Seasonal Forecasts of the Exceptional Northern Hemisphere Winter of 2020

Abstract:

The winter of 2019-20 was dominated by an extremely strong and persistent stratospheric polar vortex and positive tropospheric Arctic Oscillation (AO). Here, we analyze forecasts from 6 different models contributing to the C3S seasonal forecast database. Most models performed very strongly, with consistently high skill demonstrated by forecasts for January-March 2020 launched through October to December 2019. Although the magnitude of the anomalies was underestimated, the forecast performance was widely unprecedented for a positive AO winter in the common hindcast climate. Ensemble members which better predicted the extremely strong stratospheric vortex better predicted the extreme tropospheric state. We find a significant relationship between forecasts of the anomalous mid-latitude tropospheric wave pattern in early winter, which destructively interfered with the climatological stationary waves, and the forecast strength of the stratospheric vortex later in the winter. Our results support the role of accurate Arctic stratospheric vortex predictions for improving seasonal weather forecasts.