Preprints
https://doi.org/10.5194/egusphere-2024-2749
https://doi.org/10.5194/egusphere-2024-2749
18 Sep 2024
 | 18 Sep 2024
Status: this preprint is open for discussion.

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

Jianing Li, Qingxuan Yang, and Hui Sun

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 ΓT 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. ΓT is jointly modulated by the Ozmidov scale to the Thorpe scale ratio ROT and the buoyancy Reynolds number Reb, namely ΓTROT−4/3 · Reb1/2. The eddy diffusivity based on observed ΓT 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 ΓθF and ΓSF are explored. ΓθF scatters over two orders of magnitude with a median value of 0.47, and is mostly linearly correlated with ΓSF as ΓSF ≈ 5ΓθF. The density flux ratio for salt finger decreases sharply with density ratio Rρ smaller than 2.4 but regrows to a larger value with Rρ 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.

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Jianing Li, Qingxuan Yang, and Hui Sun

Status: open (until 13 Nov 2024)

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Jianing Li, Qingxuan Yang, and Hui Sun
Jianing Li, Qingxuan Yang, and Hui Sun

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Short summary
Osborn relation is widely used to estimate diapycnal mixing rate, but its accuracy is questioned due to the assumed constant dissipation ratio (Γ) and without identifying mixing types. We identify salt finger and turbulence in the western Pacific and midlatitude Atlantic, and find Γ is highly variable and is related to turbulence-related parameters, by which we improve mixing rate estimates. Thus, identifying mixing types and refining Γ are necessary to improve mixing parameterization accuracy.