The Impact of Salt Fingering on the Thermohaline Circulation under Mixed Boundary Conditions

Jubao Zhang Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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Raymond W. Schmitt Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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Abstract

The impact of salt fingers on the thermohaline circulation in a single hemisphere basin under mixed boundary conditions is investigated through scaling analysis and numerical experiments. By assuming that the internal density ratio is determined by the surface horizontal density ratio, the effect of a double-diffusive parameterization on the vertical diffusivity of density (Kρ) is estimated. Salt fingers reduce the magnitude of Kρ, with the extent of the reduction dependent on the magnitude of the density ratio. The reduced diapycnal mixing leads to a diminished thermohaline circulation and modifies the stability criteria for the thermal/haline mode transition under mixed boundary conditions. Quasi-equilibrium numerical experiments with the GFDL Modular Ocean Model produce results consistent with the scaling analysis in the reduction of the magnitude of the thermohaline circulation and the change in the critical freshwater forcing required for the existence of the stable thermal mode. Sensitivity experiments are also conducted on the variables in the salt finger parameterization and found to be consistent with the scaling analysis. These results indicate that salt fingers make the thermohaline circulation more susceptible to transition to the haline mode (haline catastrophe), so should not be ignored in long-term climate prediction models.

Corresponding author address: Dr. Raymond W. Schmitt, Dept. of Physical Oceanography, WHOI, MS #21, Woods Hole, MA 02543-1541.

Email: rschmitt@whoi.edu

Abstract

The impact of salt fingers on the thermohaline circulation in a single hemisphere basin under mixed boundary conditions is investigated through scaling analysis and numerical experiments. By assuming that the internal density ratio is determined by the surface horizontal density ratio, the effect of a double-diffusive parameterization on the vertical diffusivity of density (Kρ) is estimated. Salt fingers reduce the magnitude of Kρ, with the extent of the reduction dependent on the magnitude of the density ratio. The reduced diapycnal mixing leads to a diminished thermohaline circulation and modifies the stability criteria for the thermal/haline mode transition under mixed boundary conditions. Quasi-equilibrium numerical experiments with the GFDL Modular Ocean Model produce results consistent with the scaling analysis in the reduction of the magnitude of the thermohaline circulation and the change in the critical freshwater forcing required for the existence of the stable thermal mode. Sensitivity experiments are also conducted on the variables in the salt finger parameterization and found to be consistent with the scaling analysis. These results indicate that salt fingers make the thermohaline circulation more susceptible to transition to the haline mode (haline catastrophe), so should not be ignored in long-term climate prediction models.

Corresponding author address: Dr. Raymond W. Schmitt, Dept. of Physical Oceanography, WHOI, MS #21, Woods Hole, MA 02543-1541.

Email: rschmitt@whoi.edu

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  • Broecker, W. S., D. M. Peteet, and D. Rind, 1985: Does the ocean–atmosphere system have more than one stable mode of operation? Nature,315, 21–26.

  • Bryan, F., 1986: High-latitude salinity effects and interhemispheric thermohaline circulations. Nature,323, 301–304.

  • ——, 1987: On the parameter sensitivity of primitive equation ocean general circulation model. J. Phys. Oceanogr.,17, 970–985.

  • Bryan, K., 1969: A numerical method for the study of the circulation of the world ocean. J. Comput. Phys.,4, 347–376.

  • Gargett, A. E., and G. Holloway, 1992: Sensitivity of the GFDL ocean model to different diffusivities for heat and salt. J. Phys. Oceanogr.,22, 1158–1177.

  • ——, and B. Ferron, 1996: The effects of differential vertical diffusion of T and S in a box model of thermohaline circulation. J. Mar. Res.,54, 827–866.

  • Gent, P. R., and J. C. McWilliams, 1990: Isopycnal mixing in ocean circulation models. J. Phys. Oceanogr.,20, 150–155.

  • Griffies, S. M., A. Gnanadesikan, R. C. Pacanowski, V. D. Larichev, J. K. Dukowicz, and R. D. Smith, 1998: Isoneutral diffusion in a z-coordinate ocean model. J. Phys. Oceanogr.,28, 805–830.

