Anisotropy of Salt Fingers

Abstract

An understanding of the structure of the vertical temperature and salinity profiles in salt fingers is required to identify the occurrence of fingers in vertical profiles of oceanic microstructure and hence to make estimates of the contribution of fingers to vertical mixing in the ocean. With this in mind, a laboratory experiment was set up in which the vertical temperature and conductivity profiles through salt fingers were measured and compared with horizontal profiles of the same quantities. It was found that the vertical component of the temperature gradient in salt fingers had few zero crossings, so was quite different from the temperature gradient that would result from turbulence. On the other hand, the conductivity gradient (which is dominated by changes in salt concentration in our experiments) had numerous zero crossings even when the salt fingers were relatively weak. For density ratios Rp < 5 (where Rp = α??z/?S?z and ??z and S?z are the mean vertical gradients of potential temperature and salinity) the average value of the ratio of the wavenumbers where the amplitudes of the vertical and horizontal temperature gradient spectra were maximum (when the spectra were plotted in variance-preserving form) was 0.58. Scaling of the temperature variance equation shows that the ratio of the vertical to one horizontal component of the Cox number C3/C1 should vary as the square of the ratio of the horizontal to vertical finger length scales. If the wavenumber ratio described above is a measure of this length-scale ratio, then C3/C1 should be (0.58)2. This is to be compared with the measured average of C3/C1, 0.25.