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

Oregon State University, Corvallis OR

Investigators

Abstract

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 oceanographer The 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.