Climate and Forecast Standard Names
The phrase "magnitude_of_X" means magnitude of a vector X. The surface called "surface" means the lower boundary of the atmosphere. "Surface stress" means the shear stress (force per unit area) exerted by the wind at the surface. A downward stress is a downward flux of momentum. Over large bodies of water, wind stress can drive near-surface currents. "Downward" indicates a vector component which is positive when directed downward (negative upward).
Mapping to be determined
magnitude_of_surface_downward_stress