The behavior of the barotropic tide in a closed ocean basin is well understood: in plan view the basin modes appear to rotate around the periphery of the basin as Kelvin waves, in the counter- clockwise sense in the northern hemisphere. This project explores whether the coupled baroclinic response, internal basin modes of much smaller wavelength than the surface tide, can be responsible for the large internal tides that are observed on the continental shelf. In the presence of any topographic variability, the barotropic and baroclinic modes are coupled, and the tide generating potential can generate large amplitude internal basin modes as well as barotropic modes. Because the baroclinic parameters (density anomaly and stratification structure) vary in time, the baroclinic response is not expected to be locked in constant phase with the tide generating potential, as is the barotropic mode. Nonetheless there will be a response that is slowly modulated, over times that are longer than a tidal period. Observations of internal tides reveal that there exists an along-coast energy flux, and that the variables (velocities, isotherm displacements) do not conform to a simple mode one internal wave, because the sense of rotation of the horizontal velocity vector changes with depth. That behavior is however consistent with baroclinic frictional Kelvin waves over shoaling depth. This study will combine theoretical analyses, idealized and realistic simulations and analyses of previously collected data to study the generation and propagation of along-coast propagating internal tides. Theoretical techniques will be used to investigate baroclinic response along continental shelves to basin-scale barotropic tidal forcing in a stratified, rotating basin. Idealized numerical simulations will be used investigate the structure of along-coast propagating internal tides generated by barotropic Kelvin waves interacting with along-shelf bathymetric variations. The sensitivity of barotropic to baroclinic energy conversion to shelf width, stratification, amplitude of barotropic tide and amplitude of bathymetric variability will be quantified. Realistic simulations of internal tide generation by barotropic tides will be conducted for U.S. west coast shelf and slope domains to investigate barotropic to baroclinic energy conversion and the nature of along-coast internal tide propagation. Long-term time series from previous field experiments also will be analyzed to determine the spatial structure of internal tides there and the nature of along-coast propagation.
Intellectual Merit. Current variability on many continental shelves is dominated by internal tides. The nature of along-coast structure and propagation is currently not known. This study aims to provide the idealized and realistic framework with which to understand along-coast trapping and propagation of internal tides.
Broader Impacts. This study will support the development of a permanent exhibit and related curriculum at the Hatfield Marine Science Center?s Visitor Center (HMSVC) in Newport, OR. A graduate student at Scripps Institution of Oceanography will be supported by this study and will be trained in coastal physical oceanography, numerical methods, and in field data analysis techniques. The circulation induced in coastal waters by internal waves and tides are currently the focus of attention along the Pacific coast of California and Mexico. This project will also allow the PIs to continue their collaboration with Mexican colleagues in the design of an observational study of internal tides, internal waves and cross-shore fluxes off the Mexican coast.
