The problem was actually below. There are never-ending active convection currents flowing in the troposphere. When those currents hit the bottom of the stratosphere (the tropopause), the vibration thereby caused is transmitted upward through the stratosphere in the form of waves. Because the vibrations at the tropopause are basically standing vibrations, waves head both east and west at the same time. However, if there is a current present, the waves heading in the same direction as the current are selectively absorbed by the current. On the other hand, waves that head against the current penetrate it and are transmitted to a greater distance. These waves cause the current to reverse at a point far from their point of origin.