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. The strength of the stratospheric polar vortex influences thesurface weather in the Northern Hemisphere in winter; a weaker (stronger)than average stratospheric polar vortex is connected to negative (positive)Arctic Oscillation (AO) and colder (warmer) than average surfacetemperatures in northern Europe within weeks or months. This holds thepotential for forecasting in that timescale. We investigate here if thestrength of the stratospheric polar vortex at the start of the forecastcould be used to improve the extended-range temperature forecasts of theEuropean Centre for Medium-Range Weather Forecasts (ECMWF) and to findperiods with higher prediction skill scores. For this, we developed astratospheric wind indicator (SWI) based on the strength of the stratosphericpolar vortex and the phase of the AO during the following weeks. Wedemonstrate that there was a statistically significant difference in theobserved surface temperature in northern Europe within the 3–6 weeks,depending on the SWI at the start of the forecast. When our new SWI was applied in post-processing the ECMWF's 2-week meantemperature reforecasts for weeks 3–4 and 5–6 in northern Europeduring boreal winter, the skill scores of those weeks were slightlyimproved. This indicates there is some room for improving the extended-rangeforecasts, if the stratosphere–troposphere links were better captured in themodelling. In addition to this, we found that during the boreal winter, incases where the polar vortex was weak at the start of the forecast, the mean skillscores of the 3–6 weeks' surface temperature forecasts were higher thanaverage.

Adding value to extended-range forecasts in northern Europe by statistical post-processing using stratospheric observations