Collaborative research: Generation of internal waves due to the scattering of semidiurnal hybrid Kelvin-edge waves at varying continental shelf topography

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Sustaining the observed structure of the ocean and its circulation requires a certain level of mixing. Near the surface, wind is a major driver for mixing but its effectiveness diminishes with depth. Astronomic forces apply at all depths of the ocean and induce flows throughout the water column, called barotropic tides. However, being nearly uniform over large spatial scales, those flows are not very effective at stirring the ocean. This study explores the idea that these barotropic tides propagating along a wide shelf undergo a particular kind of instability near topographic variations and result in flows which vary strongly with depth, called baroclinic modes. These modes can propagate obliquely as internal waves and result in mixing on the continental shelf as well as in the interior of the ocean. They can affect the horizontal and vertical fluxes of nutrients and pollutants, sediment transport, and the carbon cycle on continental shelves. Strong internal tides can induce substantial velocity shear and represent a hazard for oil and gas drilling platforms. The results of this project may improve a description of the internal wave-induced mixing in numerical climate and general circulation models, especially in oceanic boundary regions. The project will support two PhD students (one at University of South Carolina and one at University of Southern Mississippi) and will offer training opportunities for undergraduates at the University of South Carolina Marine Science program through class work and individual research projects. An early-career scientist will be supported.In many areas of the World Ocean, barotropic tides exist in the form of long wave modes trapped by the coastline. Typically, the most energetic is the zero, fundamental mode, propagating with the coast on its right (left) in the Northern (Southern) hemisphere. This zero mode resembles a nondispersive Kelvin wave when the shelf is narrow. For wider shelves, the semidiurnal fundamental mode becomes a hybrid Kelvin-edge wave (HKEW) with group velocity changing with the wavenumber. For shelves wider than ~200 km, the HKEW group velocity at semidiurnal frequency becomes low or even zero. If a tidal wave propagating along the continental margin encounters topographic variations where its group velocity decreases, the resulting alongshore energy flux convergence causes the amplification of tidal amplitude and the radiation of tidal energy in the form of non-trapped Poincare wave modes. A good example of this phenomenon is the Patagonia Shelf (Southwest Atlantic) where the propagation of semi-diurnal tides is seemingly blocked in the vicinity of 40 deg S. The central hypothesis of this study is that the energy flux convergence in the HKEW mode encountering alongshore variations of shelf topography results in the energy conversion from barotropic to baroclinic mode. That is, there should be a strong generation of internal tides where the group velocity of barotropic tides substantially decreases in the direction of the fundamental mode phase propagation. A series of process-oriented numerical experiments will be made using Regional Ocean Modeling System (ROMS) where this wave scattering process will be studied by systematically varying the shelf and slope geometry, stratification and the incident HKEW mode amplitude. A simple parameterization will be sought to predict the fraction of the incident energy flux converted into baroclinic modes radiating from the shelf break/upper continental slope both toward the coast and offshore. The parameterization will be evaluated by comparing its predictions with state of the art tidal simulations in HYCOM for low-mode internal wave energy conversion (presumably well resolved by the current version of HYCOM). At the same time, the study will identify areas where higher mode internal wave beams can be potentially important, which are unresolved in the current version of HYCOM. Thus, results of this project will guide a further development of tidal simulations, especially on continental shelves. Nonlinear dynamics associated with the semidiurnal HKEW mode scattering into internal waves and their interaction with mean, eddying currents will also be considered. These dynamics can result in the generation of low-frequency or stationary mesoscale flows in the vicinity of the scattering region, a different mechanism for mesoscale variability than the often invoked instability of mean currents.