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

海底附近的亚中尺度不稳定性及其对海洋环流和混合的影响

• 批准号: 1756118
• 负责人: Leif Thomas
• 金额: $ 32.46万
• 依托单位: Stanford University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-09-01 至 2021-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1756118&HistoricalAwards=false
• 关键词: Submesoscale; instabilities; near; sea; floor

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.

海洋底部边界层（BBL）中的混合过程被认为对一般循环产生了强大的控制。例如，已经提出，与地形上断裂的内波相关的湍流混合的增强在关闭子午倾覆循环的深部分支中起着核心作用。同样，当沿着倾斜地形流动的大规模电流被底部摩擦减速时，BBL中的湍流混合可能会产生。重要的是，这些过程在BBL中产生温度和盐度场，这些温度和盐度场在垂直方向充分混合，但在水平上不混合，类似于海面附近的前部。这种相似性表明，许多类型的子尺度（水平尺度为0.1-10公里）的不稳定性，这些能力几乎完全在上胚方阵线的背景下进行了研究，在BBL中也可能活跃。在地面海洋中，众所周知，子尺度过程在限制表面混合层，从而抵消湍流混合以及示踪剂（例如污染物，溶解的气体和营养素）的分散和运输中起着重要作用。了解BBL中的子尺度过程可能活跃的条件，以及它们对循环和混合的影响，因此对了解深海和沿海海洋的物理和生物地球化学特性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议的大部分，并将新的科学成果纳入外展的讲座和实验室演示中，并为当地高中生的学生进行。该项目将使用理论化的数值建模组合来研究BBL在自然旋转过程中生成的BBL的动力学动力学。初步计算表明，在倾斜地形上的BBL可以支持一个尺度上的斜压斜率不稳定性模式，这是BBL与表面混合层不稳定性的对应。这些不稳定性是快速增长的，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。最初的数值模拟还表明，这些斜压稳定性可能导致垂直浮力通量，在某些条件下，这些通量可能主导其他更广泛的垂直浮力通量来源，例如在地图时内部波动的破裂。拟议的研究将使用线性稳定性分析和理想化的数值建模来扩展这些初始结果，以完全探索BBL子尺度尺度尺度不稳定性的参数依赖性，重点是浮力浮力和潜在的涡度。还将考虑到包括渠道流在内的理想化变化的边界层动态。该奖项反映了NSF的法定任务，并认为使用基金会的知识分子优点和更广泛的影响评估标准，认为值得通过评估来获得支持。