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: 1537449
• 负责人: Alexander Yankovsky
• 金额: $ 26.23万
• 依托单位: University of South Carolina at Columbia
• 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=1537449&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.

维持观测到的海洋结构及其循环需要一定程度的混合。在地表附近，风是混合的主要驱动力，但其效果随着深度的增加而减弱。天文力量作用于海洋的所有深度，并在整个水柱中诱导水流，称为正压潮汐。然而，这些流动在大的空间尺度上几乎是均匀的，在搅动海洋方面并不是很有效。这项研究探索了这样一种观点，即这些沿着宽阔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，并导致了随深度强烈变化的流动，称为斜压模。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。它们会影响营养物质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。强烈的内潮可以引起巨大的速度剪切，对石油和天然气钻井平台构成危险。这一项目的结果可能会改进数值气候和大气环流模式中对内波诱导混合的描述，特别是在海洋边界地区。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。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的形式存在。通常情况下，能量最强的是零基模，在北半球(南半球)沿其右(左)岸传播。当搁板很窄时，这种零模类似于非色散的开尔文波。对于较宽的陆架，半日基模变成群速度随波数变化的混合开尔文边缘波。对于宽度大于200公里的大陆架，HKEW半日频率的群速度变得很低，甚至为零。如果沿大陆边缘传播的潮波遇到地形变化，其群速度减小，由此产生的近岸能量通量收敛导致潮汐振幅放大，并以非捕获庞加莱波模的形式辐射潮汐能量。这种现象的一个很好的例子是巴塔哥尼亚陆架(西南大西洋)，在那里半日潮的传播似乎在40°S附近被阻止。本研究的中心假设是HKEW模式中的能量通量收敛遇到陆架地形的近岸变化导致了从正压模式到斜压模式的能量转换。也就是说，应该有一个强烈的内潮产生，其中正压潮汐的群速度在基模相位传播方向上显著减小。将利用区域海洋模式系统(ROMS)进行一系列面向过程的数值试验，通过系统地改变陆架和斜坡的几何形状、层结和入射HKEW模振幅来研究波的散射过程。将寻求一种简单的参数化来预测从陆架断裂/大陆上坡向海岸和离岸辐射转化为斜压模式的入射能量通量的比例。将通过将其预测与HYCOM中最先进的低模内波能转换潮汐模拟(假设HYCOM当前版本很好地解决了这一问题)进行比较来评估该参数化。与此同时，这项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模散射到内波以及它们与平均涡流的相互作用有关的非线性动力学。这些动力学可导致在散射区附近产生低频或稳定的中尺度流动，这是一种不同于经常引发的平均流不稳定的中尺度可变性的机制。