• Baker, M. A., and C. H. Gibson, 1987: Sampling turbulence in the stratified ocean: Statistical consequences of strong intermittency. J. Phys. Oceanogr.,17, 1817–1836.

  • Bevington, P. R., and D. K. Robinson, 1992: Data Reduction and Error Analysis for the Physical Sciences. McGraw-Hill, 328 pp.

  • Davis, R. E., 1996: Sampling turbulent dissipation. J. Phys. Oceanogr.,26, 341–358.

  • Efron, B., and G. Gong, 1983: A leisurely look at the bootstrap, the jackknife, and cross-validation. Amer. Stat.,37, 36–48.

  • Gargett, A. E., 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.

  • Haldane, J. B. S., 1952: Simple tests for bimodality and bitangentiality. Ann. Eugen.,16, 359–364.

  • Hamilton, J. M., M. R. Lewis, and B. R. Ruddick, 1989: Vertical fluxes of nitrate associated with salt fingers in the world’s oceans. J. Geophys. Res.,94, 2137–2145.

  • Huq, P., and R. E. Britter, 1995: Turbulence evolution and mixing in a two layer stably stratified fluid. J. Fluid Mech.,285, 41–67.

  • Jackett, D. R., and T. J. McDougall, 1997: A neutral density variable for the world’s oceans. J. Phys. Oceanogr.,27, 237–263.

  • Joyce, M. T., J. R. Luyten, A. Kubryakov, F. B. Bahr, and J. S. Pallant, 1998: Meso- to large-scale structure of subducting water in the subtropical gyre of the eastern North Atlantic Ocean. J. Phys. Oceanogr.,28, 40–61.

  • Kunze, E., 1987: Limits on growing, finite-length salt fingers: A Richardson number constraint. J. Mar. Res.,45, 533–556.

  • ——, 1990: The evolution of salt fingers in inertial wave shear. J. Mar. Res.,48, 471–504.

  • ——, 1994: A proposed flux constraint for salt fingers in shear. J. Mar. Res.,52, 999–1016.

  • ——, and J. M. Toole, 1997: Tidally driven vorticity, diurnal shear, and turbulence atop Fieberling Seamount. J. Phys. Oceanogr.,27, 2663–2693.

  • ——, A. J. Williams, and R. W. Schmitt, 1987: Optical microstructure in the thermohaline staircase east of Barbados. Deep-Sea Res.,34, 1697–1704.

  • 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.

  • ——, ——, and ——, 1998: Mixing of a tracer released in the pycnocline of a subtropical gyre. J. Geophys. Res.,103, 21 499–21 529.

  • Linden, P. F., 1974: Salt fingers in a steady shear flow. Geophys. Fluid Dyn.,6, 1–27.

  • Lueck, R., 1987: Microstructure measurements in a thermohaline staircase. Deep-Sea Res.,34, 1677–1688.

  • McDougall, T. J., 1991: Water mass analysis with three conservative variables. J. Geophys. Res.,96, 8687–8693.

  • ——, and J. R. Taylor, 1984: Flux measurements across a finger interface at low values of the stability ratio. J. Mar. Res.,42, 1–14.

  • ——, and B. R. Ruddick, 1992: The use of ocean microstructure to quantify both turbulent mixing and salt fingering. Deep-Sea Res.,39, 1931–1952.

  • Moum, J. N., 1996: Efficiency of mixing in the main thermocline. J. Geophys. Res.,101, 12 057–12 069.

  • Oakey, N. S., 1985: Statistics of mixing parameters in the upper ocean during JASIN phase 2. J. Phys. Oceanogr.,15, 1662–1675.

  • Osborn, T. R., 1980: Estimates of the local rate of vertical diffusion from dissipation measurements. J. Phys. Oceanogr.,10, 83–89.

  • ——, and C. S. Cox, 1972: Oceanic fine structure. Geophys. Fluid Dyn.,3, 321–345.

  • Pedlosky, J., 1996: Ocean Circulation Theory. Springer-Verlag, 453 pp.

