Collaborative research: Generation of internal waves due to the scattering of semidiurnal hybrid Kelvin-edge waves at varying continental shelf topography

维持观测到的海洋结构及其环流需要一定程度的混合。在地表附近，风是混合的主要驱动力，但其有效性随着深度而减弱。天文力作用于海洋的所有深度，并在整个水柱中产生气流，称为正压潮汐。然而，由于在大的空间尺度上几乎是均匀的，这些水流在搅动海洋方面不是很有效。这项研究探讨了沿宽大陆架传播的正压潮汐在地形变化附近经历了一种特殊的不稳定性，并导致了随深度变化强烈的流动，称为斜压模态。这些模态可以作为内波斜向传播，并在大陆架和海洋内部造成混合。它们可以影响营养物质和污染物的水平和垂直通量、沉积物运输以及大陆架上的碳循环。强烈的内部潮汐会引起大量的速度剪切，对石油和天然气钻井平台构成危险。该项目的研究结果可以改进数值气候和环流模式中，特别是海洋边界区域内波混合的描述。该项目将支持两名博士生（一名在南卡罗来纳大学，另一名在南密西西比大学），并将通过课堂作业和个人研究项目为南卡罗来纳大学海洋科学项目的本科生提供培训机会。一名早期职业科学家将得到支持。在世界海洋的许多地区，正压潮汐以被海岸线困住的长波模式存在。通常，能量最高的是零，基本模式，在北半球（南半球）沿其右（左）海岸传播。当搁架较窄时，这种零模式类似于非色散开尔文波。对于较宽的大陆架，半日基模变成混合开尔文-边缘波，群速度随波数变化。对于宽度大于~200 km的大陆架，在半日频率下的群速度变得很低甚至为零。当沿大陆边缘传播的潮汐波遇到地形变化时，其群速度减小，由此产生的沿岸能量通量辐合引起潮汐幅值的放大和潮汐能以非捕获庞加莱波模态的形式辐射。这种现象的一个很好的例子是巴塔哥尼亚大陆架（西南大西洋），那里的半日潮传播似乎在40°s附近被阻挡。本研究的中心假设是HKEW模式的能量通量辐合遇到大陆架的沿岸地形变化导致能量从正压向斜压转换。即存在强烈的内潮产生，正压潮群速度在基模相位传播方向上大幅度减小。我们将利用区域海洋模拟系统（ROMS）进行一系列面向过程的数值实验，通过系统地改变陆架和斜坡的几何形状、分层和入射HKEW模态振幅来研究这种波散射过程。将寻求一种简单的参数化方法来预测从陆架断裂/上大陆斜坡向海岸和近海辐射转化为斜压模态的入射能量通量的比例。参数化将通过将其预测结果与HYCOM的低模态内波能量转换潮汐模拟进行比较来评估（可能由当前版本的HYCOM很好地解决了）。同时，该研究将确定高模内波束可能具有潜在重要性的区域，这在当前版本的HYCOM中是无法解决的。因此，该项目的结果将指导潮汐模拟的进一步发展，特别是在大陆架上。与半昼夜HKEW模式散射到内波及其与平均涡流的相互作用有关的非线性动力学也将被考虑。这些动力学可以导致在散射区域附近产生低频或平稳的中尺度流动，这是一种不同于通常引起的平均电流不稳定的中尺度变率机制。

Sustaining the observed structure of the ocean and its circulation requires a certain level of mixing. Near the surface, wind is a major driver for mixing but its effectiveness diminishes with depth. Astronomic forces apply at all depths of the ocean and induce flows throughout the water column, called barotropic tides. However, being nearly uniform over large spatial scales, those flows are not very effective at stirring the ocean. This study explores the idea that these barotropic tides propagating along a wide shelf undergo a particular kind of instability near topographic variations and result in flows which vary strongly with depth, called baroclinic modes. These modes can propagate obliquely as internal waves and result in mixing on the continental shelf as well as in the interior of the ocean. They can affect the horizontal and vertical fluxes of nutrients and pollutants, sediment transport, and the carbon cycle on continental shelves. Strong internal tides can induce substantial velocity shear and represent a hazard for oil and gas drilling platforms. The results of this project may improve a description of the internal wave-induced mixing in numerical climate and general circulation models, especially in oceanic boundary regions. The project will support two PhD students (one at University of South Carolina and one at University of Southern Mississippi) and will offer training opportunities for undergraduates at the University of South Carolina Marine Science program through class work and individual research projects. An early-career scientist will be supported.In many areas of the World Ocean, barotropic tides exist in the form of long wave modes trapped by the coastline. Typically, the most energetic is the zero, fundamental mode, propagating with the coast on its right (left) in the Northern (Southern) hemisphere. This zero mode resembles a nondispersive Kelvin wave when the shelf is narrow. For wider shelves, the semidiurnal fundamental mode becomes a hybrid Kelvin-edge wave (HKEW) with group velocity changing with the wavenumber. For shelves wider than ~200 km, the HKEW group velocity at semidiurnal frequency becomes low or even zero. If a tidal wave propagating along the continental margin encounters topographic variations where its group velocity decreases, the resulting alongshore energy flux convergence causes the amplification of tidal amplitude and the radiation of tidal energy in the form of non-trapped Poincare wave modes. A good example of this phenomenon is the Patagonia Shelf (Southwest Atlantic) where the propagation of semi-diurnal tides is seemingly blocked in the vicinity of 40 deg S. The central hypothesis of this study is that the energy flux convergence in the HKEW mode encountering alongshore variations of shelf topography results in the energy conversion from barotropic to baroclinic mode. That is, there should be a strong generation of internal tides where the group velocity of barotropic tides substantially decreases in the direction of the fundamental mode phase propagation. A series of process-oriented numerical experiments will be made using Regional Ocean Modeling System (ROMS) where this wave scattering process will be studied by systematically varying the shelf and slope geometry, stratification and the incident HKEW mode amplitude. A simple parameterization will be sought to predict the fraction of the incident energy flux converted into baroclinic modes radiating from the shelf break/upper continental slope both toward the coast and offshore. The parameterization will be evaluated by comparing its predictions with state of the art tidal simulations in HYCOM for low-mode internal wave energy conversion (presumably well resolved by the current version of HYCOM). At the same time, the study will identify areas where higher mode internal wave beams can be potentially important, which are unresolved in the current version of HYCOM. Thus, results of this project will guide a further development of tidal simulations, especially on continental shelves. Nonlinear dynamics associated with the semidiurnal HKEW mode scattering into internal waves and their interaction with mean, eddying currents will also be considered. These dynamics can result in the generation of low-frequency or stationary mesoscale flows in the vicinity of the scattering region, a different mechanism for mesoscale variability than the often invoked instability of mean currents.