The generic nature of the tropospheric response to sudden stratospheric warmings

Official version

The tropospheric response to mid-winter sudden stratospheric warmings (SSWs) is examined using an idealised model. SSW events are triggered by imposing high-latitude stratospheric heating perturbations of varying magnitude for only a few days, spun-off from a free-running control integration (CTRL). The evolution of the thermally-triggered SSWs are then compared with naturally-occurring SSWs identified in CTRL. By applying a heating perturbation, with no modification to the momentum budget, it is possible to isolate the tropospheric response directly attributable to a change in the stratospheric polar vortex, independent of any planetary-wave momentum torques involved in the initiation of a SSW. Zonal-wind anomalies associated with the thermally-triggered SSWs first propagate downward to the high-latitude troposphere after ∼ 2 weeks, before migrating equatorward and stalling at midlatitudes, where they straddle the near-surface jet. After ∼ 3 weeks, the circulation and eddy fluxes associated with thermally-triggered SSWs evolve very similarly to SSWs in CTRL, despite the lack of initial planetary-wave driving. This suggests that at longer lags, the tropospheric response to SSWs is generic and it is found to be linearly governed by the strength of the lower-stratospheric warming, whereas at shorter lags, the initial formation of the SSW potentially plays a large role in the downward coupling. In agreement with previous studies, synoptic waves are found to play a key role in the persistent tropospheric jet shift at long lags. Synoptic waves appear to respond to the enhanced midlatitude baroclinicity associated with the tropospheric jet shift, and preferentially propagate poleward in an