Comparison of mixing efficiency and vertical diffusivity models from temperature microstructure

Due to the many difficulties associated with measuring buoyancy flux directly in the field, parameterizations for the flux involving eddy diffusivities are commonly used. These parameterizations are often cast in terms of a mixing efficiency (), which itself is often assumed to be a constant throughout the water column (0.17-0.2). Alternatively, can be calculated in terms of turbulence parameters which attempt to capture the varying mixing dynamics throughout the water column. In this paper three separate parameterizations are used to calculate the eddy diffusivities in a dynamically evolving stratified water column in the Gulf of Aqaba. Furthermore, we use two different approaches for calculating by Ivey and Imberger (1991) and Shih et al. (2005) for these diffusivity parameterizations and compare the results obtained using each approach. Further work on the Shih et al. (2005) parameterization by D. Bouffard and L. Boegman (personal communication, 2011) has also been used in this analysis. While direct measurements of buoyancy flux are needed to validate the actual accuracy of the mixing efficiency parameterizations, we can make the following conclusions. First, we found that in some circumstances the eddy diffusivity models produce very different estimates from one another and the estimates are themselves sensitive to the choice of parameterization for . Second, when the stratification is weak and temperature variance is small, parameterizations involving calculations of the Thorpe scale, _T as well as the turbulent Froude number and Reynolds numbers should be treated with great caution. Third, under the same circumstances, the Shih et al. (2005) approach for applied to the parameterization for vertical diffusivity seems to be the best vertical diffusivity (based on a comparison to ).