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A robust feature of the observed response to El Nino-Southern Oscillation (ENSO) is an altered circu- lation in the lower stratosphere. When sea surface temperatures (SSTs) in the tropical Pacific are warmer there is enhanced upwelling and cooling in the tropical lower stratosphere and downwelling and warming in the midlatitudes, while the opposite is true of cooler SSTs. The midlatitude lower stratospheric response to ENSO is larger in the Southern Hemisphere (SH) than in the Northern Hemisphere (NH). In this study the dynamical version of the Canadian Middle Atmosphere Model (CMAM) is used to sim- ulate25 realizations ofthe atmospheric responsetothe 1982/83 ElNinoandthe 1973/74La Nina. Thisversion of CMAM is a comprehensive high-top general circulation model that does not include interactive chemistry. The observed lower stratospheric response to ENSO is well reproduced by the simulations, allowing them to be used to investigate the mechanisms involved. Both the observed and simulated responses maximize in December-March and so this study focuses on understanding the mechanisms involved in that season. The response in tropical upwelling is predominantly driven by anomalous transient synoptic-scale wave drag in the SH subtropical lower stratosphere, which is also responsible for the compensating SH midlatitude response.Thisalteredwavedragstemsfromanalteredupwardfluxofwaveactivityfromthetroposphereinto the lower stratosphere between 208 and 408S. The alteredflux of wave activity can be divided into two distinct components.In the Pacific, the acceleration of the zonal wind in the subtropics from the warmer tropicalSSTs results in a region between the midlatitude and subtropical jets where there is an enhanced source of low phase speed eddies. At other longitudes, an equatorward shift of the midlatitude jet from the extratropical tropospheric response to El Nino results in an enhanced source of waves of higher phase speeds in the subtropics. The altered resolved wave drag is only apparent in the SH and the difference between the two hemispheres can be related to the difference in the climatological jet structures in this season and the pro- jection of the wind anomalies associated with ENSO onto those structures.

Dynamics of the Lower Stratospheric Circulation Response to ENSO