Hemispheric divergent trends in winter temperature drop days

As global temperatures rise, widespread warming and fewer cold extremes are generally observed. However, using the metric “temperature drop days”, we identify hemispheric divergent trends in winter cold weather over the past four decades. North America experiences increasing temperature drop days, whereas Europe sees a decline, forming a “Cold North America–Warm Europe” pattern. These opposing trends are linked to atmospheric circulation changes: a weakened but asymmetric tropospheric polar vortex has favored more stable circulation over Europe, while a southward-tilting, meandering polar jet enhances daily circulation variability over North America, increasing temperature drop days. Further analysis indicates that this asymmetry primarily arises from tropical Pacific cooling, with contributions from Arctic sea-ice loss, anthropogenic aerosols, and volcanic forcing. In contrast, greenhouse gas forcing mainly drives the overall weakening of the tropospheric polar vortex. Together, this interplay highlights how internal variability and external forcings shape regional winter cold weather. Occurrence of winter temperature drop days increased in North America but declined in Europe over the past four decades due to changes in tropospheric polar vortex, revealed by attribution analyses based on climate models.