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。在靠近地表的地方，风是混合的主要驱动力，但其有效性随着深度的增加而减弱。 天文力作用于海洋的所有深度，并引起整个水柱的流动，称为正压潮汐。然而，由于在大的空间尺度上几乎是均匀的，这些流动在搅动海洋方面不是很有效。这项研究探讨的想法，这些正压潮汐传播沿着一个广泛的货架经历了一种特殊的不稳定性附近的地形变化，并导致流量随深度变化强烈，称为斜压模式。这些模式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和污染物的水平和垂直通量、沉积物迁移以及大陆架上的碳循环。强烈的内潮会引起大量的速度切变，对石油和天然气钻井平台构成危险。该项目的结果可能会改善数值气候和大气环流模式中内波引起的混合的描述，特别是在海洋边界区域。该项目将资助两名博士生（一名在南卡罗来纳州，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本科生提供南卡罗来纳州海洋科学方案的培训机会。在世界海洋的许多地区，正压潮汐以长波模式的形式存在，被海岸线捕获。通常，最有活力的是零，基本模式，传播与海岸在其右（左）在北方（南）半球。当大陆架狭窄时，这种零模类似于非色散开尔文波。对于较宽的陆架，半日基本波模成为混合开尔文边缘波（HKEW），群速度随波数变化。对于宽于~200 km的陆架，半日频率的HKEW群速度变低甚至为零。如果潮波沿着大陆边缘传播，遇到地形变化，群速度降低，沿岸能流会聚，引起潮振幅放大，潮能以非陷波Poincare波模的形式辐射。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），半日潮的传播似乎在南纬40度附近受阻。本研究的中心假设是HKEW模态中的能量通量辐合遇到陆架地形的沿岸变化，导致能量从正压模态转换为斜压模态。也就是说，应该有一个强大的内潮的产生，其中正压潮汐的群速度基本上在基模相位传播的方向上减小。将使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几何形状、分层和入射HKEW模式振幅来研究波浪散射过程。将寻求一个简单的参数化，以预测入射能量通量转换成斜压模式辐射从陆架坡折/上大陆坡向海岸和近海的部分。参数化将通过将其预测与HYCOM中最先进的潮汐模拟进行比较来评估，以进行低模式内波能转换（推测HYCOM的当前版本已很好地解决了这一问题）。同时，该研究将确定高模内波束可能具有潜在重要性的领域，这些领域在当前版本的HYCOM中尚未解决。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。还将考虑与半日HKEW模式散射成内波及其与平均涡流的相互作用相关的非线性动力学。这些动态可能会导致在散射区附近产生低频或静止的中尺度流，这是一种不同于通常引起的平均流不稳定性的中尺度变率机制。

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.