 Likewise, to the south of the faster flow the air is spun in a clockwise direction (faster air (jet)to the north with slower air to the south, spin is created).Thus, to the south of the faster winds will be an area of negative vorticity.When these areas of negative and positive vorticity are moved (advected) they produce areas of negative vorticity advection (NVA) and positive vorticity advection (PVA) respectively, downstream from the trough base.