Abrupt Stratospheric Vortex Weakening Associated With North Atlantic Anticyclonic Wave Breaking

The sudden stratospheric warming (SSW) of 12 February 2018 was not forecast by any extended-range model beyond 12days. From early February, all forecast models that comprise the subseasonal-to-seasonal (S2S) database abruptly transitioned from indicating a strong stratospheric polar vortex (SPV) to a high likelihood of a major SSW. We demonstrate that this forecast evolution was associated with the track and intensity of a cyclone in the northeast Atlantic, with an associated anticyclonic Rossby wave break, which was not well forecast. The wave break played a pivotal role in building the Ural high, which existing literature has shown was a precursor of the 2018 SSW. The track of the cyclone built an anomalously strong sea level pressure dipole between Scandinavia and Greenland (termed the S-G dipole), which we use as a diagnostic of the wave break. Forecasts that did not capture the magnitude of this event had the largest errors in the SPV strength and did not show enhanced vertical wave activity. A composite of 49 similarly strong wintertime (November-March) S-G dipoles in reanalysis shows associated anticyclonic wave breaking leading to significantly enhanced vertical wave activity and a weakened SPV in the following days, which occurred in 35% of the 15-day periods preceding observed major SSWs. Our results indicate a particular transient trigger for weakening the SPV, complementing existing results on the importance of tropospheric blocking for disruptions to the Northern Hemisphere extratropical stratospheric circulation. Plain Language Summary During winter, a large circulation 10-50 km above the pole (known as the stratospheric polar vortex) can influence the day-to-day weather patterns in the troposphere beneath from weeks to months later. Thus, being able to predict the behavior of the stratospheric polar vortex is important for predicting the weather on longer time frames. In February 2018, the Northern Hemisphere stratospheric polar vortex broke apart in an event known as a sudden stratospheric warming, which was not well forecast. This event led to unusually cold conditions across Eurasia. In this article we find the poor predictability of the event was due to a poorly forecast weather system in the Atlantic. We also show that this pattern was present in previously observed cases where the stratospheric polar vortex has weakened. Our results demonstrate a trigger mechanism for these extreme events and have implications for our ability to predict the weather at longer ranges.