The response of the tropical lower stratosphere to ENSO

In response to anomalies in tropical pacific sea surface temperatures, both observations and models consistently show a temperature signal in the tropical lower stratosphere of the opposite sign to the SST anomaly. During warm ENSO conditions the tropical lower stratosphere is cooler and vice-versa. This temperature signal is associated with changes in tropical upwelling which also acts to alter stratospheric ozone and water vapour concentrations.

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.