Quantifying the contribution of stratospheric processes to extreme weather events is a challenge. This study breaks new ground in the quantitative attribution of weather extremes to dynamical processes in the stratosphere. It applies two complementary statistical indicators (relative risk and quantile shift) to a coordinated set of experiments with several models to study the role of the stratosphere in three detailed case studies. The paper has the potential to be an authoritative study in this area due to its combination of rigor, depth and range of impacts.

This study describes the implementation and evaluation of a stochastically perturbed parameter (SPP) scheme applied to the sea ice component of ECMWF's Integrated Forecast System (IFS). Applying the SPP scheme in seasonal forecasts enhances ensemble spread for both sea ice concentration and thickness by approximately 10%. The study then concisely catalogues atmospheric effects of introducing stochastic sea ice perturbations and shows that they improve seasonal forecast skill beyond the Arctic region, in particular in Europe during winter. Hypotheses are well presented and tested with different experiments, statistical tests, and toy models.