Influence of bottom frictional effects in sill regions upon lee wave generation and implications for internal mixing |
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Authors: | Jiuxing Xing Alan M Davies |
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Institution: | (1) Proudman Oceanographic Laboratory, 6 Brownlow Street, Liverpool, L3 5DA, England, UK |
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Abstract: | A cross-sectional nonhydrostatic model using idealized sill topography is used to examine the influence of bottom friction
upon unsteady lee wave generation and flow in the region of a sill. The implications of changes in shear and lee wave intensity
in terms of local mixing are also considered. Motion is induced by a barotropic tidal flow which produces a hydraulic transition,
associated with which are convective overturning cells, wave breaking, and unsteady lee waves that give rise to mixing on
the lee side of the sill. Calculations show that, as bottom friction is increased, current profiles on the shallow sill crest
develop a highly sheared bottom boundary layer. This enhanced current shear changes the downwelling of isotherms downstream
of the sill with an associated increase in the hydraulic transition, wave breaking, and convective mixing in the upper part
of the water column. Both short and longer time calculations with wide and narrow sills for a number of sill depths and buoyancy
frequencies confirm that increasing bottom friction modifies the flow and unsteady lee wave distribution on the downstream
side of a sill. Associated with this increase in bottom friction coefficient, there is increased mixing in the upper part
of the water column with an associated decrease in the vertical temperature gradient. However, this increase in mixing and
decrease in temperature gradient in the upper part of the water column is very different from the conventional change in near-bed
temperature gradient produced by increased bottom mixing that occurs in shallow sea regions as the bottom drag coefficient
is increased. |
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