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

潮汐是由整个水柱上的天文体力产生的，但在分层海洋中，它们与地形或类似机制的相互作用也会产生内部潮汐，表现为各种密度水平从正常静止位置垂直方向的周期性位移。这些内部潮汐可以在远离地形的地方传播很远的距离，它们的消散导致海洋混合，这被认为有助于维持深海层结和颠覆环流。大量关于内潮产生的数值和观测研究，要么集中在洋脊和孤立的地形地貌，要么集中在开阔海岸的产生，而不是在半封闭盆地，如海湾、海湾、边缘海和海峡。在适当的盆地配置下，潮汐强迫可以引起对适当的盆地几何形状的共振响应，然而，这种“适当的”配置可能是什么，人们还知之甚少。以往很少有工作考虑正压或两层模式中的盆地模式共振与潮汐强迫。然而，垂直层结及其与沿海水深测量的相互作用很可能对盆地内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，以调查在盆地的哪些物理特征下，内潮可以被描述为共振盆地模式，而不是自由传播的波。将使用基于卫星的内部潮汐高程(幅度和相位)估计来调查在哪些盆地观测到共振的内部潮汐，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上，但结果和分析将有更广泛的跨学科应用。例如，由于内部潮汐对密度层的移动而造成的垂直输送可以将营养物质输送到表面或使底层水域富氧。这种垂直混合对于盆地的生物生产力很重要，特别是如果内部潮汐幅度以及随后的垂直漂移能够被共振放大的话。这项研究的分析将突出哪些盆地的内潮可能发挥重要作用，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根据卫星观测估计内部潮汐幅度的模型至关重要。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，为估计内潮提供了进一步的前景。在模型中使用SWOT数据将对现有卫星没有很好采样的小盆地和沿海地区特别有用。这项研究的理论结果将阐明新的内部潮汐动力学的物理机制，这对于验证和理解基于卫星的估计将是重要的。此外，这一拟议项目将支持职业生涯早期的女海洋学家的专业发展。拟议项目解决了有关内部潮汐的文献中的一个重大空白：它们在小流域中的动力学。以前的许多研究工作要么集中在内潮与孤立的海底地形的相互作用以及对公海深海混合的影响，要么集中在沿海陆架的内潮产生上，而研究半封闭盆地(如海湾、海湾、边缘海和海峡)对潮汐强迫的响应的研究很少。然而，由于它们的波长很长(与水平盆地的尺度相当)，内潮可能与自然盆地的模式共振，这可以解释以前在一些盆地观测到的大幅度内潮。这项研究将通过系统地增加数值模拟的复杂性来调查盆地特征(例如，大小、地形、层化)对其响应的影响，从而建立对盆地对潮汐强迫的响应的理解。这些结果将阐明盆地的标准，在那里内部潮汐响应可能被放大，从而潜在地导致更大的垂直输送和混合。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.