The surface called "surface" means the lower boundary of the atmosphere. A velocity is a vector quantity. "Northward" indicates a vector component which is positive when directed northward (negative southward). "Geostrophic" indicates that geostrophic balance is assumed. "Water" means water in all phases. surface_geostrophic_northward_sea_water_velocity is the sum of a variable part, surface_geostrophic_northward_sea_water_velocity_assuming_sea_level_for_geoid, and a constant part due to the stationary component of ocean circulation.