Vertical turbulent transport of density (mass) in a system of stable stratification ∂p/∂z < 0 (z positive upward) is often modelled by an “eddy” diffusivity Kv ≡ −/(∂p/∂z), normally assumed to be constant. Recent evidence from stratified lakes, fjords and oceans suggests that Kv may be more accurately described as a decreasing function of buoyancy frequency N ≡ (–g(o)–1 (∂p/∂z))1/2. A main purpose of this paper is to review available estimates of Kv from a variety of stratified geophysical systems. Particular emphasis is placed upon the degree to which these estimates are dependent upon underlying models used to derive values for Kv from observable quantities. Most techniques reveal a disagreeable degree of model-dependence, frequently providing only upper bounds to the magnitude of Kv. I have coupled the functional dependence which emerges from the least model-dependent of available techniques with ensemble-averaged values of oceanic turbulent kinetic energy dissipation rate per unit mass ε as a function of N, and show that the resulting parameterization for Kv is consistent with a wide range of present oceanic data. Finally, brief re-examination of a simple vertical advection/diffusion model of thermohaline circulation illustrates possible dynamical significance of a stratification-dependent Kv.
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