Characteristics of Northern Hemisphere cold spells with respect to the state of the stratospheric polar vortex

Abstract:

Persistent cold spells during the boreal winter can heavily strain the economy and significantly impact everyday life. Stratospheric dynamic activity may serve as a potential precursor for tropospheric extreme events. Recently, vertically propagating Rossby waves that are reflected (absorbed) by the Stratosphere during sudden stratospheric warmings have been linked to extremely cold temperatures in North America (Eurasia). However, persistent cold spells in both regions may occur during the full range of stratospheric variability. Hence, adding to these recent findings, we investigate persistent cold events in North America and Eurasia with respect to the prevailing stratospheric polar vortex regime prior to their onset, with a particular focus on extremely weak and strong stratospheric winds. By performing (lagged) composite analysis based on ERA5 reanalysis data we find upward (downward) wave activity fluxes to be linked to Eurasian (North American) cold spells, which is consistent with previous studies. In addition, we show that within each region, cold events exhibit significant differences in their respective statistical as well as dynamical behavior depending on whether they follow a strong or a weak stratospheric vortex event. A comprehensive understanding of these surface cold events and the processes involved in their generation is crucial for extended-range predictability that is highly sought after by various industrial sectors and society.