Stratospheric impacts on weather regimes following the 2018 and 2019 sudden stratospheric warmings

Major disruptions of the stratospheric polar vortex can improve subseasonal forecast skill for surface climate, as negative North Atlantic Oscillation-like (NAO−) states can follow sudden stratospheric warmings (SSWs). Yet most insights come from observational studies or large operational forecast archives. Here we use Stratospheric Nudging And Predictable Surface Impacts (SNAPSI) project experiments, which applies stratospheric nudging to forecasts of two SSWs (2018 and 2019) followed by differing tropospheric evolutions. We show that SSWs systematically shift the atmosphere toward negative NAO-like regimes (stronger Greenland anticyclone) in both the North Atlantic-European and North American regions. Comparisons among nudged, free, and control runs quantify the benefits of improving and removing SSW representation in diagnosing tropospheric regime shifts. In 2018, accurate stratospheric representation improved weather regime forecasts. However, in 2019, despite persistent observed ridged regimes, nudged SSWs still induced negative NAO-like patterns, implying that subseasonal models sometimes underrepresent other teleconnections and overrepresent stratosphere-troposphere coupling.