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Based on numerical calculations we demonstrate that small changes in thesmooth climatological background atmosphere may lead to an unstable meanzonal wind distribution in the summer middle atmosphere. We relate thesechanges to small ones because locations and power of the main circulationstructures are conserved, except for the acceleration of the easterly jet inthe stratosphere/mesosphere. The instability forces oscillations propagatingwestward 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 theexcitation and transient propagation of these waves, and the numerical resultscorrespond to features of these variations observed in experimental studies.The growing waves tend to remove the source of excitation. This process iseffective enough to reduce the strong easterly jet and to remove the strongnegative gradient of the zonal mean potential vorticity in the region of theinstability. Therefore, when these parameters are calculated as mean valuesover a long time interval, the obtained values are too small to provide theinstability. Strong 2-day waves, in turn, are unstable and can generatesecondary waves with longer periods and lower zonal wave numbers. This effectis only significant for extremely strong 2-day waves. Another process isfound to be more effective to produce secondary waves. We demonstrated thatthe 2-day wave with s=3 forced by nonlinear interaction between the 10-14day planetary waves and the 2-day wave of zonal wave number 4 is unstable.This wave instability generates secondary waves with amplitudes that arelarge enough to be observed by ground-based radars, for example

Quasi-two-day wave in an unstable summer atmosphere - some numerical results on excitation and propagation