Can stratospheric nudging improve surface predictability? Insights from the 2019 Southern Hemisphere sudden stratospheric warming

This study isolates the role of the 2019 Southern Hemisphere (SH) sudden stratospheric warming (SSW) for surface climate in eight Stratospheric Nudging And Predictable Surface Impacts project (SNAPSI) models. The novel nudging experiments allow for a clearer disentanglement of the SSW’s impact on surface climate than the standard free-running forecasts, in which the entire atmospheric system evolves freely and deviates from observed reality. SNAPSI models capture the downward propagation of the negative Southern Annular Mode (SAM) from the stratosphere when the zonally symmetric stratospheric circulation is nudged toward the observations. In addition, zonally asymmetric stratospheric variations amplify warm anomalies and dry conditions over eastern Australia, enhancing the Australian local high-pressure system. The composite analysis reveals that the zonally asymmetric stratospheric variations driven by the westward-shifted vortex influence the eastern Australian precipitation forecast by strengthening the downward coupling of the SAM and enhancing the meridional ridge-trough structure over Australia. Westward-shifted vortex events are identified in over 60% more ensemble members in the nudged run than in the free run, associated with precipitation closer to observed values in the nudged run. The wildfire weather potential risk in Australia is assessed using the Fraction Attributable Risk (FAR) of the Hot Dry Windy (HDW) wildfire weather index. The positive FAR of the HDW wildfire weather index indicates that the nudged stratosphere contributes up to ~30% to the increased wildfire weather risk along eastern and southern Australia, highlighting the contribution of the 2019 SSW event to Australian extreme weather from mid-October to mid-November.