Turbulent Diffusion and Entrainment in Two-Layer Flow

A layer of freshwater overriding a pool of seawater may extract salt from the pool by two mechanisms: (1) diffusion; and (2) entrainment. While diffusion is a two-way transport by which equal volumes are exchanging places, entrainment is a one-way, advectival type of transport. The rates of the two vertical transports depend on the turbulence in the surface layer and in the pool. The more turbulent water mass will erode the less turbulent one and entrain it. On two occasions fluxes of water and salt past two vertical cross sections of the surface layer were measured in a fjord. In one case there was only about 10% entrainment between the two sections, in the other about 100%.