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).
This concept was harvested from the controlled vocabulary at http://vocab.nerc.ac.uk/collection/P07/current/ on 2022-06-10. Information was pulled as-is for the following properties: dcterms:created, skos:definition, skos:exactMatch, skos:related and skos:notation. The URI of the concept is generated using SHA256 on the CF standard name (in the skos:notation) and concatenated with the TERN controlled vocabularies' base URI (http://linked.data.gov.au/def/tern-cv/).
TERN is supported by the Australian Government through the National Collaborative Research Infrastructure Strategy, NCRIS.