  • Huang, R. X., 1993: Real freshwater flux as the upper boundary condition for the salinity balance and thermohaline circulation forced by evaporation and precipitation. J. Phys. Oceanogr.,23, 2428–2446.

  • ——, 1994: Thermohaline circulation: Energetics and variability in a single-hemisphere basin model. J. Geophys. Res.,99 (C6), 12 471–12 485.

  • ——, 1995: Advances in Theories of Wind-Driven and Thermohaline Circulation, A Lecture Series. University of Hawaii at Manoa, SOEST, 213 pp.

  • Iselin, C. O. D., 1939: The influence of vertical and lateral turbulence on the characteristics of the water at mid-depths. Trans. Amer. Geophys. Union,20, 414–417.

  • Keigwin, L. A., G. A. Jones, S. J. Lehman, and E. A. Boyle, 1991: Deglacial meltwater discharge, North Atlantic deep circulation, and abrupt climate change. J. Geophys. Res.,96, 16 811–16 826.

  • Ledwell, J. R., A. J. Watson, and C. S. Law, 1993: Evidence for slow mixing across the pycnocline from an open-ocean tracer-release experiment. Nature,364, 701–703.

  • Manabe, S., and R. J. Stouffer, 1994: Multiple-century response of a coupled ocean–atmosphere model to an increase of atmospheric carbon dioxide. J. Climate,7, 5–23.

  • Pacanowski, R. C., 1995: MOM2 documentation, user’s guide and reference manual. GFDL Ocean Group Tech. Rep. 3, 671 pp. [Available online at http://www.gfdl.gov/∼kd/momwebpages/momver2.html.].

  • Polzin, K. L., J. M. Toole, J. R. Ledwell, and R. W. Schmitt, 1997: Spatial variability of turbulent mixing in the abyssal ocean. Science,276, 93–96.

  • Rahmstorf, S., 1995: Bifurcations of the Atlantic thermohaline circulation in response to changes in the hydrological cycle. Nature,378, 145–149.

  • Schmitt, R. W., 1981: Form of temperature–salinity relationship in the central water: Evidence for double-diffusive mixing. J. Phys. Oceanogr.,11, 1015–1026.

  • ——, 1988: Mixing in a thermohaline staircase. Small-Scale Turbulence and Mixing in the Ocean. J. Nihoul and B. Jamart, Eds., Elsevier Oceanogr. Ser., Vol. 46, Elsevier, 435–452.

  • ——, 1990: On the density ratio balance in the Central Water. J. Phys. Oceanogr.,20, 900–906.

  • ——, 1994: Double diffusion in oceanography. Annu. Rev. Fluid Mech.,26, 255–285.

  • ——, 1998: Double-diffusive convection: Its role in ocean mixing and parameterization schemes for large scale modeling. Ocean Modeling and Parameterization, E. P. Chassignet and J. Verron, Eds., Kluwer Academic, 215–234.

  • ——, P. S. Bogden, and C. E. Dorman, 1989: Evaporation minus precipitation and density fluxes for the North Atlantic. J. Phys. Oceanogr.,19, 1208–1221.

  • St. Laurent, L., and R. W. Schmitt, 1999: The contribution of salt fingers to vertical mixing in the North Atlantic tracer release experiment. J. Phys. Oceanogr.,29, 1404–1424.

  • Stern, M. E., 1960: The “salt fountain” and thermohaline convection. Tellus,12, 172–175.

  • Stommel, H. M., 1961: Thermohaline convection with two stable regimes of flow. Tellus,13, 224–230.

  • Weaver, A. J., and T. M. C. Hughes, 1992: Stability and variability of the thermohaline circulation and its link to climate. Trends Phys. Oceanogr.,1, 15–70.

  • Zhang, J., 1998: Impacts of double-diffusive processes on the thermohaline circulation. Ph.D. thesis, Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 157 pp. [Available from Education Office, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.].

  • ——, R. W. Schmitt, and R. X. Huang, 1998: Sensitivity of GFDL Modular Ocean Model to the parameterization of double-diffusive processes. J. Phys. Oceanogr.,28, 589–605.

  • ——, ——, and ——, 1999: The relative influence of diapycnal mixing and hydrologic forcing on the stability of the thermohaline circulation. J. Phys. Oceanogr.,29, 1096–1108.

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