Internal tides in straits and small ocean basins: resonant modes vs. propagating waves

潮汐是由整个水柱上的天体力产生的，但在分层的海洋中，它们与地形或类似机制的相互作用也产生了内部潮汐，这表现为各种密度水平垂直于其正常静止位置的周期性位移。这些内部潮汐可以在远离地形的地方传播很远的距离，它们在远离产生地点的消散导致海洋混合，这被认为有助于维持深海分层和翻转环流。大量关于内部潮汐生成的数值和观测研究，要么集中在洋脊和孤立地形上的生成，要么集中在开阔海岸上的生成，而不是在海湾、海湾、边缘海和海峡等半封闭盆地上的生成。在适当的盆地结构下，潮汐强迫可以引起适当的盆地几何形状的共振响应，然而这种“适当”的结构可能是什么，人们还知之甚少。在正压或双层模式中，很少有文献考虑盆地模式对潮汐强迫的共振。然而，垂直分层及其与海岸测深的相互作用可能对盆地内波的产生及其动力学产生重大影响。该项目将进行数值模拟，以研究在盆地的什么物理特征下，内部潮汐可以被表征为共振盆地模式，而不是自由传播的波。基于卫星的内部潮汐高程（振幅和相位）估计将用于研究在哪些盆地观测到共振内部潮汐，并将这些估计与数值模拟结果进行比较。虽然本项目的范围主要集中在内部潮汐动力学的物理机制上，但其结果和分析将具有更广泛的跨学科应用。例如，由内部潮汐引起的密度层位移引起的垂直运输可以将营养物质输送到表面或向底部水域提供氧气。这种垂直混合对一个盆地的生物生产力是重要的，特别是如果内部潮汐振幅和随后的垂直偏移可以被共振放大。本研究的分析将强调盆地内部潮汐可能发挥重要作用，特别是与风应力引起的混合有关。此外，对物理机制的理论理解对于改进从卫星观测估计内部潮汐振幅的模式至关重要。即将发射的表面海洋和水地形（SWOT）卫星任务具有前所未有的高分辨率和广泛的空间覆盖范围，为估计内部潮汐提供了进一步的希望。在模型中使用SWOT数据对现有卫星取样不足的小流域和沿海地区特别有用。本研究的理论结果将揭示内部潮汐动力学的新物理机制，这将对验证和理解基于卫星的估计非常重要。此外，这个拟议的项目将支持早期职业女性海洋学家的专业发展。拟议的项目解决了关于内部潮汐的文献中的一个重要空白：它们在小盆地中的动力学。以前的许多研究工作都集中在内部潮汐与孤立的海底地形的相互作用以及对开放海洋深海混合的影响或沿海大陆架内部潮汐的产生，而对半封闭盆地（如海湾、海湾、边缘海和海峡）对潮汐强迫的反应的研究却很少。然而，由于其波长较长（与水平盆地尺度相当），内部潮汐可能与自然盆地模式共振，这可以解释先前在某些盆地中观测到的大振幅内部潮汐。本研究将通过系统地增加数值模拟的复杂性来研究盆地特征（如大小、地形、分层）对其响应的影响，从而建立对盆地对潮汐强迫响应的理解。这些结果将阐明盆地的标准，在这些盆地中，内部潮汐反应可能被放大，从而可能导致更大的垂直输送和混合。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Tides are generated by astronomic body forces on the entire water column but in the stratified ocean, their interaction with topography or similar mechanisms also generate internal tides, which manifest themselves as a periodic displacement of the various density levels vertically from their normal resting position. These internal tides can propagate large distances away from the topography, and their dissipation away from the generation site results in ocean mixing that is thought to contribute to sustaining abyssal stratification and overturning circulation. A large number of both numerical and observational studies regarding internal tide generation have been focused on either generation at ocean ridges and isolated topographical features or generation at the open coast, but not in semi-enclosed basins, such as bays, gulfs, marginal seas, and straits. Under appropriate basin configuration, tidal forcing can elicit a resonant response for an appropriate basin geometry, yet what such “appropriate” configuration may be is yet poorly understood. Few previous works considered basin mode resonance to tidal forcing in barotropic or two-layer models. However, vertical stratification and its interaction with coastal bathymetry are likely to have a significant impact on the generation of internal waves and their dynamics within a basin. This project will conduct numerical simulations to investigate under what physical characteristics of a basin, internal tides can be characterized as resonant basin modes rather than freely-propagating waves. Satellite-based estimates of internal tide elevation (amplitude and phase) will be used to investigate in which basins resonant internal tides are observed and these estimates will be compared with numerical simulation results. Although the scope of this project is primarily focused on physical mechanisms of internal tide dynamics, the results and analysis will have broader interdisciplinary application. For instance, vertical transport due displacements of density layers by the internal tides can deliver nutrients to the surface or oxygenate the bottom waters. Such vertical mixing is important for biological productivity of a basin, especially if internal tide amplitudes, and subsequently vertical excursions, can be resonantly amplified. Analysis in this study will highlight in which basins internal tides might play an important role, especially relative to wind stress-induced mixing. Furthermore, theoretical understanding of the physical mechanisms is crucial to improving models that estimate internal tide amplitudes from satellite observations. The forthcoming launch of Surface Ocean and Water Topography (SWOT) satellite mission, which has unprecedentedly high resolution and wide spatial coverage, provides further promise for estimating internal tides. Using SWOT data in models will be particularly useful for small basins and coastal regions that are not well sampled by existing satellites. Theoretical results from this study will shed light on the new physical mechanisms for internal tide dynamics, which will be important for validating and understanding the satellite-based estimates. Additionally, this proposed project will support the professional development of an early career female oceanographerThe proposed project addresses a significant gap in literature regarding internal tides: their dynamics in small basins. Much of the previous research efforts have focused either on the interaction of internal tides with isolated bottom topography and effects on abyssal mixing in the open ocean or the generation of internal tides at coastal shelves, while there is a paucity of studies investigating the response to tidal forcing in semi-enclosed basins, such as bays, gulfs, marginal seas, and straits. However, because of their long wavelengths (comparable to horizontal basin scales), internal tides may be resonant with natural basin modes, which would explain previous observations of large amplitude internal tides in some basins. This study will build an understanding of the basin response to tidal forcing by systematically adding complexity to numerical simulations to investigate the effects of basin characteristics (e.g., size, topography, stratification) on its response. These results will shed light on the criteria for basins, where internal tide response may be amplified, thus potentially resulting in greater vertical transport and mixing.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.