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Diffusivity Within a Few Meters of the Bottom

*Chris Rehmann and Tim Duda*

*AOPE Dept., Woods Hole Oceanographic Institution*

*Woods Hole, MA USA*
*13 July 1999*

Results from towing very near the bottom (70 m depth)
are summarized by this plot. Profiles of temperature gradient,
thermal variance dissipation rate "chi", diffusivity (K), and upward
heat flux are shown. In the low-gradient near-bottom layer the heat
flux estimate is subject to sensitive to errors in the mean gradient estimate
and has large uncertainty. The final plot of this page shows
K as a function of N instead of depth.

The figures to follow show how the near-bottom
results were obtained. First is an example tow track, followed by estimates
of ``chi'' from another of our tows. From ``chi'' and the temperature
gradient one can estimate eddy diffusivity *(K)* and heat flux using
the Osborn-Cox model for diapycnal heat diffusion.

*Depth record of tow Deep-8.*

*``Chi'' (linear and log) and the depth-track for tow
Deep-5.*

The next two figures show results from tow Deep-6, performed
near the BASS bottom tripod near the CMO central site.

The statistics of thermal dissipation rate ``chi'' from
tow Deep-6 show a wide dynamic range (histogram, lower frame) and a nearly
log-normal probability distribution (upper frame). The variable examined
here is the greater of two ``chi'' estimates resulting from the ``white
spectrum'' correction method (Washburn et al, JAOT 1996). The two estimates
have different assumptions about T/S coherence, which is uncertain in the
thermal diffusive rolloff region.

Below, the uncorrected (left) and corrected upper-bound
(blue) and lower-bound (red) estimates of ``chi'' (right) from the
microconductivity probe are shown. The corrections depend on the local
TS relation. Mean profiles of salinity (green) and temperature (red)
are on the left. The results are averaged for all 10 tows in layers
(defined by temperatures) to get the mean profiles shown above.

Summary plot of deep-towing results (in blue), with bootstrap
confidence

intervals (95%) and the dye result (magenta, near N=20).

Coastal and Ocean Fluid Dyn. Lab Webpage

Ocean Acoustics Lab Webpage
(tfd formal affiliation.)

CMO experiment Webpage