Case-to-Case Variability in the Tropospheric Response to Sudden Stratospheric Warmings Revealed by Ensemble Re-Forecasts

Abstract. Stratospheric extreme events during Northern winter and spring have been shown to sometimes enhance the subseasonal predictability of large-scale tropospheric circulation patterns such as the North Atlantic oscillation (NAO) and Greenland/European blocking. We aim to quantify the highly variable downward influence of sudden stratospheric warming (SSW) events on the troposphere in numerical simulations. With this aim, we construct a model climatology using the ICON global numerical weather prediction (NWP) model consisting of possible realistic stratosphere-troposphere evolutions of the coupled troposphere-stratosphere system during winter months. The resulting simulations demonstrate clear stratosphere-troposphere coupling, consistent with observational findings from previous studies. Ensemble re-forecasts, centred around selected SSW events, reveal significant variability in surface responses and robustly show that on average across SSW events, the lower stratosphere serves as a mediator between the upper/mid-stratosphere and the tropospheric flow. We show that the mean tropospheric response to SSWs based on composites and ensemble re-forecasts is heavily case dependent and ties in with the strength of the lower stratospheric anomaly. Our results establish an increased likelihood of developing Greenland blocking with an anomalous lower stratospheric evolution. Moreover, we present indications that the height of wave reflection surfaces can be decisive in establishing persistent lower-stratospheric anomalies and the associated tropospheric response. Overall, we show that individual SSW events differ significantly in their likelihood to induce a canonical tropospheric response, and this likelihood can be predicted at the onset of the SSW.