Subseasonal Reversal of Extreme Cold Temperature Frequencies in Northeast China: Possible Mechanism and Prediction

Subseasonal reversals of extreme cold days (ECDs) over Northeast China during the winters of 1980-2022 are investigated in this study. Almost half of the years (18 of the 43) experienced subseasonal reversals between early winter (December 1 to January 15) and late winter (January 16 to February 28), characterized by either "more-to-fewer ECDs (ECD+−)" or "fewer-to-more ECDs (ECD−+)." Subseasonal shifts of the North Atlantic Oscillation, the Scandinavian-like pattern, and the stratospheric polar vortex are associated with ECD+−/−+ years. Previous autumn sea surface temperature anomalies and Siberian snow cover anomalies can excite significant atmospheric circulation anomalies or Rossby wave trains that contribute to the subseasonal reversal of ECDs. Statistical forecast models based on physical mechanisms skillfully predict the early/late winter ECD index, with temporal correlation coefficient skill of 0.74/0.46 during the cross-validation period of 1980-2002 and 0.54/0.54 during the independent hindcast period of 2003-2022; moreover, extreme values of the ECD index are also reasonably predicted. The findings of this study offer new insights regarding the mechanism and prediction of subseasonal ECDs over Northeast China.