Sub seasonal variations of weak stratospheric polar vortex in December and its impact on Eurasian air temperature

Weak stratospheric polar vortex (WPV) events during winter months were investigated. WPV events were identified as being weakest in December, accompanied by the most dramatic increase in geopotential height over the polar region. After the onset of a December WPV event, the dynamic processes influencing Eurasian temperature can be split into two separate periods. Period I (lag of 0-25 days) is referred to as the stratosphere-troposphere interactions period, as it is mainly characterized by stratospheric signals propagating downwards. In Period I, a stratospheric negative Northern Annular Mode (NAM) pattern associated with the WPV propagates downwards, inducing a negative NAM in the troposphere. The anomalous low centers over the Mediterranean and North Pacific bring cold advection to northern Eurasia, resulting in a north-cold-south-warm dipole pattern over Eurasia. The zero line between negative and positive temperature anomalies moves southwards during days 5-20. Stratospheric cold anomalies at midlatitudes propagate downwards to high latitudes in the troposphere and contribute to the dipole structure. During Period II (lag of 25-40 days), as downward signals from the stratosphere have vanished, the dynamic processes mainly take place within the troposphere. Specifically, a wave train is initiated from the North Atlantic region to northern Europe. The propagation of wave activity flux intensifies a cyclonic anomaly over northern Europe, which brings cold advection to Scandinavia and warm advection to central Asia. Therefore, a northwest-cold-southeast-warm dipole structure occupies Eurasia and migrates southeastwards during this period.