The Polar Annular Mode (PAM) is a recently discovered stratospheric mode linked to the north-south movement of the stratospheric polar vortex during boreal winter.  The structure, physics and tropospheric impact of PAM is studied via composite analyses of large amplitude PAM events.  PAM events are initiated in the stratosphere via the breaking of anomalous Rossby wave activity emanating from the troposphere. PAM onset is subsequently characterized by an abrupt change in the circumpolar jet stream having a coherent structure that extends from the mid- stratosphere downward through the troposphere to the Earth’s surface.   The tropospheric impact of PAM events persists for 2-3 weeks with a significant regional impact upon surface air temperature and midlatitude storm activity.