The weakening of the jetstream that holds this in place has allowed cold air to spill further south into much of the United States and Canada, while bringing above-average temperatures to parts of Europe.
The knock-on effects of the vortex follow one of the mildest Decembers in a century in Nordic countries. Ketil Isaksen, a scientist at the Norwegian Meteorological Institute, said the country had been 4.2C above the mean temperature for December with parts of Oslo and south-eastern Norway experiencing the third warmest December on record. "It was very unusual to see no snow in large areas where it is normal in December. Only in the mountains and certain parts of Norway could you find snow."
Much of the precipitation in lowland and populated areas had fallen as rain instead of snow, he said. "In general it was a very wet December. Large parts of Norway had up to three times as much rain as normal and the country as a whole had 180% more than average."
Finland too has seen heavy rain, with flooding in western coastal areas and the majority of Finland's lakes containing record volumes of water. Temperatures exceeded their normal seasonal average by 4-5C nationwide, with Helsinki and southern Finland recording the mildest second half of December in 30 years.
Temperatures in parts of Sweden have fluctuated greatly, at Nikkaluokta falling from 4.7C on 3 December to -40.8C on 9 December, then rising two days later to 7.7C. Many locations measured their warmest December temperatures on record. "In the north, winter has arrived, but in the south it's autumn according to the meteorological definition," the Swedish Meteorological and Hydrological Institute said.
