Despite several decades of research, there is still much ambiguity about the importance of deep ocean topographic scattering of the low mode internal tide. Theoretical studies based on infinitesimal topography predict a modest 6-10% scattering efficiency (Muller & Xu 1992; StLaurent & Garrett 2002), and recent field data reveals long range propagation of the low mode internal tide northeast from Hawaii (Zhao et al. 2010). A recent theoretical study using finite-amplitude random topography, however, indicates that scattering can provide a rapid decay mechanism for internal tides (Buhler & Holmes-Cerfon 2011), and the currently underway NSF EXperimental study of Internal Tide Scattering (EXITS) field program is focusing on the Line Islands Ridge, where satellite altimetry data indicates 37% of the mode-1 flux propagating southwest from Hawaii is dissipated (Johnston, Merrfield & Holloway 2003).

The intellectual merit of this project is in the advancement and utilization of an analytical Green function method to make reasonable predictions of topographic internal tide scattering in the deep ocean. The method, which the PI has already used for internal tide generation studies, is not subject to many of the idealizations of previous theoretical approaches (e.g. infinitesimal topography and/or constant stratification). The theoretical model will be validated through comparison with laboratory experiments that employ a novel internal tide generator; a proof-of-concept experiment has already been performed to demonstrate the feasibility of the experimental program (Peacock et al. 2010). The work will be integrated with the field studies and numerical simulations of the NSF-funded EXITS program.

The primary broader impact of this research is a robust and quantitative assessment of the efficiency of deep ocean topographic scattering of low mode internal tides. The analytical method, validated by laboratory experiments, will provide a benchmark for testing numerical models and interpreting field data; it will also be integrated into a MATLAB code called iTide that will be made freely available to oceanographers. The project will support the training of a postdoctoral researcher and an undergraduate MIT researcher. The theoretical method and experimental results will be incorporated in graduate and undergraduate courses on wave mechanics, fluid mechanics and instrumentation. The results of this study will be disseminated through journal articles, invited seminars and the PI?s website. A workshop on internal waves at the Aspen Center for Physics will be proposed.

Project Report

This project is focused on a study of the scattering of internal waves - waves in the ocean density structure - by the sea floor topography. Of particular interest is the efficiency of sea floor topography in scattering energy from long to short wave length internal tides (internal waves at the tidal frequency), the latter being more amenable to instability and mixing processes. We developed an advanced analytical model to investigate this process and found that although typical small amplitude topography on the ocean floor is relatively inefficient at inducing scattering, modest amplitude topography that is around half the ocean depth is quite efficient. This is significant because it means that one only needs one significant topographic feature in the path of a propagating internal tide to extract substantial energy that can potentially drive instability and mixing in the ocean. Thus, scattering by deep ocean topography can be an important process, particularly in regions with many seamounts, such as the Western Pacific. These results were verified by comparisons with numerical simulations, and a software code called iTides has been made publicly available. Members of the project also participated in the NSF funded field study entitled Experimental Study of Internal Tide Scattering (EXITS) and are currently involved in supporting the data analysis. As a by-product of this project, a new method to analyze the modal structure of ocean internal wave data was also produced.

National Science Foundation (NSF)
Division of Ocean Sciences (OCE)
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Eric C. Itsweire
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Massachusetts Institute of Technology
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