Localized stirring in a field of salt-fingers

In a series of laboratory experiments, a partially mixed patch was produced in thick linear concentration gradients favorable to salt-finger convection. Salt-fingers, which give rise to an up-gradient flux of buoyancy, can reduce and invert the density gradient in the initial imposed patch. This leads to overturning convection within the patch if (a) the ratio of ambient T and S gradients, Rρ αT_z/βS_z, is near one; (b) the initial imposed turbulence results in a nearly well-mixed patch; and (c) the patch thickness is large enough that convective eddies are able to transport T and S faster than salt-fingers. Once overturning occurs, subsequent turbulent entrainment can lead to growth of the patch thickness. Experimental results for one-dimensional patches (layers) agree well with the theoretical prediction. This thickening is in contrast to the collapse that a partially mixed three-dimensional patch would experience due to lateral intrusion in a wide tank.