Dissipation ratio and eddy diffusivity of turbulent and salt finger mixing derived from microstructure measurements

Abstract. Eddy diffusivity is usually estimated by using the Osborn relation assuming a constant dissipation ratio of 0.2. In this study, we examine dissipation ratios and eddy diffusivities of turbulent mixing and salt finger mixing based on microstructure datasets. We find the dissipation ratio of turbulence Γ^T is highly variable with a median value clearly greater than 0.2, which shows strong seasonal variation and decreases slightly with depth in the western equatorial Pacific, but obviously increases in vertical in the midlatitude Atlantic. Γ^T is jointly modulated by the Ozmidov scale to the Thorpe scale ratio R_OT and the buoyancy Reynolds number Re_b, namely Γ^T ∝ R_OT^−4/3 · Re_b^1/2. The eddy diffusivity based on observed Γ^T is larger than that estimated with 0.2, and presents a much stronger bottom enhancement. The eddy diffusivities of heat and salt for salt finger are calculated by two "analogical" Osborn equations; and their corresponding "effective" dissipation ratios Γ_θ^F and Γ_S^F are explored. Γ_θ^F scatters over two orders of magnitude with a median value of 0.47, and is mostly linearly correlated with Γ_S^F as Γ_S^F ≈ 5Γ_θ^F. The density flux ratio for salt finger decreases sharply with density ratio R_ρ smaller than 2.4 but regrows to a larger value with R_ρ exceeding 2.4. The salt finger-induced eddy diffusivities become more comparable or even stronger than the turbulent diffusivities with depth. This study highlights the influences of variable dissipation ratios and different mixing types on eddy diffusivity estimates, and should help further improvement of mixing estimate and parameterization.