Dynamics of the lower stratospheric circulation response to ENSO

A robust feature of the observed response to anomalies in tropical Pacific sea surface temperatures (SSTs) is a change in temperature of the low latitude lower stratosphere. This consists of a cooling in the tropics and a warming in mid-latitudes for warm ENSO and vice-versa. These temperature anomalies are associated with an anomalous lower stratospheric circulation that can also alter ozone and water vapour concentrations.

Here, the mechanism for production of this low latitude stratospheric circulation response will be investigated with SST perturbation experiments using the dynamical version of the Canadian Middle Atmosphere Model (CMAM). It is found that the anomalous upwelling is predominantly associated with altered resolved wave drag in the southern hemisphere sub-tropical lower stratosphere. It is proposed that the reason for this anomalous wave drag is associated with the altered zonal wind structure in the troposphere. For warm ENSO conditions an acceleration of the sub-tropical jet and an equatorward shifted mid-latitude jet results in improved conditions for vertical wave propagation into the lower stratosphere, related to a quasi-geostrophic refractive index anomaly. The opposite is true of cold ENSO conditions.

This altered wave propagation and drag is only apparent in the southern hemisphere. The difference between the two hemispheres can be related to the difference in their tropospheric zonal wind anomalies which is likely a consequence of the difference in climatological jet structures.