Deep ocean internal wave generation in weak stratification

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PI: Crockett, JulieProposal Number: 1606040The concentration of salt in the ocean changes with depth. This fact generates waves within the water that are called internal waves. The breaking of these waves drives much of the mixing in the deep ocean, defining mixing rates of pollutants, greenhouse gases, ocean nutrients and maintaining a balanced temperature distribution. It has been estimated that 2.1 terawatts of mixing power is required to maintain critical ocean mixing. Over half of the necessary power is postulated to come from internal waves generated by tidal flow over rough topography. The waves propagate through interactions with other waves and with ocean currents, until they convert to smaller scale, breaking waves. The focus of this proposal is to use laboratory experiments to study this process of internal wave generation and to understand how internal waves are generated and propagated within the ocean system. Findings from this research could lead to the development of new large scale ocean models that are important for the prediction of aquatic life migration patterns, for the prediction of the dispersion of pollutants in the ocean (like the dispersion of the radioactive spill from the Fukushima accident) and for the prediction of weather and climate patterns.An unexpected contributor to internal wave generation is tides flowing over topography in regions where the stratification is too weak for internal waves to be generated directly above the topography, yet internal waves can be observed well above the topography in a region where the stratification has increased. These waves may contribute to the conversion of tidal energy from large to small scales, which has important environmental consequences for wave-breaking and resultant mixing. The work proposed here is an experimental and analytical effort to characterize a relatively unexplored method of internal wave generation - the generation of internal waves due to tidal flow over topography in regions of weak stratification (where internal waves are not predicted to propagate) which radiate into strong stratification regions with implications for characterizing the internal wave energy spectrum throughout the ocean. It is proposed to: 1) explore internal wave generation by both uniform and tidal flow over topography in a two-layer stratification profile using experimentation; 2) characterize the effect of variations in stratification profile on internal wave generation, where for all stratification profiles the topography is in a region of weak stratification, such that internal waves are not expected, with a stronger stratification above; and 3) create an analytical model of the energy propagation from the topography through the variable stratification including the region representing evanescent to propagating waves. The project will contribute to the development of experimental methodology and understanding of stratified flow dynamics which will inform large scale modeling systems, resulting in better wave generation, propagation, and dissipation estimates. On the educational side, the project will support a research program of a professor from an under-represented group and training of graduate and undergraduate students in the ideas, concepts and practice of fluid dynamics, physical oceanography, internal wave theory, experimental methodology, and data analysis. The research will also be highlighted in high school and lower-division outreach activities and in upper-division courses.