Two sets of experiments are describe that use a stratosphere mesosphere model to investigate the response of the Northern Hemisphere stratospheric flow regime to changes in tropospheric wave forcing and to changes in equatorial wind direction. An ensemble approach is employed so that changes in variability of the winters can also be assessed. Results suggest that under intermediate planetary wave forcing there is no strong tropospheric control of the stratospheric response. Under these circumstances the ambient background flow of the stratosphere into which the planetary waves propagate is important. The ambient background flow can be influenced by the equatorial wind direction and strength and also by the 'initial' conditions in autumn / early winter as the vortex is established. The role of travelling anticyclones in the build-up of the Aleutian High and the subsequent sudden stratospheric warming is highlighted. These anticyclones are features of the equatorial / subtropical upper stratosphere. They have a relatively shallow structure and maximum amplitude at the stratopause. E-P flux diagnostics suggest that flow modification in the upper stratosphere is a key aspect of the early stages of the stratospheric warming.