Forecast-based attribution of the role of stratospheric variability in weather extremes

Abstract. Variability of the stratospheric polar vortex, particularly its dramatic breakdown during sudden stratospheric warming (SSW) events, has been linked to a number of surface weather extremes. However, attributing the role of stratospheric variability in a specific observed weather extreme, rather than an abstracted class of extremes, has proved highly challenging. Here we use an ensemble of subseasonal forecast simulations from 7 forecast systems participating in the Stratospheric Nudging and Predictable Surface Impacts (SNAPSI) project to carry out this task. By comparing the likelihood of extreme events in free-running forecasts to those with the zonal-mean stratospheric state nudged towards its observed or climatological evolution (while the troposphere is freely-evolving), we are able to calculate the changes in the risk and severity of extremes due to the occurrence, or non-…