The seasonal and interannual variations of the stratospheric circulation are investigated using a quasi-geostrophic, β-plane channel model of the middle atmosphere. In the horizontal the model truncation retains at most two zonal and meridional wavenumbers. There are 27 layers in the vertical. All conditions at the model's lower boundary in the lower stratosphere are constant in time. No interannual variations are found if the flow is forced to a radiative equilibrium condition that is annually periodic with a radiative damping time of ~20 days in the lower stratosphere. The seasonal evolution of the stratospheric circulation differs for different amplitudes of zonal wavenumbers 1 and 2 prescribed at the model's lower boundary (hB1 and hB2, respectively). When hB2 is small (50 m), variations are weak during winter and strong during spring, thus resembling the southern stratosphere. When hB2 is larger (150 m), variations have significant intensity during the entire winter, thus resembling the northern stratosphere. It is demonstrated that interannual variations can be obtained during winter and spring if two requirements are met. First, the radiative damping time is doubled in the lower stratosphere to a value in closer agreement with observational estimates. Second, if the model's truncation includes the antisymmetric mode about the channel center. The two requirements complement each other in the following way: Larger radiative damping times imply that the system can remember different conditions at the beginning of the previous summer, when otherwise the system memory is lost. Nonsymmetric conditions about the channel center imply that the critical level of the mean zonal wind in early winter does not extend across the entire domain. It is concluded that intraseasonal and interannual variations in the stratospheric circulation could exist even in the presence of planetary waves with constant amplitude in the troposphere and radiative forcing in the stratosphere, which is annually periodic. ¿ 2001 American Geophysical Union