  • Polzin, K. L., 1996: Statistics of the Richardson number: Mixing models and finestructure. J. Phys. Oceanogr.,26, 1409–1425.

  • ——, and E. T. Montgomery, 1996: Microstructure profiling with the High Resolution Profiler. Proc. ONR Workshop on Microstructure Sensors, Mt. Hood, OR, 109–115.

  • Robinson, A. R., and H. Stommel, 1959: The oceanic thermocline and the associated thermohaline circulation. Tellus,11, 295–308.

  • Rohr, J. J., E. C. Itsweire, and C. W. Van Atta, 1984: Mixing efficiency in stably-stratified decaying turbulence. Geophys. Astrophys. Fluid Dyn.,29, 221–236.

  • Ruddick, B., D. Walsh, and N. Oakey, 1997: Variations in apparent mixing efficiency in the North Atlantic Central Water. J. Phys. Oceanogr.,27, 2589–2605.

  • Schmitt, R. W., 1979a: The growth rate of super-critical salt fingers. Deep-Sea Res.,26, 23–40.

  • ——, 1979b: Flux measurements on salt fingers at an interface. J. Mar. Res.,37, 419–436.

  • ——, 1981: Form of the temperature-salinity relationship in the Central Water: Evidence for double-diffusive mixing. J. Phys. Oceanogr.,11, 1015–1026.

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

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

  • ——, and D. L. Evans, 1978: An estimate of the vertical mixing due to salt fingers based on observations of North Atlantic Central Water. J. Geophys. Res.,83, 2913–2919.

  • ——, H. Perkins, J. D. Boyd, and M. C. Stalcup, 1987: C-SALT: An investigation of the thermohaline staircase in the western tropical North Atlantic. Deep-Sea Res.,34, 1655–1665.

  • ——, J. M. Toole, R. L. Koehler, E. C. Mellinger, and K. W. Doherty, 1988: The development of a fine- and microstructure profiler. J. Atmos. Oceanic Technol.,5, 484–500.

  • Shen, C. Y., 1993: Heat-salt finger fluxes across a density interface. Phys. Fluids A,5, 2633–2643.

  • ——, 1995: Equilibrium salt-fingering convection. Phys. Fluids A,7, 706–717.

  • Spall, M. A., 1999: A simple model of the large scale circulation of Mediterranean Water and Labrador Sea Water. Deep-Sea Res.,46, 181–204.

  • Stern, M. E., 1969: Collective instability of salt fingers. J. Fluid Mech.,35, 209–218.

  • ——, 1975: Ocean Circulation Physics. Academic Press, 246 pp.

  • ——, and J. S. Turner, 1969: Salt fingers and convecting layers. Deep-Sea Res.,16, 497–511.

  • Taylor, J., and P. Bucens, 1989: Laboratory experiments on the structure of salt fingers. Deep-Sea Res.,36, 1675–1704.

  • Toole, J. M., K. L. Polzin, and R. W. Schmitt, 1994: Estimates of diapycnal mixing in the abyssal ocean. Science,264, 1120–1123.

  • ——, R. W. Schmitt, K. L. Polzin, and E. Kunze, 1997: Near-boundary mixing above the flanks of a midlatitude seamount. J. Geophys. Res.,102, 947–959.

  • Turner, J. S., 1967: Salt fingers across a density interface. Deep-Sea Res.,14, 599–611.

  • Yamazaki, H., and T. R. Osborn, 1990: Dissipation estimates for stratified turbulence. J. Geophys. Res.,95, 9739–9744.

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

All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 823 230 24
PDF Downloads 444 193 15

The Contribution of Salt Fingers to Vertical Mixing in the North Atlantic Tracer Release Experiment

View More View Less
  • 1 MIT/WHOI Joint Program, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
  • | 2 Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
Restricted access

