 Shear vorticity is caused by the difference in wind speed between air moving through the trough base (typically a jet or jet finger) and slower moving air on either poleward and equatorward side of the faster flow.Consider that slower air to the poleward side will be imparted counter-clockwise spin (picture faster moving air (jet) south and slower air to the north, spin is created).Thus, to the north (poleward) of the trough base an air parcel will experience positive vorticity.