Interannual variations in the stratospheric circulation with a perfectly steady troposphere

The seasonal and interannual variations of the stratospheric circulation ar e investigated using a quasi-geostrophic, p-plane channel model of the midd le atmosphere. In the horizontal the model truncation retains at most two z onal and meridional wavenumbers. There are 27 layers in the vertical. All c onditions at the model's lower boundary in the lower stratosphere are const ant 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 similar to 20 days in the lower stratosphere. The seasonal evolution of the stratospheric circulation differs for different amplitude s of zonal wavenumbers 1 and 2 prescribed at the model's lower boundary (h( B1) and h(B2), respectively). When h(B2) is small (50 m), variations are we ak during winter and strong during spring, thus resembling the southern str atosphere. When h(B2) is larger (150 m), variations have significant intens ity 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 i s doubled in the lower stratosphere to a value in closer agreement with obs ervational estimates. Second, if the model's truncation includes the antisy mmetric 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 wi nd in early winter does not extend across the entire domain. It is conclude d that intraseasonal and interannual variations in the stratospheric circul ation could exist even in the presence of planetary waves with constant amp litude in the troposphere and radiative forcing in the stratosphere, which is annually periodic.