Shallow water tidal currents in close proximity to the seafloor and boundary-induced turbulence
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Velocity measurements with vertical resolution 0.02 m were conducted in the lowest 0.5 m of the water column using acoustic Doppler current profiler (ADCP) at a test site in the western part of the East China Sea. The friction velocity u * and the turbulent kinetic energy dissipation rate ε wl(ζ) profiles were calculated using log-layer fits; ζ is the height above the bottom. During a semidiurnal tidal cycle, u * was found to vary in the range (1–7) × 10−3 m/s. The law-of-the-wall dissipation profiles ε wl(ζ) were consistent with the dissipation profiles ε mc(ζ) evaluated using independent microstructure measurements of small-scale shear, except in the presence of westward currents. It was hypothesized that an isolated bathymetric rise (25 m height at a 50-m seafloor) located to the east of the measurement site is responsible for the latter. Calculation of the depth integrated internal tide generating body force in the region showed that the flanks of the rise are hotspots of internal wave energy that may locally produce a significant turbulent zone while emitting tidal and shorter nonlinear internal waves. This distant topographic source of turbulence may enhance the microstructure-based dissipation levels ε mc(ζ) in the bottom boundary layer (BBL) beyond the dissipation ε wl(ζ) associated with purely locally generated turbulence by skin currents. Semi-empirical estimates for dissipation at a distance from the bathymetric rise agree well with the BBL values of ε mc measured 15 km upslope.
KeywordsTidal current Bottom boundary layer Friction velocity Turbulent kinetic energy dissipation rate Logarithmic layer Law of the wall Boundary-induced turbulence
The authors are grateful to scientists and students at Ocean University of China led by Prof. Hao Wei for arranging logistics of data collection. The effort was supported by the Major State Program of China for Basic Research (grant 2006CB400602). The analysis presented in this paper was supported by the US Office of Naval Research (grant N00014-05-1-0245), the National Natural Science Foundation of China (Z. Liu, grants 41006017 and 41076001), the Fundamental Research Funds for the Central Universities (Z. Liu, grant 2010121031), Arizona State University (J. Armengol as a visiting researcher), and by the Spanish Ministry of Education and Science (E. Roget, grant FIS2008-03608).
- Korotenko KA (1995) On the formation of mesoscale inhomogeneities of the matter concentration field in a local upwelling zone of the ocean. Oceanol Eng Transl 34(4):445–451Google Scholar
- Kunze E, Llewellyn Smith SG (2004) The role of small scale topography in turbulent mixing of the global ocean. Oceanography 17:51–60Google Scholar
- Lozovatsky ID (1999) Seamount turbulence: generation and decay. Abstracts of XXII General Assembly of IUGG, Birmingham, UKGoogle Scholar
- Nabatov VN, Ozmidov RV (1988) A study of turbulence over underwater mountains in the Atlantic Ocean. Oceanology 28:210–217 (in Russian)Google Scholar
- Thorpe SA (2005) The turbulent ocean. Cambridge University Press, CambridgeGoogle Scholar