The combined impact of subtropical wave train and Polar-Eurasian teleconnection on the extreme cold event over North China in January 2021

An extreme cold event occurred in North China (NC) during 4-9 January 2021, with a cooling anomaly in most area of NC reaching more than 10 degrees C. During this event, NC average daily surface air temperature was below - 1 standard deviation, and the daily minimum/maximum temperature was lower than the 10th percentile threshold. The combined effect of the Polar-Eurasian teleconnection (POL) characterized by a geopotential height seesaw with positive anomalies over the Arctic and negative anomalies over Northeast Asia and the subtropical wave train originating from the North Atlantic on the extreme cold event was investigated based on observational, reanalysis datasets and numerical simulation. Results showed that diabatic heating released by the increased North Atlantic precipitation acted as a heat source which excited the eastward propagating subtropical wave train. Additionally, sea ice loss in Barents Sea produced an upward net heat flux anomaly and then triggered an anticyclonic anomaly, which spread southeastward and formed the POL. Both the subtropical wave train and POL had a cyclonic anomaly over East Asia associated with stronger anomalous northerly wind over NC, leading to the extreme cold event in NC. The linear baroclinic model simulations also reproduced the generation and propagation of the subtropical wave train and POL, which verified the role of Atlantic anomalous precipitation and Barents Sea sea-ice loss in triggering this extreme event. The above findings can give guidance for the prediction of extreme cold events in NC from the perspective of both mid-latitude and Arctic factors.