Abstract
Salt fingers, which can occur in a fluid with stable temperature stratification and unstable salinity stratification, have larger fluxes when the ratio of the contributions of temperature and salinity to the density gradient (i.e., the density ratio) is close to one. This study investigated whether differential diffusion—or the preferential transport of temperature in a weakly turbulent, strongly stratified flow—can decrease an initially large density ratio enough to strengthen salt fingers. Laboratory experiments were conducted in which a stratification favorable for salt fingers was stirred with oscillating arrays of vertical rods. The density ratio decreased slightly when its initial value was large, as expected in differential diffusion, and it increased when its initial value was small, as expected for salt fingers. The mixing efficiency was less than 4%, and in two of the runs with low initial density ratio, it started negative. A one-dimensional eddy diffusion model in which the overall eddy diffusivities simply sum the contributions of salt fingers and mechanically generated turbulence described the evolution well and predicted an equilibrium state once the two eddy diffusivities became equal.
Article highlights
-
The evolution of the density ratio illustrated the importance of salt fingers and differential diffusion.
-
The mixing efficiency was less than 4%, and it showed features of differential diffusion and fingering.
-
A model that adds the mixing from mechanical turbulence and fingering predicts equilibrium.
Similar content being viewed by others
Data availability
The data are available from the corresponding author.
Code availability
Not applicable.
References
Gargett AE (2003) Differential diffusion: an oceanographic primer. Prog Oceanogr 56:559–570. https://doi.org/10.1016/s0079-6611(03)00025-9
Gargett AE (2019) The effects of KT ≠ KS in a Stommel-like model of the upper Atlantic Meridional Overturning Circulation under steady surface flux forcing. J Mar Res 77:243–266. https://doi.org/10.1357/002224019826887353
Gargett AE, Ferron B (1996) The effects of differential vertical diffusion of T and S in a box model of thermohaline circulation. J Mar Res 54:827–866. https://doi.org/10.1357/0022240963213628
Gargett AE, Holloway G (1992) Sensitivity of the GFDL ocean model to different diffusivities for heat and salt. J Phys Oceanogr 22:1158–1177. https://doi.org/10.1175/1520-0485(1992)022%3c1158:SOTGOM%3e2.0.CO;2
Hebert D (1999) Intrusions: What drives them? J Phys Oceanogr 29:1382–1391. https://doi.org/10.1175/1520-0485(1999)029%3c1382:iwdt%3e2.0.co;2
Huppert HE, Linden PF (1979) On heating a stable salinity gradient from below. J Fluid Mech 95:431–464. https://doi.org/10.1017/s0022112079001543
Inoue R, Yamazaki H, Wolk F, Kono T, Yoshida J (2007) An estimation of buoyancy flux for a mixture of turbulence and double diffusion. J Phys Oceanogr 37:611–624. https://doi.org/10.1175/JPO2996.1
Jackson PR, Rehmann CR (2003) Kinematic effects of differential transport on mixing efficiency in a diffusively stable, turbulent flow. J Phys Oceanogr 33:299–304. https://doi.org/10.1175/1520-0485(2003)033%3c0299:KEODTO%3e2.0.CO;2
Jackson PR, Rehmann CR (2003) Laboratory measurements of differential diffusion in a diffusively stable, turbulent flow. J Phys Oceanogr 33:1592–1603. https://doi.org/10.1175/2405.1
Jackson PR, Rehmann CR (2009) Theory for differential transport of scalars in sheared stratified turbulence. J Fluid Mech 621:1–21. https://doi.org/10.1017/s0022112008004308
Jackson PR, Rehmann CR (2014) Experiments on differential scalar mixing in turbulence in a sheared, stratified flow. J Phys Oceanogr 44(10):2661–2680. https://doi.org/10.1175/jpo-d-14-0027.1
Kimura S, Smyth W, Kunze E (2011) Turbulence in a sheared, salt-fingering-favorable environment: anisotropy and effective diffusivities. J Phys Oceanogr 41:1144–1159. https://doi.org/10.1175/2010jpo4543.1
Kunze E (1987) Limits on growing, finite-length salt fingers: a Richardson number constraint. J Mar Res 45:533–556. https://doi.org/10.1357/002224087788326885
Linden PF (1971) Salt fingers in the presence of grid-generated turbulence. J Fluid Mech 49(3):611–624. https://doi.org/10.1017/s0022112071002283
Linden PF (1974) Salt fingers in a steady shear flow. Geophys Fluid Dyn 6:1–27. https://doi.org/10.1080/03091927409365785
