The surface called "surface" means the lower boundary of the atmosphere. A velocity is a vector quantity. "x" indicates a vector component along the grid x-axis, when this is not true longitude, positive with increasing x. "Geostrophic" indicates that geostrophic balance is assumed. surface_geostrophic_sea_water_x_velocity is the sum of a variable part, surface_geostrophic_sea_water_x_velocity_assuming_sea_level_for_geoid, and a constant part due to the stationary component of ocean circulation.