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.


Fig

Depth record of tow Deep-8.



Fig

``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.
 


Fig


 




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.
 


Fig



Summary plot of deep-towing results (in blue), with bootstrap confidence
intervals (95%) and the dye result (magenta, near N=20).


Fig


 





Coastal and Ocean Fluid Dyn. Lab Webpage
Ocean Acoustics Lab Webpage (tfd formal affiliation.)
CMO experiment Webpage