Subseasonal swing of cold and warm extremes between Eurasia and North America in winter of 2020/1: Initiation and physical process


 Eurasia and North America experienced a robust subseasonal swing of surface air temperature (SAT) extremes in 2020/21 winter, featuring severe cold (warm) extremes over Eurasia before (after) January 15 and conversely over North America. This sharp subseasonal swing of intercontinental cold and warm extremes exerted considerable severe impacts on human activities and the global economy. Here we examined the initiation and physical process based on data analyses. Our results show annual cycle (AC) anomalies of SAT caused this subseasonal alternating of temperature extremes in two regions. The AC anomalies of SAT are regulated by the phase transition of the North-Pacific-Oscillation-like (NPO-like) circumglobal Rossby wave (CRW) train. Unprecedented midlatitude Northwest Atlantic warming sea surface temperature (SST) in early winter initiated a positive phase of the NPO-like CRW train, via eddy-mediated physical processes and the resultant feedback of sea ice loss over the Barents-Kara Seas. While, the subsequent downward feedback of stratospheric processes resulted in the negative phase of the NPO-like CRW pattern in late winter. This work advances the understanding of the subseasonal predictability of SAT extremes from impacts of AC anomalies and intercontinental seesawing.