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

合作研究：由于半日混合开尔文边缘波在不同大陆架地形上的散射而产生内波

• 批准号: 1537158
• 负责人: Maarten Buijsman
• 金额: $ 22.71万
• 依托单位: University of Southern Mississippi
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-09-15 至 2020-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1537158&HistoricalAwards=false
• 关键词: Collaborative; research; Generation; internal; waves

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.

维持观察到的海洋结构及其循环需要一定水平的混合。在表面附近，风是混合的主要驱动力，但其有效性会随着深度而减小。天文力在海洋的所有深处都施加，并在整个水柱中诱导流动，称为正压潮。但是，在大空间尺度上几乎均匀，这些流在搅动海洋方面并不是很有效。这项研究探讨了这样一种观念，即这些正面潮汐沿宽阔的架子传播，经历了一种特殊的不稳定性近地形变化，并导致流动随深度急剧而变化，称为斜压模式。这些模式可以倾斜地作为内波传播，并在大陆架以及海洋内部混合。它们可以影响养分和污染物，沉积物传输以及大陆架子上的碳循环的水平和垂直通量。强烈的内部潮汐可以引起大量的速度剪切，并代表了石油和天然气钻井平台的危害。该项目的结果可能会改善对数值气候和一般循环模型中内波诱导的混合的描述，尤其是在海洋边界区域。该项目将支持两名博士生（一名在南卡罗来纳大学，一名在南密西西比大学），并通过课堂工作和个人研究项目为南卡罗来纳大学海洋科学计划的本科生提供培训机会。将支持一名早期职业科学家。在世界上许多海洋地区，正压潮汐以被海岸线捕获的长波模式的形式存在。通常，最有活力的是零，基本模式，在北半球（南）半球的右（左）上传播了海岸。当架子狭窄时，这种零模式类似于非分散性开尔文波。对于较宽的架子，半潮汐基本模式变成了杂种开尔文 - 边缘（HKEW），随着波数的速度变化。对于架子宽于200 km的范围较大，半繁殖频率下的HKEW组速度变低甚至零。如果沿着大陆边缘传播的潮汐会遇到其组速度降低的地形变化，则沿沿岸能量通量收敛产生的潮汐幅度会导致潮汐幅度的扩增和潮汐能量的辐射。这种现象的一个很好的例子是巴塔哥尼亚的架子（西南大西洋），其中半潮汐潮汐的传播似乎在40摄氏度的附近被阻塞。这项研究的中心假设是，在hekew模式下，能量通量融合在沿海货币多样性的hekew模式中产生了shelf toposhion controspospon的hekew模式。也就是说，应该有强烈的内部潮汐产生，在基本模式相传播的方向上，正压潮的组速度大大降低。将使用区域海洋建模系统（ROMS）进行一系列面向过程的数值实验，其中将通过系统地改变架子和坡度几何形状，分层和入射HKEW模式振幅来研究此波散射过程。将寻求一个简单的参数化，以预测从架子断裂/上大陆斜坡辐射到海岸和近海的入射能通量的比例。参数化将通过将其预测与HYCOM中的最先进的潮汐模拟的状态进行比较，以评估低模式内波能量转换（大概由当前版本的HYCOM解决了）。同时，该研究将确定较高模式内波束可能非常重要的区域，而当前版本的HYCOM在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，尤其是在大陆货架上。还将考虑与半潮期HKEW模式相关的非线性动力学，并将其与平均涡流电流相互作用。这些动力学可能导致在散射区域附近产生低频或固定的中尺度流，这是一种与通常所调用的平均电流不稳定性相比，一种用于中尺度变异性的机制不同。