Numerical study of tide-induced mixing over rough bathymetry in the abyssal ocean |
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| Authors: | Nobuyuki Iwamae Toshiyuki Hibiya |
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| Institution: | (1) Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan;(2) Present address: Civil Structure Group, Kajima Technical Research Institute, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan; |
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| Abstract: | Locally enhanced turbulent mixing over rough bottom bathymetry is one of the candidates that might make up for the lack of
diapycnal diffusivity in maintaining the global overturning circulation. In the present study, using a two-dimensional vertical
numerical model for the Brazil Basin, we numerically examine the intensity and vertical structure of tide-induced mixing over
multi-beam bottom bathymetry via the comparison with those over somewhat smoothed bottom bathymetry. Note that even this smoothed
bottom bathymetry is finer than in commonly used datasets. In comparison to the response over the smoothed bottom bathymetry,
energy dissipation rates are enhanced within a few hundred meters over the multi-beam bottom bathymetry. In spite of several
limitations of the two-dimensional vertical numerical model, the magnitude and vertical distribution of the calculated dissipation
rates agree well with those from microstructure measurements. We find that tidal interaction with fine-scale (≤2 km) bottom
bathymetry efficiently generates high wavenumber internal waves, which are subject to local energy dissipation and hence strongly
control the abyssal mixing; the most important finding is that the intensity and vertical decay scale of abyssal mixing are
in a trade-off relationship with each other, which is not taken into account in the existing parameterizations. |
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