The downward influence of stratospheric sudden warmings

Abstract

The coupling between the stratosphere and the troposphere following two major stratospheric sudden warmings is studied in the Canadian Middle Atmosphere Model using a nudging technique by which the zonal mean evolution of the reference sudden warmings are artificially induced in an ∼100 member ensemble spun off from a control simulation. Both reference warmings are taken from a freely-running integration of the model. One event is a displacement, the other a split, and both are followed by extended recoveries in the lower stratosphere. The methodology permits a statistically robust study of their influence on the troposphere below. The nudged ensembles exhibit a tropospheric annular-mode response closely analogous to that seen in observations, confirming the downward influence of sudden warmings on the troposphere in a comprehensive model. This tropospheric response coincides more closely with the lower stratospheric annular mode anomalies than with the mid-stratospheric wind reversal. In addition to the expected synoptic scale eddy feedback, the planetary-scale eddies also reinforce the tropospheric wind changes, apparently responding directly to the stratospheric anomalies. Furthermore, despite the zonal symmetry of the stratospheric perturbation, a highly zonally asymmetric near surface response is produced, corresponding to a strongly negative phase of the North Atlantic Oscillation with a much weaker response over the Pacific basin which matches composites of sudden warmings from the ERA-Interim reanalysis. The Coupled Model Intercomparison Project phase 5 models exhibit a similar response, though in most models its magnitude is under-represented.