A global, quasigeostrophic model of planetary wave interaction with the mean zonal flow has been used to simulate sudden stratospheric warmings forced by mean tropospheric zonal winds interacting with orography. Planetary waves m=1 and 2 are simultaneously computed and interact with each other only through the mean zonal flow. Orographic forcing is determined by the tropospheric wind field. An increase in the jet stream maximum from 36 ms-1 to 48 ms-1 is sufficient to initiate repeated major warmings with an ~80 day cycle. These results are in good agreement with the observed anti-correlation of the annual variation of winter monthly averaged jet stream strength and polar night jet strentgh, which is interpreted to be a direct consequence of the increased planetary wave stresses on the mean flow in the stratosphere.