On the Zonal Asymmetry of Tropospheric Anomalies Forced by Stratospheric Polar Vortex Variability

Abstract The observed surface response to stratospheric warming largely resembles the negative phase of the North Atlantic Oscillation (NAO), a regional mode of the tropospheric intrinsic variability. We use an idealized general circulation model to show that zonal asymmetries in the tropospheric response to stratospheric variability are closely linked to zonal asymmetries in the tropospheric intrinsic variability. A SYMMETRIC model configuration with zonally homogeneous forcing and annular tropospheric variability is compared against a LOCALIZED configuration with stationary wave sources and localized NAO-like variability. Warming the stratosphere in both configurations produces a surface response that resembles the corresponding leading mode of tropospheric intrinsic variability. This suggests that the same eddy feedbacks responsible for the intrinsic variability also play a central role in zonally localizing the tropospheric response to stratospheric forcing. We rule out the zonally averaged secondary circulation (“downward control”) as the primary pathway for transmitting dynamical signals from the stratosphere down to the troposphere. In SYMMETRIC, downward control is responsible for less than a third of the total tropospheric response; we infer that transient planetary waves account for the remaining fraction. In LOCALIZED, stationary planetary waves produce strong two-way coupling between the NAO and the stratosphere that dominates any coupling mediated by downward control. We find evidence that stationary planetary waves also mediate two-way coupling in reanalysis output, although it suggests the upward coupling in LOCALIZED is too strong. In light of these results, we propose “planetary waves with eddy feedback (PWEF)” as an alternative paradigm to “downward control with eddy feedback (DCWEF)” to understand stratosphere–troposphere coupling.