Arctic and Pacific Ocean Conditions Were Favourable for Cold Extremes over Eurasia andNorth America during Winter 2020/2

A sequence of extreme cold events occurred across mid-high-latitude Eurasia and western North America during winter 2020/2021. Here, we explore the causes and associated mechanisms for the extremely cold temperatures using both ERA5 observations and large-ensemble atmospheric model simulations. Experiments were conducted with observed ocean surface boundary conditions prescribed globally, and regionally to discern the specific influence of Arctic, tropical Pacific and North Pacific drivers. Increased likelihood of daily cold extremes in mid December to mid January are found in Eurasian midlatitudes in response to reduced Arctic sea ice. Tropical sea surface temperature anomalies, more specifically the La Nia pattern, increased probability of extreme cold over high-latitude Eurasia in early January to early Febraury. Both reduced Arctic sea ice and La Nia increased the probability of daily cold extremes over western North America in late January to late Febraury. We conclude that a combination of reduced Arctic sea ice, La Nia, and a sudden stratospheric warming in January 2021 were factors in the February 2021 extreme cold-wave that caused huge societal disruptions in Texas and the Southern Great Plains. Although the magnitude of the simulated cold extremes are relatively small when compared with observed anomalies, the Arctic and Pacific Ocean surface conditions in winter 2020/21 increased the probability of cold days as cold or colder than observed by approximately 17%~43%. Extreme cold events occurred across Eurasia and North America during winter 2020/21 were made more likely by a combination of reduced Arctic sea ice, La Nia, and a sudden stratospheric warming.