Generation of density fine structure by salt fingers in a spatially periodic shear

Vertical shear is ubiquitous in the ocean thermocline. The temperature and salinity gradient are also often favorable to the growth of salt fingers. We investigate the implications of the superposed shear and salt fingering for the formation of fine structure and show that a depth-dependent spatially periodic shear leads to the development of variations in the density profile on the same wavelength as the shear. Laboratory experiments were carried out in a continuous density gradient with a spatially periodic shear produced by exciting a low-frequency baroclinic mode of vertical wavelength 60 mm. The density gradient consisted of opposing salt and sugar gradients favorable to salt fingers (an analogue to the oceanic heat/salt system). Where the shearing was large the salt finger buoyancy fluxes were smaller. Changes in salinity gradient due to the resulting flux divergence were self-amplifying until a steady state was reached in which the spatial variations in the ratio of salt and sugar gradients were such that the flux divergence vanished. Thus, along with reducing the mean salt finger buoyancy flux a spatially varying shear can also lead to the formation of density structure.