Air quality and radiative impacts of downward propagating sudden stratospheric warmings (SSWs).

Sudden stratospheric warmings (SSWs) are abrupt disturbances to the Northern Hemisphere wintertime stratospheric polar vortex that can lead to pronounced regional changes in surface temperature and precipitation. SSWs also strongly impact the distribution of chemical constituents within the stratosphere, but the implications of these changes for stratosphere-troposphere exchange (STE) and radiative effects in the upper troposphere-lower stratosphere (UTLS) have not been extensively studied. Here we show, based on a specified-dynamics simulations from the EMAC chemistry-climate model, that SSWs lead to a pronounced increase in high-latitude ozone just above the tropopause (>25 % relative to climatology), persisting for up to 50 days for the ~50 % events classified as downward propagating following Hitchcock et al. (2013). This anomalous feature in lowermost stratospheric ozone is verified from ozone-sonde soundings and using the Copernicus Atmospheric Monitoring Service (CAMS) atmospheric composition reanalysis product. A significant dipole anomaly (>±25 %) in water vapour also persists in this region for up to 75 days, with a drying signal above a region of moistening, also evident within the CAMS reanalysis. Resultant enhanced STE leads to a significant 5–10 % increase in ozone of stratospheric origin over the Arctic, with a typical time-lag of 50 days. The signal also propagates to mid-latitudes leading to significant enhancements in UTLS ozone, and, of weakening strength, also in free tropospheric and near-surface ozone up to 90 days after the event. In quantifying the potential significance for surface air quality breaches above ozone regulatory standards, a risk enhancement of up to a factor of 2 to 3 is calculated following such events. The chemical composition perturbations in the Arctic UTLS result in radiatively-driven Arctic stratospheric temperature changes of around 2 K. An idealised sensitivity evaluation highlights the changing radiative importance of both ozone and water vapour perturbations with seasonality. Our results imply that SSW-related transport of ozone needs to be accounted for when studying the drivers of surface air quality. Accurate representation of UTLS composition (namely ozone and water vapour), through its effects on local temperatures, may also help improve numerical weather prediction forecasts on sub-seasonal to seasonal timescales. [ABSTRACT FROM AUTHOR]