Shear of small vertical scale observed in the permanent oceanic thermocline
Van Leer, John Cloud
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KeywordOceanic mixing; Thermoclines; Atlantis II (Ship : 1963-) Cruise AII45; Atlantis II (Ship : 1963-) Cruise AII47
Two shear experiments performed in the permanent thermocline are described and analyzed in this thesis. The first employed dye streak techniques to gain fractional meter vertical resolution. Shears with small vertical scales with frequent reversals in direction and changes of magnitude were observed in every experiment between 160 and 900 meters depth. The ratio of the standard deviation of shear magnitude to the mean shear magnitude was computed at the depth of each dye experiment. These ratios decreased exponentially with depth suggesting a decay of surface supplied energy. The vertical wave number of the shear is not inconsistent with high mode inertio-gravitational internal waves or interleaving layers or salt-driven convection. The second experiment used sensor pairs to measure shear, temperature gradient, and salinity gradient with three meter vertical spacing. A weak but significant negative correlation between shear magnitude and density gradient magnitude was found in most of the records whether density gradient was computed from temperature and salinity or estimated from temperature alone. This result disagrees with a simple linear internal wave model developed for a layered medium. The expected positive correlation is strong enough to cause shear instability to occur first in regions of strongest static stability. This prediction again disagrees with observed shear density data in which the smallest Richardson numbers occur in weakly stratified regions. This negative correlation was observed to be strongest in regions of weak static stability. Perhaps we are observing the results of mixing processes in the main thermocline which cannot be described by the simple linear equations of motion. Two likely sources for the energy of mixing are suggested. Inertial motions are shown to have sufficient energy at thermocline depth and are known to have high enough vertical wave number to have shears comparable to those observed. Salt fingers are known to be able to release enough energy from an unstable salt buoyancy field to form convective layers under laboratory conditions. Since the temperature and salinity in the main thermocline near Bermuda both decrease with depth and have nearly equal and opposite buoyancy contributions, salt fingers must be considered likely. In the main oceanic thermocline no single mixing process seems likely to dominate everywhere or perhaps anywhere. The data collected in this thesis and elsewhere are not yet sufficient to define the statistics of these mixing processes or even to uniquely separate one from another at one location. A time series of experiments combining the two techniques developed in this thesis should be able to establish how shears vary in direction with time and vary with density gradient. These questions are at the heart of the thermocline mixing problem.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1971
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