Predictability of Arctic Sudden Stratospheric Warmings investigated by the ensemble predictions of the ECMWF's integrated forecast system

The temporal evolution of the northern polar vortex exhibits remarkable interannual variability. Operational forecasts of the ECMWF ensemble prediction system (EPS) are analysed to provide quantitative measures of the reliability of the stratospheric forecasts. In order to quantify the predictive skill, we analyse the performance of the 50 members of the EPS for two different lead times during the recent four winters. These winters include extreme cases as the very stable polar vortex and late breakup during 2010-2011 as well as the very early major sudden stratospheric warming (SSW) during the most recent winter 2012-2013.

In an early investigation, it was shown that the EPS forecasts for the onset of the major SSW in the winter 2009-2010 were very skilful and the ensemble spread was very small (Dörnbrack et al., 2012). However, the ensemble spread increased dramatically after the major SSW, displaying the strong non-linearity and internal variability involved in the SSW event. Here, we will present results for the recent four winters and discuss differences and similarities.

Dörnbrack, A., Pitts, M. C., Poole, L. R., Orsolini, Y. J., Nishii, K., and Nakamura, H.: The 2009-2010 Arctic stratospheric winter - general evolution, mountain waves and predictability of an operational weather forecast model, Atmos. Chem. Phys., 12, 3659-3675, doi:10.5194/acp-12-3659-2012, 2012.