Saturation of internal tide generation and wake vortex energy partitioning in shallow fjord-like topography
Date
2025
Authors
Chang, Jia-Xuan
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Abstract
Internal tides and wake vortices are key processes driving energy dissipation and mixing in coastal environments. While internal tide generation over supercritical slopes has been extensively studied in deep ocean settings, much less is known about its behavior in shallow coastal systems, where barotropic tidal forcing is strong, and non-linear effects dominate. This dissertation investigates the tidal energy pathways and dissipation associated with internal tides and wake vortices in such environments, using numerical simulations to explore isolated and complex three-dimensional to- pographies.
A novel phenomenon of energy saturation in internal tide generation is identi- fied, where the energy converted from barotropic tides ceases to scale quadratically with tidal velocity under highly non-linear conditions. This study characterizes the qualitative flow features associated with saturation, revealing that the flow resem- bles approach-controlled flow. Saturation is found to occur when the mean speed at the crest equals the mode-1 phase speed (Frc = Uc/c1 = 1). Moreover, the results challenge the conventional understanding that internal tides are not generated when F rc > 1; instead, they are generated but reach a saturation state.
In complex topographies featuring headlands, the energy partitioning between internal tides and wake vortices is analyzed. We identify additional energy losses fed to vortices, providing a systematic framework to estimate these losses using the bluff body law. Our results highlight that ridge-constricted flow and the cross-sectional area between the ridge crest and the top of headland yield reliable estimates. While wake vortices consume most of the energy that was fed into locally, their presence does not influence the dissipation or outward propagation of internal tide energy. The dissipation and propagation pathways of both processes are quantified, offering insights into their independent roles in coastal energy budgets.
These findings challenge theoretical scaling law and highlight the distinct dynam- ics of tidal energy in shallow coastal systems. By improving our understanding of tidal energy distribution and dissipation, this research contributes to refining param- eterizations in ocean models and advancing knowledge of coastal mixing processes.
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Keywords
Internal Tides, Fjord Oceanography, Tidal Energy Loss, Ocean Mixing, Headland Vortex