Near-Surface Conversions of Semi-Diurnal Internal Tide Beam Energy

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As the gravitationally-forced tides flow over topography they disturb the ocean's internal stratification (e.g., layers of warm over cold water), creating internal oscillations (internal tides) with relatively strong currents and short spatial scales. The energy of the internal tides plays an important role in mixing the deep ocean with subsequent impacts on temperature, salinity and nutrient distributions that have far-reaching consequences for ocean dynamics and biogeochemistry, including climate-time-scale variability. The distribution of the energy in the internal tides is therefore a subject of considerable consequence. This project is focused on elucidating how, and how much, internal tide energy reaches the deep ocean. The investigators will undertake a set of numerical investigations to characterize and quantify the processes occurring when an upward-propagating, semi-diurnal, internal tide interacts with the variable stratification near the surface of the ocean and reflects back downward. The overarching question is "how much energy is 'lost' from the internal tide during its transits and reflection?"Even though this loss of internal tide energy in the upper ocean has received little attention, it may result in a significant reduction in the amount of energy available to ocean mixing, which is critical for maintaining the stratification at depth, as well as the supply of nutrients for primary production near the surface. Proper representation of such processes in oceanic and climate numerical models continues to be a subject of intense research due to the sensitivity of these models to mixing distributions. . A graduate student, Ms. Sherry Chou, who has provided much material for the proposal resulting in this project, will continue her training in internal wave science and numerical modeling techniques, and will complete her dissertation. The numerical modeling results will also inform how best to apply oceanographic instrumentation to study intricate internal wave processes in the future. The project's principal objectives are as follows: (1) To ascertain the range of the fluxes of energy from the internal tide into other phenomena (e.g., solitary waves; sub-harmonic waves; super-harmonic waves) as it interacts with the upper ocean stratification and free surface under different environmental conditions (such as different small-scale and large-scale stratification structures); and, (2) To explore from the numerical investigations how best to try to discriminate and quantify the different phenomena, which may all be present at once during the internal tide's transit through the real ocean, given the spatial and temporal sampling limitations of oceanographic instrumentation.