The surface called "surface" means the lower boundary of the atmosphere. "Northward" indicates a vector component which is positive when directed northward (negative southward). "Downward" indicates a vector component which is positive when directed downward (negative upward). "Downward northward" indicates the ZY component of a tensor. A downward northward stress is a downward flux of northward momentum, which accelerates the lower medium northward and the upper medium southward. The surface downward stress is the windstress on the surface.