Assessing stratospheric contributions to subseasonal predictions of precipitation after the 2018 sudden stratospheric warming from the Stratospheric Nudging And Predictable Surface Impacts (SNAPSI) project

The sudden stratospheric warming (SSW) event in February 2018 was followed by dry spells in Scandinavia and record-breaking rainfall over the Iberian Peninsula through the following March. Here, we study the role of the 2018 SSW in subseasonal-to-seasonal (S2S) prediction of the "Wet Iberia and Dry Scandinavia"precipitation signal, using a new database of S2S forecasts generated by the Stratospheric Nudging And Predictable Surface Impacts (SNAPSI) project. This database includes three sets of forecast ensembles: a free ensemble in which the atmosphere evolves freely, a nudged ensemble in which the stratosphere is nudged to the observed zonal-mean evolution of the 2018 SSW, and a control ensemble in which the stratosphere is nudged to climatology. Each set of ensembles has two initialization dates: 25 January and 8 February 2018, both before the onset of the SSW on 12 February. We find that the "Wet Iberia and Dry Scandinavia"pattern was captured by the late free ensemble (initialized at 8 February) that successfully predicted the stratospheric warming but not by the early free ensemble (initialized at 25 January) that predicted a stratospheric cooling. Unlike the early free ensemble, the early nudged ensemble successfully captured the "Wet Iberia and Dry Scandinavia"pattern, indicating that an accurate forecast of stratospheric variability can improve S2S predictability of precipitation. While the pattern of European precipitation anomalies is evidently connected to the stratosphere, we estimate that only roughly a quarter of the amplitude was expected, given the stratospheric anomalies. Nonetheless, Iberian rainfall extremes of equal strength or stronger than the one observed are twice as likely in the nudged ensemble than in the control ensemble. The increased likelihood in the nudged ensemble suggests that the weakened stratospheric polar vortex can increase the risk of Iberian rainfall extremes.