Fig. 1: Atmospheric conditions of A-RTDW events.

a Synoptic anomalous atmospheric conditions from the initial day (A-RTDW day −3, i.e., 3 days before the A-RTDW event occurs) to the peak date (A-RTDW day 0, i.e., the day when the A-RTDW event occurs) of A-RTDW events, including 1000 hPa temperature increment anomalies from the prior day to the peak date (colored, interval: 1, units: °C; positive in red and negative in blue), 500 hPa horizontal wind anomaly (blue vector, units: m·s⁻¹) and geopotential height (black contours, interval: 100, units: m) averaged from A-RTDW day −3 to A-RTDW day 0. The purple line, purple solid dots and purple numbers −3, −2, −1, 0 in (a) represent the storm trajectory, centers of the storm and 3, 2, 1, 0 days before the A-RTDW event occurs, respectively. Anomalies of 500 hPa geopotential height (colored, interval: 3, units: m) and horizontal wind (black vector, units: m s⁻¹) in DJF obtained by regression on the (b) A-RTDW index and (c) DJF EMI index during 1950–2018, respectively. In (b) and (c), only vectors with meridional component anomalies at the significance level of p < 0.1 are shown. The EMI index in (c) has been multiplied by –1 for display purposes. The dashed purple boxes in (b) and (c) mark the region (60°–90°N, 30°W–20°E). d Anomalies of 850 hPa eddy kinetic energy (EKE; colored, interval: 0.3, units: m² s⁻²) in DJF obtained by regression on the DJF V index during 1950–2018. The dots indicate significance at p < 0.1.