The downward inﬂuence of stratospheric sudden warmings
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.