The exact mechanisms for the production of this lower stratospheric response remain uncertain. Here, SST perturbation experiments with the dynamical version of the Canadian Middle Atmosphere Model (CMAM) will be used to investigate the mechanisms responsible for the change in circulation of the tropical lower stratosphere in response to SST anomalies. While some recent evidence suggests that altered gravity wave drag in the northern hemisphere sub-tropics may be important, in these experiments changes in resolved wave drag in the summer hemisphere are found to dominate. The reason for this anomalous wave drag and why the summer hemisphere dominates will be discussed. Furthermore the mechanism for the production of the tropical upwelling response to ENSO will be compared and contrasted with that in response to global warming.