Predicting Sudden Stratospheric Warming 2018 and Its Climate Impacts With a Multimodel Ensemble

Sudden stratospheric warmings (SSWs) are significant source of enhanced subseasonal predictability, but whether this source is untapped in operational models remains an open question. Here we report on the prediction of the SSW on 12 February 2018, its dynamical precursors, and surface climate impacts by an ensemble of dynamical forecast models. The ensemble forecast from 1 February predicted 3 times increased odds of an SSW compared to climatology, although the lead time for SSW prediction varied among individual models. Errors in the forecast location of a Ural high and underestimated magnitude of upward wave activity flux reduced SSW forecast skill. Although the SSW's downward influence was not well forecasted, the observed northern Eurasia cold anomaly following SSW was predicted, albeit with a weaker magnitude, due to persistent tropospheric anomalies. The ensemble forecast from 8 February predicted the SSW, its subsequent downward influence, and a long-lasting cold anomaly at the surface. Plain Language Summary Predicting climate anomalies more than 2 weeks ahead remains a challenging task. However, there are periods when climate is more predictable. In particular, it is known that in the wintertime Northern Hemisphere midlatitudes, climate may be more predictable during several weeks following sudden stratospheric warmings (SSWs), when a cold stratospheric polar vortex suddenly breaks and the stratosphere warms. In this article we evaluate quality of monthly forecasts by several up-to-date forecast systems during a sudden stratospheric warming observed in February 2018 focusing both on predictability of the stratospheric anomalies and on predictability of their surface impacts. We find that models predicted enhanced probability of sudden stratospheric warming at least 11 days before the observed event. Four days before the event it was forecast by all models with a very high certainty. We further show that successful prediction of the SSW required correct forecasting of anticyclonic anomaly over Ural and associated upward propagation of planetary-scale atmospheric waves. The observed cold anomaly across Eurasia in February 2018 was predicted more than 2 weeks ahead although at this lead time the predictability was more likely associated with persistence of tropospheric anomalies in the forecast, with smaller downward influence of the stratospheric anomalies.