James Hamilton

Exploring the Role of Salt Fingers in Vertical Mixing: Theory and Observation

Thesis Approved March 1991

A model which considers the vertical mixing properties of a salt finger system is developed and compared to a shear-driven turbulent mixing model. Although the Osborn-Cox relation can be used to calculate heat fluxes from microstructure measurements without having to distinguish between these two mixing processes, the model predictions of vertical eddy diffusivities of salt (and nutrients) are quite different. Consequently, vertical flux estimates may be in error by as much as a factor of three if an improper assumption about the mixing process is made.

Measurement of the "scaled dissipation ratio" *, is proposed as a method of discriminating between mixing processes because of the difference in the two model predicted values. Measurements in a thermohaline staircase are supportive of the predicted salt finger model * value. The method is used to examine the significance of salt fingers in an area of low density ratio and "intermittent steppiness" in T and S profiles. The observations there are not consistent with the turbulent mixing model, although they indicate that turbulent mixing is the dominant process. Assuming that the higher than turbulent values of * are due to salt fingers, a computed vertical salt flux enhancement factor [McDougall and Ruddick, 1991] suggests that salt fingers enhanced mixing by 50%.