In physical oceanography, this is a consequence of the double diffusion phenomena. If a layer of warmer, saltier water overlies a layer of cooler, fresher water such that the density of the upper layer is equal to or less than the density of the lower layer, the saltier water at the interface will lose heat to the cooler water below faster than it will lose salt because of the differences in molecular diffusivities. This may cause the water immediately above/below the interface to become denser/lighter which will cause it to sink/rise to the lower/upper layer. These fallings and risings occur in thin columns and the process is referred to as salt fingering, and the process of the water actually moving as double diffusive convection (also thermohaline or thermosolutal convection).