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The Structure of Tidal Mixing on Georges Bank

Russ Burgett, Dave Hebert and Neil Oakey

Our work to date has consisted of two parts. In part one
microstructure and acoustic Doppler current profiler data are used
to parameterize the bottom stress in terms of a quadratic drag
coefficient, and to estimate the vertical extent of the near-bottom
constant stress layer. Semidiurnal tidal phase-averaged profiles of
turbulent kinetic energy (TKE) dissipation rate derived from
velocity shear data, were used to produce friction velocity profiles
at various times throughout the tidal cycle at each site for both
field studies. These friction velocity profiles were used to
estimate the bottom stress and the vertical extent of the constant
stress layer. The drag coefficient was computed from the bottom
stress and current magnitude at 12 m height above the bottom. At
the shallow site, the drag coefficient varied from 0.0006 to 0.0013.
At the deep site, a range of 0.0006 to 0.0009 was observed. The
drag coefficient decreased with increasing current speed at both
sites (Figure 1).
In Part Two, the measurements are used to study the vertical
structure of turbulent mixing on the bank
(Figure 2). At both sites
the dominant variability in the turbulent kinetic energy dissipation
rate was related to the semi-diurnal, M2, tide. In the well-mixed
portions of the water column a phase delay between TKE dissipation
rate and the current speed which increased with height above the
bottom was observed. Analysis of the vertical structure of the M2
tide shows that a phase lag between tidal velocity and vertical
shear exists and has a depth dependence which is similar to that of
the TKE dissipation rate-speed phase lag. A simple, one-dimensional
tidal model also produces a shear-velocity phase lag. Both the data
analysis and the model results can be explained in terms of the
analytical solution to the horizontal equations of motion with
constant eddy viscosity, forced by an oscillating pressure gradient.