The summer-to-winter-pole residual circulation in the upper meosphere is driven by gravity waves. In the northern winter hemisphere the westward gravity-wave drag is strongly modulated by the variability of the polar vortex which in turn is controlled by quasi-stationary Rossby waves. An analogous modulation of the eastward gravity-wave drag in the summer mesopause region is not possible since quasi-stationary Rossby waves are filtered by the westward stratospheric jet in summer. Nevertheless, the gravity-wave drag in summer can be modulated by the polar vortex in winter through interhemispheric coupling. It is therefore likely that the gravity-wave driving in summer is also affected through other mechanisms. In this contribution we focus on modulations of the gravity-wave driving that are induced by 1) solar proton events, 2) thermal tides, and 3) an altered radiation budget caused by increased CO2-concentrations. All model results are deduced from sensitivity experiments with the Kühlungsborn Mechanistic general Circulation Model (KMCM). In the first two examples, the perturbations give rise to variations of the background wind that cause eastward gravity waves to attain shorter vertical wavelengths. This implies a downward shift of the equatorward branch of the residual circulation and hence an anomalous dynamical warming of the mesopause region. In the third example, the overall radiative cooling in the middle atmosphere is reversed in the region of the cold summer mesopause, inducing an anomalous westward background wind component in response to CO2 doubling. In turn, the gravity wave drag maximum is shifted to higher altitudes, inducing a dynamic cooling which partly offsets the radiative perturbation. This mechanism should be contrasted to the effect of enhanced tropospheric gravity-wave sources which may also be expected to arise from climate change and lead to a cooling of the polar summer mesosphere that peaks around the mesopause.