Abstract

The North Atlantic Tracer Release Experiment (NATRE) was performed in an area moderately favorable to salt fingers. However, the classic finger signature of a distinct thermohaline staircase caused by upgradient density flux was absent. This is likely because mixing by turbulence was sufficiently strong to disrupt the formation of permanent step and layer systems. Despite the lack of a staircase, optical shadowgraph profiles revealed that small-scale tilted laminae, previously observed in a salt-finger staircase, were abundant at the NATRE site. Using microstructure observations, the strength of salt-finger mixing has been diagnosed using a nondimensional parameter related to the ratio of the diffusivities for heat and buoyancy (Γ, “the dissipation ratio”). By examining the dissipation ratio in a parameter space of density ratio (Rρ) and Richardson number (Ri), the signal of salt fingers was discerned even under conditions where turbulent mixing also occurred. While the model for turbulence describes most dissipation occurring when Ri < 1, dissipation at larger Ri is better described by the salt-finger model. Based on the results of the parameter space analysis, a method is proposed for estimating the salt-finger enhancement of the diapycnal haline diffusivity (ks) over the thermal diffusivity (kθ). During April 1992 at the NATRE site, it was found that kθ = (0.08 ± 0.01) cm2 s−1 and ks = (0.13 ± 0.01) cm2 s−1 for the neutral density surface local to the tracer release isopycnal (σθ ∼ 26.75 kg m−3, z ∼ 300 m). The flux divergence of buoyancy was also computed, giving the diapycnal advection w∗ = −(1.7 ± 1.2) m yr−1. Moreover, divergence of vertical buoyancy flux was dominated by the haline component. For comparison, the tracer release method gave a diffusivity of ks = (0.12 ± 0.02) cm2 s−1 (May–November 1992) and a diapycnal velocity of w∗ = −(3 ± 1) m yr−1 (May 1992–November 1994) at this site. The above numbers are contrasted to diffusivity estimates derived from turbulence theory alone. Best agreement between tracer-inferred mixing rates and microstructure based estimates is achieved when the salt-finger enhancement of ks is taken into account.

Corresponding author address: Louis St. Laurent, MIT/WHOI Joint Program, MS #21, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.

Email: lstlaurent@whoi.edu

Abstract

The North Atlantic Tracer Release Experiment (NATRE) was performed in an area moderately favorable to salt fingers. However, the classic finger signature of a distinct thermohaline staircase caused by upgradient density flux was absent. This is likely because mixing by turbulence was sufficiently strong to disrupt the formation of permanent step and layer systems. Despite the lack of a staircase, optical shadowgraph profiles revealed that small-scale tilted laminae, previously observed in a salt-finger staircase, were abundant at the NATRE site. Using microstructure observations, the strength of salt-finger mixing has been diagnosed using a nondimensional parameter related to the ratio of the diffusivities for heat and buoyancy (Γ, “the dissipation ratio”). By examining the dissipation ratio in a parameter space of density ratio (Rρ) and Richardson number (Ri), the signal of salt fingers was discerned even under conditions where turbulent mixing also occurred. While the model for turbulence describes most dissipation occurring when Ri < 1, dissipation at larger Ri is better described by the salt-finger model. Based on the results of the parameter space analysis, a method is proposed for estimating the salt-finger enhancement of the diapycnal haline diffusivity (ks) over the thermal diffusivity (kθ). During April 1992 at the NATRE site, it was found that kθ = (0.08 ± 0.01) cm2 s−1 and ks = (0.13 ± 0.01) cm2 s−1 for the neutral density surface local to the tracer release isopycnal (σθ ∼ 26.75 kg m−3, z ∼ 300 m). The flux divergence of buoyancy was also computed, giving the diapycnal advection w∗ = −(1.7 ± 1.2) m yr−1. Moreover, divergence of vertical buoyancy flux was dominated by the haline component. For comparison, the tracer release method gave a diffusivity of ks = (0.12 ± 0.02) cm2 s−1 (May–November 1992) and a diapycnal velocity of w∗ = −(3 ± 1) m yr−1 (May 1992–November 1994) at this site. The above numbers are contrasted to diffusivity estimates derived from turbulence theory alone. Best agreement between tracer-inferred mixing rates and microstructure based estimates is achieved when the salt-finger enhancement of ks is taken into account.

Corresponding author address: Louis St. Laurent, MIT/WHOI Joint Program, MS #21, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.

Email: lstlaurent@whoi.edu

Save