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

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 (R?) 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 (k?). During April 1992 at the NATRE site, it was found that k? = (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 (σ? ? 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.