Abstract. Based on numerical calculations we demonstrate that small changes in the smooth climatological background atmosphere may lead to an unstable mean zonal wind distribution in the summer middle atmosphere. We relate these changes to small ones because locations and power of the main circulation structures are conserved, except for the acceleration of the easterly jet in the stratosphere/mesosphere. The instability forces oscillations propagating westward with a period of about 2 days and zonal wave numbers s=3 and/or 4. There are variations in the mean zonal wind distribution due to the excitation and transient propagation of these waves, and the numerical results correspond to features of these variations observed in experimental studies. The growing waves tend to remove the source of excitation. This process is effective enough to reduce the strong easterly jet and to remove the strong negative gradient of the zonal mean potential vorticity in the region of the instability. Therefore, when these parameters are calculated as mean values over a long time interval, the obtained values are too small to provide the instability. Strong 2-day waves, in turn, are unstable and can generate secondary waves with longer periods and lower zonal wave numbers. This effect is only significant for extremely strong 2-day waves. Another process is found to be more effective to produce secondary waves. We demonstrated that the 2-day wave with s=3 forced by nonlinear interaction between the 10-14 day planetary waves and the 2-day wave of zonal wave number 4 is unstable. This wave instability generates secondary waves with amplitudes that are large enough to be observed by ground-based radars, for example.