Diffusivity is also sometimes known as the coefficient of diffusion. Diffusion occurs as a result of a gradient in the spatial distribution of mass concentration, temperature or momentum. The diffusivity may be very different in the vertical and horizontal directions. The construction vertical_X_diffusivity means the vertical component of thediffusivity of X due to motion which is not resolved on the grid scale of the model. "Due to background" means caused by a time invariant imposed field which may be eitherconstant over the globe or spatially varying, depending on the ocean model used. The specification of a physical process by the phrase due_to_process means that the quantity named is a single term in a sum of terms which together compose the general quantity named by omitting the phrase.