Laboratory experiments on the structure of salt fingers

Abstract

We investigated the structure of salt fingers in a laboratory tank using horizontal and vertical conductivity and temperature profiles; similar measurements have been made of salt finger microstructure in the ocean. Visualization of the salt fingers using fluorescent dye mixed into the upper layer showed that they were disordered, with new fingers being formed at the edge of the gradient region then growing into the gradient. Because of the disordered state of the fingers the average coherence between the signals for two vertically separated sensors was small, even though the separation of the sensors was of the order of the finger width. The peak in horizontal gradient spectrum was close to both the wavenumber of salt fingers with the maximum growth rate and to the wavenumber of fingers that maximize the buoyancy flux in Howard and Veronis' (1987, Journal of Fluid Mechanics, 183, 1–23), salt finger model. Assuming that the vertical advection of the mean temperature gradient within an individual finger was balanced by horizontal heat diffusion, we derived an estimate for the buoyancy flux due to heat from the variance of the horizontal temperature gradient. On average, this estimate for the flux was 0.6 that determined from the rate of change of the mean layer properties, and our result supports the use of this technique for estimating salt finger fluxes in the ocean. We also derived the buoyancy flux ratio, defined as the ratio of the buoyancy flux due to heat to that due to salt, from the ratio of the variances of the horizontal temperature and salinity profiles. Our estimate for the flux ratio from horizontal profiles was in agreement with that derived from the vertical profiles. At comparable stability ratios the salt flux and buoyancy flux ratio determined from the present experiments were closer to those presented by Turner (1967, Deep-Sea Research, 14, 599–611) and Schmitt (1979a, Journal of Marine Research, 37, 419–436) than to the later results of McDougall and Taylor (1984, Journal of Marine Research, 42, 1–14).