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Internal Waves and the Nepheloid Layer on the Continental Shelf in the Gulf of Mexico
David W. McGrail, Richard Rezak (1)
The nepheloid layer on the south Texas continental shelf exhibits a distinct relationship with a mixed (isohaline-isothermol) layer at the base of the water column. This mixed layer is a boundary phenomenon produced by turbulance generated in the adjustment of the various modes of motion to the no slip condition at the shelf-sea interface. The boundary layer is capped by strongly stratified water which inhibits further upward mixing. Sediment either advected from near shore regions or resuspended locally is primarily limited to this mixed layer. There are, however, instances where layers containing suspended sediment maxima occur above and isolated from the mixed layer. These usually occur at some step in the temperature and salinity profiles. A simple flow visualization experiment carried out proximal to Southern and Hospital Banks offers both an explanation for the isolated maxima and a mechanism for substantial vertical diffusion of suspended sediment. The experiment consisted of recording the behavior of plumes shed from dye packets deployed within a meter of the bottom on both super 8mm movie film and video tape from the DSR/V Diaphus. Analysis of these recordings indicate that unusually high frequency internal waves (periods less than one minute) occurred at both locations. Similar motions were also picked up as isolated events on the Precision Depth Recorder. At Hospital Bank these motions dominated the flow whereas at Southern Bank the waves were superimposed on a unidirectional current. Classical theory and laboratory experiments have shown that when these waves propagate on to a slope they may become unstable, break and inject mixed fluid from the boundary seaward. This would account for the isolated suspended sediment maxima and their occurrence in stepped profiles of temperature and salinity. Both the internal waves and intermittent burst of turbulence were seen to cause upward diffusion of suspended sediment which would maintain the nepheloid layer.
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