The distinct roles of the Stratospheric Polar Vortex during the initiation and intensification stages of the 2016 East Asian Cold Air Outbreak

Using ERA-Interim reanalysis data and ensemble forecasts from the Global Ensemble Forecast System (GEFS), we explore the roles of the Arctic stratospheric polar vortex throughout the life cycle of the 2016 East Asian cold air outbreak (CAO). Our results reveal the distinct roles of the stratosphere in the initiation and intensification stages of this CAO. The occurrence of this CAO was directly related to an exceptionally strong Ural blocking ridge that advected cold air mass toward East Asia. Notably, the strengthening of Ural blocking is largely influenced by the behavior of the stratospheric polar vortex. Specifically, the contraction of the polar vortex toward the western Urals paved the way for reflecting planetary waves originating from the North Atlantic via changing the buoyancy frequency just above the tropopause. The reflected waves converged over the western Urals and acted to strengthen Ural blocking that in turn affected this CAO. Subsequently, an elongation of the polar vortex toward East Asia was observed, leading to a significant descent of the tropopause over East Asia. According to the invertibility theory of potential vorticity, the descending tropopause likely induced a cyclonic anomaly over East Asia, further contributing to the intensification of this CAO. However, the impact of this cyclonic anomaly induced by the polar vortex elongation is relatively limited compared to the influence of Ural blocking. In summary, our study highlights that the geometry of the polar vortex, rather than its strength, is crucial in understanding the stratospheric impacts on the 2016 East Asian CAO.