The Downward Influence of Sudden Warmings

We argue, on the basis of a set of experiments with a comprehensive stratosphere-resolving model, that the presence of the zonally-symmetric anomalies associated with long-lived recoveries from stratospheric sudden warmings is sufficient to produce the equatorward shift of the tropospheric mid-latitude jets seen in composites of these events. In particular, we demonstrate quantitatively that the `direct' response (via zonally-symmetric adjustment) to the zonal forcing and heating required to produce and maintain the lower stratospheric adjustments is not sufficient to cause the full zonal mean response. We argue further that this is true even if eddy-feedbacks produced in response to this symmetric adjustment are included.

However, the planetary-scale eddies in the model also respond in the presence of the lower stratospheric anomalies, and are weakened during the extended recovery period even in the troposphere. This suppression does not resemble the planetary wave response to annular mode variability produced internally by the model's troposphere. While the anomalous tropospheric momentum fluxes associated with the planetary-scale eddies are weak compared to those of the synoptic scale eddies, we provide evidence that the response of the latter can be explained as a feedback to the former, and that the combined effect is sufficient to explain the zonal mean shift of the tropospheric jet.