Ma YC, Peltier WR (2021) Parametrization of irreversible diapycnal diffusivity in salt-fingering turbulence using DNS. J Fluid Mech. https://doi.org/10.1017/jfm.2020.1018
Martin JE, Rehmann CR (2006) Layering in a flow with diffusively stable temperature and salinity stratification. J Phys Oceanogr 36:1457–1470. https://doi.org/10.1175/JPO2920.1
McDougall TJ, Ruddick BR (1992) The use of ocean microstructure to quantify both turbulent mixing and salt-fingering. Deep-Sea Res Part A-Oceanogr Res Pap 39:1931–1952. https://doi.org/10.1016/0198-0149(92)90006-f
Merryfield WJ (2002) Intrusions in double-diffusively stable Arctic waters: Evidence for differential mixing? J Phys Oceanogr 32:1452–1459. https://doi.org/10.1175/1520-0485(2002)032%3c1452:iiddsa%3e2.0.co;2
Merryfield WJ, Holloway G, Gargett AE (1999) A global ocean model with double-diffusive mixing. J Phys Oceanogr 29:1124–1142. https://doi.org/10.1175/1520-0485(1999)029%3c1124:AGOMWD%3e2.0.CO;2
Montgomery DC, Runger GC (2011) Applied statistics and probability for engineers, 5th edn. Wiley, New York
Padman L, Dillon TM (1987) Vertical heat fluxes through the Beaufort Sea thermohaline staircase. J Geophys Res 92:10799–10806. https://doi.org/10.1029/JC092iC10p10799
Radko T, Edwards E (2016) Diapycnal velocity in the double-diffusive thermocline. Fluids. https://doi.org/10.3390/fluids1030025
Rehmann CR, Koseff JR (2004) Mean potential energy change in stratified grid turbulence. Dyn Atmos Oceans 37:271–294. https://doi.org/10.1016/j.dynatmoce.2003.09.001
Schmitt RW (1981) Form of the temperature-salinity relationship in the Central Water: evidence for double-diffusive mixing. J Phys Oceanogr 11:1015–1026. https://doi.org/10.1175/1520-0485(1981)011%3c1015:fottsr%3e2.0.co;2
Schmitt RW (2003) Observational and laboratory insights into salt finger convection. Prog Oceanogr 56:419–433. https://doi.org/10.1016/s0079-6611(03)00033-8
Shibley NC, Timmermans ML (2019) The formation of double-diffusive layers in a weakly turbulent environment. J Geophys Res 124:1445–1458. https://doi.org/10.1029/2018jc014625
Smyth WD, Kimura S (2011) Mixing in a moderately sheared salt-fingering layer. J Phys Oceanogr 41:1364–1384. https://doi.org/10.1175/2010jpo4611.1
Smyth WD, Ruddick B (2010) Effects of ambient turbulence on interleaving at a baroclinic front. J Phys Oceanogr 40:685–712. https://doi.org/10.1175/2009jpo4297.1
St Laurent L, Schmitt RW (1999) The contribution of salt fingers to vertical mixing in the North Atlantic Tracer Release Experiment. J Phys Oceanogr 29:1404–1424. https://doi.org/10.1175/1520-0485(1999)029%3c1404:TCOSFT%3e2.0.CO;2
Taylor J (1991) Laboratory experiments on the formation of salt fingers after the decay of turbulence. J Geophys Res 96:12497–12510. https://doi.org/10.1029/90jc02313
Vladoiu A, Bouruet-Aubertot P, Cuypers Y, Ferron B, Schroeder K, Borghini M, Leizour S, Ben Ismail S (2019) Mixing efficiency from microstructure measurements in the Sicily Channel. Ocean Dyn 69:787–807. https://doi.org/10.1007/s10236-019-01274-2
Wells MG, Griffiths RW (2003) Interaction of salt finger convection with intermittent turbulence. J Geophys Res. https://doi.org/10.1029/2002JC001427
Wykes MSD, Dalziel SB (2014) Efficient mixing in stratified flows: experimental study of a Rayleigh–Taylor unstable interface within an otherwise stable stratification. J Fluid Mech 756:1027–1057. https://doi.org/10.1017/jfm.2014.308
Zhang JB, Schmitt RW, Huang RX (1998) Sensitivity of the GFDL modular ocean model to parameterization of double-diffusive processes. J Phys Oceanogr 28:589–605. https://doi.org/10.1175/1520-0485(1998)028%3c0589:sotgmo%3e2.0.co;2
Funding
This material is based upon work supported by the National Science Foundation under Grant No. 1034221. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
Author information
Authors and Affiliations
Corresponding author
Ethics declarations
Conflict of interest
The authors declare that they have no conflict of interest.
Additional information
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.
About this article
Cite this article
Willard, I.P., Rehmann, C.R. Interaction between double diffusion and differential diffusion in a stratified turbulent flow. Environ Fluid Mech 23, 1099–1113 (2023). https://doi.org/10.1007/s10652-022-09900-2
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s10652-022-09900-2