Local Generation of Nonlinear Internal Waves on the Continental Shelf: Beams to Modes

该项目研究在海洋内部形成的波浪，在具有不同温度和/或盐度特性的水团的界面上形成。这种波被称为内波，当潮汐流与海洋内部的不规则水深或陡峭的大陆斜坡相互作用时就会产生。这些内部潮汐波倾向于沿着类似于光束的特定路径集中能量，并几乎垂直地在水柱中传播。在内潮传播过程中，会产生另一种具有不同特征的波，称为内孤子波。它们水平繁殖，并与垂直混合、物质和幼虫的跨大陆架运输以及沉积物动员有关。这项研究的重点是理解从主要的束状内潮到较短的非线性波的转变，特别是在沿海海洋中。实验室、实验工作与理论和建模活动相结合，将用于了解这些高能海浪的生命周期，并为沿海环境的建模和解释提供信息，促进我们对海洋混合的理解。在这项工作的过程中，首席研究员将通过伍兹霍尔海洋研究所（WHOI）暑期学生奖学金计划，就与内波产生和传播直接相关的研究课题向本科生提供建议。此外，本研究将为每年在WHOI举行的地球物理流体动力学（GFD）夏季项目做出贡献，并将涉及本研究的各个方面。该提案还将有助于维持WHOI GFD实验室，这是科学研究界的宝贵资源。岸上传播的大振幅内孤立波在世界各大洋的大陆架上无处不在。这些波浪在向岸上传播的过程中，由于内部潮汐的非线性变陡而呈包状出现。这一过程的一个未被充分认识的方面是，辐射的内潮通常由大陆坡上产生的垂直传播波束主导，而涌现的孤立波则以单一的垂直模式结构水平传播。从主要的梁状内潮到具有模态结构的较短的非线性波浪的转变是如何发生的还不是很清楚，特别是在沿海环境中。特别是，出现的时间和长度尺度以及由此产生的孤立波的性质令人感兴趣，并将作为本研究的一部分进行检查。将采用实验室实验和数值模拟工作相结合的方法来检查大陆斜坡上的内部潮汐波束的产生，以及波束在大陆架的封闭地形上传播时的后续演变，在那里它可能最终导致水平传播的模态内部孤立波的出现。这项工作将考虑理想化的分层、地形和强迫（例如单一强迫频率），以便关注潜在的动力学，提高我们对这种转变的理解，以及促进或抑制内部潮汐产生这些波的条件。实验组件将用于探索内部波束在架子上的产生和初始传播，并为随后的建模提供测试。建模将解决实验室工作固有的局限性，并使用两种互补的方法。第一种是理想化的多模态Korteweg-de Vries （KdV）模型，该模型推广了先前的内部潮汐产生的线性流体静力学模型，并捕获了孤立波动力学。第二种是使用MIT一般循环模型，该模型允许探索kdv型模型中未考虑的其他因素。

This project studies waves that are formed in the interior of the ocean, at the interface of water masses with different temperature and/or salinity properties. Such waves, called internal waves are generated when tidal flows interact with bathymetric irregularities in the interior of the ocean or the steep continental slope. These internal tidal waves tend to concentrate their energy along specific paths that resemble that of a light beam and propagate almost vertically in the water column. During the internal tide propagation, another type wave is generated, with different characteristics, called internal soliton waves. They propagate horizontally and have been linked to vertical mixing, cross-shelf transport of materials and larvae, and sediment mobilization. This study focuses on understanding the transition from principally beam-like internal tides to shorter nonlinear waves, especially in the coastal ocean. Laboratory, experimental work combined with theoretical and modelling activities will be used to understand the life-cycle of these energetic waves and inform modeling and interpretation of the coastal environment advancing our understanding of ocean mixing. Over the course of this work, the principal investigator will advise undergraduate students through the Woods Hole Oceanographic Institution (WHOI) Summer Student Fellowship Program on research topics directly related to the internal wave generation and propagation. Additionally, this study will contribute to the summer program in Geophysical Fluid Dynamics (GFD), held annually at WHOI, and it will involve GFD fellows with aspects of this research. This proposal will also contribute to maintaining the WHOI GFD Laboratory, a valuable resource to the scientific research community.Onshore propagating large-amplitude internal solitary waves are ubiquitous on continental shelves throughout the world's oceans. These waves emerge in packets from the nonlinear steepening of the internal tide as it propagates onshore. An under-appreciated aspect of this process is that the radiated internal tide is often dominated by vertically propagating beams generated on the continental slope, while the emergent solitary waves propagate horizontally with a single vertical mode structure. How the transition from principally beam-like internal tides to shorter nonlinear waves with modal structure occurs is not well understood, especially in the coastal setting. In particular, the time and length scales for emergence and the resulting properties of the solitary waves are of interest and will be examined as part of this study. A combination of laboratory experiments and numerical modeling work will be used to examine internal tidal beam generation at the continental slope and subsequent evolution of the beam as it propagates onto the confining topography of the shelf where it may eventually lead to the emergence of horizontally propagating modal internal solitary-like waves. The work will consider idealized stratifications, topography, and forcing (e.g. as single forcing frequency) in order to focus on the underlying dynamics and improve our understanding of this transition and the conditions that promote, or inhibit, the generation of these waves from the internal tide. The experimental component will be used to explore the generation and initial propagation of an internal wave beam onto a shelf and to provide a test for the subsequent modeling. The modeling will address limitations inherent in the laboratory work and use two complementary approaches. The first is an idealized multi-mode Korteweg-de Vries (KdV) model that generalizes previous linear, hydrostatic models of internal tide generation and captures the solitary wave dynamics. The second is to use the MIT general circulation model that allows exploration of additional factors not considered in KdV-type models.