In physical oceanography, this is a consequence of the double diffusion phenomena. If a layer of colder, fresher water overlies a layer of warmer, saltier water, the differences in molecular diffusivities between salt and heat will cause the water just above/below the interface to become lighter/heavier than that above/below it and thus it will tend to rise/sink. The phenomenon, called layering, can lead to fairly homogeneous layers separated by thinner regions with large gradients.