Collaborative Research: The Internal Tide and Mixing in Monterey and Ascension Canyons

University Of Washington, Seattle WA

Investigators

Abstract

Intellectual Merit Based on theory, laboratory experiments and field observations, canyons have long been identified as sites of intense internal waves. Turbulence measurements in Monterey Canyon (MC) and density overturns in Kaoping Canyon (Taiwan), which has similar size and shape, confirm very strong mixing. The processes generating the mixing, however, have not been determined, nor has mixing been measured in enough canyons to assess its variability from one to another. Observations and recent modeling indicate that the internal tide is a major source of energy in Monterey Canyon, coming from offshore as well as being generated locally. In addition, ridges and constrictions along the canyon make it likely that lee waves and hydraulic jumps are also major sources of mixing, as has been inferred for abyssal canyons. The project will undertake modeling and intensive observations for Monterey Canyon and exploration of nearby Ascension Canyon which is much straighter than Monterey and apparently lacks a significant offshore source for the internal tide. The goals are to: 1) understand how the internal tide propagates in canyons, and in particular how the M2 energy flux is steered around bends and when turns are so sharp that the energy is dissipated, 2) determine whether mixing locations and levels can be predicted by convergences in the along-canyon M2 energy flux, 3) identify and quantify mixing produced by internal wave scattering, lee waves and hydraulic jumps, and 4) assess the effects that canyon shape, smoothness and alignment with offshore M2 sources have on mixing and baroclinic energy by measuring mixing and the internal tide in a relatively short, straight canyon ending on the outer shelf. Broader Impacts U.S. coasts have dozens of canyons, and they account for almost half of the western continental slope from 45◦N to Alaska. Though they appear to be biologically rich and export both water and sediments, dynamical processes within canyons are too poorly understood to make a general assessment of the importance of canyons or whether they require special representation in large-scale or regional models. One set of measurements cannot resolve all questions about canyon dynamics, but the proposed work should provide qualitative and quantitative bases for assessing mixing in other canyons.. If fully successful, this project may unable the parameterization of some of the mixing or at least be able to point the way to parameterizations. As part of the proposed effort the investigators have also begun talks with regional modelers to incorporate intense canyon mixing into the models and to do sensitivity runs to define how accurately the mixing must be known to predict its effect on flows and structures outside the canyon. Two post-doc will be hired, one at APL/UW and another at the University of Hawaii. Both will participate in all phases of the work for their postdoctoral training. Coordination and collaboration with the regular MBARI sampling of Monterey Bay will benefit this project, and vice versa. Outreach activities will combine earlier canyon measurements and the future measurements.