Cross-Frontal Exchange and Scotian Shelf Cross-over Workshops 10-12 October, 2000 Holiday Inn, Falmouth, Massachusetts |
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Overview
Cross-Frontal Exchange
Presentations
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Dye Dispersion on the South Flank Isopycnal Dispersion on the South Flank Jim Ledwell (WHOI) Dye patches were released in the pycnocline outside the
tidal mixing front on both the South and North Flanks of Georges Bank in May
and June of 1999 during R/V Endeavor Cruises 323 and 324. Mean advection of dye in the pycnocline
toward or away from the front was subtle, as was true of the drogued drifters
discussed by Jim Churchill. Ultimately
we may be able place limits of around 1 cm/s, positive and negative on this
mean advection. The evolution of the
dye patches does give a clear measurement of epipycnal (along isopycnal)
dispersion, however. For example, a dye
patch released in the pycnocline on the South Flank grew dramatically over the
course of 117 hours. The growth of the area of the patch is fairly well described by the mixing diagrams of Okubo [1982], even though our data are from a stratified layer while most of the data synthesized by Okubo were from surface layers, directly driven by wind and surface energy fluxes. Okubo's diagrams indicate a scale-dependent diffusivity giving:
Where The dye patch shown in the above figure is still outside the tidal mixing front even during the last survey. Does our mixing formula hold right across this front? I can't say, not knowing well the dynamics at the front, especially the effective eddy viscosities. What does seem to be implied by our data is that a patch of plankton or other material found between the tidal mixing front and the shelf/slope front will disperse towards both fronts with a speed of 1 cm/s. That is, significant parts of the patch will reach the tidal mixing front at that speed, greatly diluted by surrounding water, and significant parts will also go away from the front at that speed. |