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

潮汐是由作用在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会产生内潮，内潮表现为各种密度水平从正常静止位置垂直的周期性位移。这些内潮可以传播到远离地形的很远的地方，它们在远离生成地点的地方消散，导致海洋混合，这被认为有助于维持深海分层和翻转环流。大量的数值和观测研究都集中在内潮的生成在洋脊和孤立的地形特征或生成在开放的海岸，而不是在半封闭的盆地，如海湾，海湾，边缘海和海峡。在适当的盆地配置下，潮汐力可以引起适当的盆地几何形状的共振响应，但这种“适当”的配置可能是还知之甚少。在正压模式或两层模式中，很少有研究考虑了海盆模态共振对潮汐强迫的影响。然而，垂直分层及其与沿海测深的相互作用可能对内波的生成及其在盆地内的动态产生重大影响。该项目将进行数值模拟，以研究在盆地的哪些物理特性下，内部潮汐可以被描述为共振盆地模式，而不是自由传播的波。将利用卫星对内潮高程（振幅和相位）的估计来调查在哪些盆地观测到共振内潮，并将这些估计与数值模拟结果进行比较。虽然这个项目的范围主要集中在内部潮汐动力学的物理机制，结果和分析将有更广泛的跨学科应用。例如，由于内潮引起的密度层位移引起的垂直迁移可以将营养物输送到表面或输送到底部沃茨。这种垂直混合对于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。在这项研究中的分析将突出在盆地内潮汐可能发挥重要作用，特别是相对于风应力引起的混合。此外，对物理机制的理论理解对于改进根据卫星观测估计内部潮汐振幅的模型至关重要。即将发射的表层海洋和水域地形（SWOT）卫星使命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数据对于现有卫星没有很好采样的小流域和沿海地区特别有用。这项研究的理论结果将揭示内部潮汐动力学的新的物理机制，这对于验证和理解基于卫星的估计将是重要的。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。拟议的项目解决了有关内部潮汐的文献中的一个重大空白：它们在小流域的动态。以前的研究工作主要集中在内潮与孤立的海底地形的相互作用和对开阔洋深海混合的影响，或在沿海陆架产生内潮，而很少有研究调查半封闭盆地，如海湾，海湾，边缘海和海峡的潮汐强迫的反应。然而，由于它们的长波长（相当于水平盆地尺度），内潮汐可能与自然盆地模式共振，这将解释以前在一些盆地中观察到的大振幅内潮汐。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（例如，大小，地形，分层）对它的反应。这些结果将阐明盆地的标准，其中内部潮汐响应可能会被放大，从而可能导致更大的垂直运输和mixing.This奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.