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. "Northward" indicates a vector component which is positive when directed northward (negative southward). Northward transport by diffusion means the part due to horizontal or isopyncal diffusion schemes in an ocean model, but not including the "bolus" or Gent-McWilliams velocity.