James Burke

Analysis of Microstructure Data from the North Atlantic Tracer Release Experiment

Thesis Approved August 1994

Field work for the North Atlantic Tracer Release Experiment (NATRE) took place between April, 1992 and May, 1993. During this experiment, a tracer (sulfur hexafluoride) was injected along a single isopycnal and its vertical dispersion was tracked over the following year. In doing this, direct estimates of the vertical diffusivity were obtained which were 1.1 x 10^{-5} m^2/s for the summer and 1.8 x 10^{-5} m^2/s for the winter. In conjunction with this tracer study, an intense series of microstructure measurements was taken in the same area over the same period of time. The acquisition of these microstructure measurements, in conjunction with the tracer results, presents a unique opportunity to test various models for estimating the vertical eddy diffusivity, and to study the physics in mixing. This thesis presents a summary of the analysis and preliminary interpretation of the microstructure data.

The parameters epsilon and chi were observed to be lognormally distributed in the pycnocline. Two of the most popular microstructure models were found to give results which were consistent with the results obtained from the tracer. The Osborn-Cox model yielded a vertical eddy diffusivity for heat which was in agreement with that obtained from the tracer estimate (1.4 x 10^{-5} m^2/s for the fall and 2.54 x 10^{-5} m^2/s for the spring); the Osborn model yielded estimates of the vertical eddy diffusivity for density which were higher than those of either the tracer estimate or the Osborn-Cox estimate (3.14 x 10^{-5} m^2/s for the fall and 3.78 x 10^{-5} m^2/s for the spring). As a result of this, it is believed that the value of Gamma = 0.25 that is often used may be too high a value to be used in the pycnocline; furthermore, it is quite likely that Gamma is not even constant but depends upon the mixing conditions.

Jim Burke is now teaching physics at Riverview Rural High School in Sydney, Nova Scotia. He also serves on the provincial physics committee. He can be reached by e-mail at jfburke@syd.eastlink.ca.