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 for January–March
2020 from forecasts launched through October–December 2019. Although
the magnitude of the anomalies was underestimated, the performance of
most prediction systems was extremely high for a positive AO winter
relative to 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 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.