In February 2012 temperatures in Berlin, Germany, plummeted to –19.6 °C, and the monthly mean temperature was just –1.7 °C. During the same month temperatures in Moscow, Russia, plunged to –28.5 °C and the monthly mean was less than –10 °C. Continuing eastwards, temperatures in Beijing, China, dropped to –7.8 °C and the cold spell persisted, producing a monthly mean of –5.5 °C.

This deep chill was not just a one-off. Similar episodes were recorded across Eurasia in January 2010, December 2009, January 2008 and January 2006. In each case the cold conditions were widespread and extreme, causing serious disruption, damage to infrastructure and loss of life.

"These cold events can influence the entire Eurasian continent, as well as Southeast Asia and Japan, with record temperatures sometimes more than 10 °C colder than the long-term average," explained Xiangdong Zhang, a climate scientist at the University of Alaska Fairbanks, US.

Without a doubt, extreme cold-winter events have become more frequent across Eurasia in recent years, and climate scientists have been rather taken by surprise: frozen winters in Eurasia were not predicted by climate models.

To better understand these Eurasian cold anomalies, Zhang and his colleagues Chuhan Lu and Zhaoyong Guan from Nanjing University of Information Science and Technology, China, analysed winter cyclone and anticyclone activities, and their interplay with the semi-permanent Siberian-high circulation pattern. Studying more than thirty years' worth of data, they found a persistent weakening of both cyclones and anticyclones between the 1990s and early 2000s, and a pronounced intensification of anticyclone activity afterwards.

"The intensified anticyclones bring north or northeast winds over Eurasia, which transport cold polar air to mid-latitude Eurasia," said Zhang. Furthermore, intensified anticyclones keep a tight lid on the atmosphere. "The strengthened descent of air mass prevents cloud formation and keeps skies clear, which enhances radiative cooling," explained Zhang, whose findings are published in Environmental Research Letters.

So why have winter anticyclones been strengthening over Eurasia? Melting sea ice in the Arctic and changing weather patterns are both possible drivers of the new winter weather trends. "Decreased sea-ice cover favours further extension of warm air into the central Arctic Ocean," explained Zhang. "When this warm air propagates to the lower-latitude Eurasian continent, it gets cooled due to radiative heat loss. Anticyclones accordingly form or intensify." Meanwhile, weakened tropospheric mid-latitude westerlies and aligned south–north winds over the North Atlantic increase heat transport from low to high latitudes.

Ironically it seems that Eurasia's extreme cold winters are partly a consequence of global warming, with warmer global temperatures driving sea-ice melt and changing weather patterns. Until now, climate models have failed to capture these extreme cold-winter events, but Zhang and his colleagues hope that their findings will help to improve future predictions. "We need to evaluate whether climate models can realistically capture weather-scale physical processes, such as clear-sky-induced radiative cooling, descent strength and anticyclone generation, which are critically important for extreme cold event formation," said Zhang.