| Summary: | A sequence of extreme cold events occurred across Eurasia and North America during winter2020/2021. Here, we explore the causes and associated mechanisms for the extremely coldtemperatures using both observations and large-ensemble simulations. Experiments wereconducted with observed ocean surface boundary conditions prescribed globally, and regionallyto discern the specific influence of Arctic, tropical Pacific and North Pacific drivers. Increasedlikelihood of daily cold extremes in mid December to mid January are found in Eurasianmidlatitudes in response to reduced Arctic sea ice. Tropical sea surface temperature anomalies,more specifically the La Niña pattern, increased probability of extreme cold over high-latitudeEurasia in early January to early Febraury. Both reduced Arctic sea ice and La Niña increased theprobability 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 Niña, and a sudden stratosphericwarming in January 2021 were factors in the February 2021 extreme cold-wave that caused hugesocietal disruptions in Texas and the Southern Great Plains. Although the magnitude of thesimulated cold extremes are relatively small when compared with observed anomalies, the Arcticand Pacific Ocean surface conditions in winter 2020/21 increased the probability of cold days ascold or colder than observed by approximately 17%~43%.
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