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 is 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 midstratospheric 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 nearsurface response is produced, corresponding to a strongly negative phase of the North Atlantic Oscillation with a much weaker response over the Pacific basin that matches composites of sudden warmings from the Interim ECMWF Re-Analysis (ERA-Interim). Phase 5 of the Coupled Model Intercomparison Project models exhibit a similar response, though in most models the response's magnitude is underrepresented.

Late Twenty-First-Century Changes in the Midlatitude Atmospheric Circulation in the CESM Large Ensemble

“…Note that in section 3e, we describe a strong connection between the strength of the polar vortex and the North Atlantic zonal index. This supports recent studies suggesting that stratosphere-troposphere coupling projects more strongly onto the NAO than the NAM (e.g., Hitchcock and Simpson 2014;Smith and Polvani 2014).…”

“…This connection between the zonal index over the North Atlantic sector (i.e., the NAO) and the stratospheric polar vortex is also found in CMIP5 models, adding credence to this finding (Manzini et al 2014;Cattiaux et al 2016). Further studies will be necessary to understand whether changes in the lower stratosphere modulate the midlatitude circulation in the troposphere or whether they are forced by changes in the midlatitude flow and in the NAO (Hitchcock and Simpson 2014;Kidston et al 2015).…”

“…7e) except over the PA/AM sector, where a larger AA is associated with larger negative ZON anomalies. In contrast, the connection to PST is only significant over the AT sector (Table 2), revealing a connection between changes in the lower polar stratosphere and NAO anomalies in the troposphere (Hitchcock and Simpson 2014;Smith and Polvani 2014). A high correlation between changes in PST and in the midlatitude atmospheric circulation has also been reported in the CMIP5 intermodel spread (Manzini et al 2014;Cattiaux et al 2016), supporting the finding that PST is strongly tied to the midlatitude circulation in late twenty-first-century RCP8.5 projections.…”

Satellite observations of middle atmosphere gravity wave absolute momentum flux and of its vertical gradient during recent stratospheric warmings

“…It has been found that the circulation anomalies of SSWs have influence on surface weather and climate (e.g., Baldwin and Dunkerton, 2001;Kodera et al, 2016), as well as on sea surface temperature (O'Callaghan et al, 2014). Near-surface effects of SSWs are not limited to the polar region, but can also extend to low latitudes (e.g., Hitchcock and Simpson, 2014).…”

Variations of Kelvin waves around the TTL region during the stratospheric sudden warming events in the Northern Hemisphere winter

“…We subtracted planetary scale wave components with zonal wave numbers k smaller than 4 (i.e. k = 1, 2, 3) (Hitchcock and Simpson, 2014) from extratropical meridional wind and temperature field, which were obtained from ERA-Interim Reanalysis between 200 and 5 hPa, to calculate the eddy heat flux. The overbar denotes zonal mean.…”