度和盐度场，它们在垂直方向上混合得很好，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，到目前为止几乎只在上层大洋锋面的背景下进行了研究，可能也在BBL中活跃。在表层海洋中，次中尺度过程在表层混合层的再分层中发挥重要作用，从而对抗湍流混合，并在污染物、溶解气体和营养物质等示踪剂的扩散和传输中发挥重要作用。因此，了解次中尺度过程在大气层中可能活跃的条件及其对环流和混合的影响，对于理解深海和沿海海洋的物理和生物地球化学性质具有潜在的深远意义。该项目将支持一位构思和撰写了该提案大部分内容的有前途的早期职业科学家，新的科学成果将被纳入为当地高中生提供的外展讲座和实验室演示中。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在斜坡上的海流摩擦自旋期间产生的大气层中亚中尺度不稳定性的动力学。初步计算表明，倾斜地形上的边界层边界层可以支持次中尺度斜压不稳定模式，这是与地面混合层不稳定相对应的边界层边界层不稳定模式。这些不稳定是快速增长的，并且对地形坡度相对不敏感，这表明它们可能是海洋边界层的一个共同特征。最初的数值模拟还表明，这些斜压不稳定可以导致垂直浮力通量，在某些条件下，这些垂直浮力通量可能主导其他更广泛研究的垂直浮力通量来源，例如地形上的内波破裂。这项研究将利用线性稳定性分析和理想化数值模拟对这些初步结果进行扩展，以提供对BBL亚中尺度不稳定性的参数依赖性的完整探索，重点是浮力和位涡的湍流通量。该奖项反映了NSF的法定使命，通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为是值得支持的。

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.