The spatial and temporal characteristics of the month-to-month variability of the polar night jet and its relationship with tropospheric circulation is investigated for both the Northern and Southern Hemispheres. The variabilities of the hemispheres have many common characteristics of the Polar Night Jet Oscillation (PJO). These common characteristics include the following: (1) the anomalous zonal-mean zonal winds shift poleward and downward; (2) the anomalous polar temperatures propagate downward from the stratopause to the upper troposphere; and (3) they are made through a wave-mean flow interaction with mainly the planetary wave of zonal wave number one. Annular modes associated with the PJOs appear in both hemispheres when the zones of maximum polar temperature anomaly descend to the lowermost stratosphere and upper troposphere. The major difference in the PJOs of the two hemispheres is found in their temporal characteristics. In the Southern Hemisphere, the phase of the PJO is closely locked to the annual cycle, while in the Northern Hemisphere it exhibits quasiperiodic variability with its envelope controlled by the annual cycle. The origin of the differences between the PJOs is discussed based on the theory of the wave-mean flow interaction. ¿ 2001 American Geophysical Union