Tidal mixing in sill regions: influence of sill depth and aspect ratio |
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| Authors: | Jiuxing Xing Alan M Davies |
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| Institution: | (1) National Oceanographic Centre, 6 Brownlow Street, Liverpool, Merseyside, L3 5DA, UK |
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| Abstract: | A non-hydrostatic model in cross-sectional form with an idealized sill is used to examine the influence of sill depth (h
s) and aspect ratio upon internal motion. The model is forced with a barotropic tide and internal waves and mixing occurs at
the sill. Calculations using a wide sill and quantifying the response using power spectra show that for a given tidal forcing
namely Froude number F
r as the sill depth (h
s) increases the lee wave response and vertical mixing decrease. This is because of a reduction in across sill velocity U
s due to increased depth. Calculations show that the sill Froude number F
s based on sill depth and across sill velocity is one parameter that controls the response at the sill. At low F
s (namely F
s ≪ 1) in the wide sill case, there is little lee wave production, and the response is in terms of internal tides. At high
F
s, calculations with a narrow sill show that for a given F
s value, the lee wave response and internal mixing increase with increasing aspect ratio. Calculations using a narrow sill
with constant U
s show that for small values of h
s, a near surface mixed layer can occur on the downstream side of the sill. For large values of h
s, a thick well-mixed bottom boundary layer occurs due to turbulence produced by the lee waves at the seabed. For intermediate
values of h
s, “internal mixing” dominates the solution and controls across thermocline mixing. |
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| Keywords: | |
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