Abrupt Stratospheric Vortex Weakening Associated With North Atlantic Anticyclonic Wave Breaking. Issue 15 (8th August 2019)

• Record Type: Journal Article
• Authors: Lee, S. H.; Charlton‐Perez, A. J.; Furtado, J. C.; Woolnough, S. J.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 15(2019)
• Page Start: 8563
• Page End: 8575
• Publication Date: 2019-08-08
• Subjects: sudden stratospheric warming -- subseasonal‐to‐seasonal prediction -- stratosphere‐troposphere coupling -- stratospheric polar vortex -- Rossby wave breaking; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2019JD030940
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: The sudden stratospheric warming (SSW) of 12 February 2018 was not forecast by any extended‐range model beyond 12 days. 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. Key Points: In early February 2018, forecasts abruptly transitioned from indicating a strong stratospheric polar vortex to sudden stratospheric warming This was due to the predictability of a cyclone in the North Atlantic, which was associated with driving an anticyclonic Rossby wave break Similar historical cases show this as a mechanism for weakening the stratospheric polar vortex, which can lead to major sudden warmings