Altered pressure patterns bring Eurasia intense iciness

People enjoying the winter sun – typical of an anticyclone, or high pressure, weather system – on the frozen Landwehrkanal in Berlin-Kreuzberg, during February 2012, when Berlin set a record for extreme cold. Credit: onnola via Flickr

Extreme cold that has left Europe and Asia snowbound, shivering and asking, “What global warming?” in recent years has been driven by intensified high pressure patterns. That’s according to Xiangdong Zhang at the University of Alaska, Fairbanks, who has been studying how such cold snaps fit in with increasing average temperatures worldwide. “Extreme cold weather events can occur in a particular region and short time period in a warming global climate,” Xiangdong pointed out. “This may highly disrupt daily life, damage infrastructure, and impact ecosystems and environment.”

Xiangdong started thinking about extreme cold events because climate studies usually use monthly temperature averages, which overlook them. “This cannot reflect extreme cold temperatures occurring on a particular day because daily temperature changes are filtered out by the average,” he told me. “For example, the monthly averaged temperature in February 2012 was -4.9°C in Berlin. But the coldest daily temperature in the same month at the same location was -19.6°C. We don’t directly feel the monthly average temperature in our daily life. What we feel is day-by-day changes in temperature. But if we can understand mechanisms of daily temperature changes, we would be able to better understand why there is colder or warmer monthly average temperature.”

Outside of tropical areas weather patterns known as cyclones, which would be called low pressure on a weather forecast, and anticyclones, or high pressure, drive those daily temperature changes. Xiangdong had previously been part of a team that adapted an automated cyclone spotting method to look at each one separately. Last year, with researchers from Nanjing University of Information Science and Technology in China, he used that method to study records from across Europe and Asia between 1978-2012. They brought together sea level pressure data recorded every six hours by a global collection network, and daily minimum air temperatures recorded at 1337 meteorological stations.

Siberian hello

University of Alaska, Fairbanks' Xiangdong Zhang and Nanjing University of Information Science and Technology's Chuhan Lu talk about their studies into what's contributed to extreme cold events in Eurasia. Credit: IOP Publishing, via Creative Commons license, see citation below.

“First we found each cyclone or anticyclone by using sea level pressure data and calculated its intensity,” Xiangdong explained. “Then we summed up the intensities for each month. After this, we found out the daily minimum surface air temperature measured at the meteorological stations. We finally examined the daily minimum temperatures when cyclone/anticyclone activities became stronger or weaker to identify the links between them.”

In a paper published in the scientific journal Environmental Research Letters late in 2012 that analysis revealed significant long-term changes. “Cyclone activities have weakened during the last decades over the Eurasian continent,” Xiangdong said. “Anticyclone activities also weakened in the 1980s and 1990s but have substantially intensified since the late 1990s.”

The researchers found that more intense anticyclones drove a quadruple-whammy of cooling effects. The clear skies they brought meant that, without a blanket of clouds, the planet’s surface lost more heat directly into space. Stronger anticyclones have also slowed down winds that bring warmer, wetter air from west to east. They have increased cold air transport southward from Arctic regions, and finally reshaped a semi-permanent area of cold, dry air known as the Siberian High. “The intensified anticyclone activities caused a strengthening and north-westward expansion of the winter Siberian High,” Xiangdong underlined.

Right now, it’s not clear why these changes have happened, or how important each factor is in causing extreme cold events, Xiangdong conceded, but he is investigating this. “We are still working on finding out which mechanisms caused the intensity changes of the cyclone and anticyclone,” he said. “I would like to hypothesize that interplay between the global-warming effect and atmospheric internal variability has changed the atmospheric circulation.”

Wintertime cyclone intensity (CI; dark green line), anticyclone intensity (ACI; red line) and Siberian High Intensity (SBH; gray column) from 1978/79 to 2011/12. Each measurement has been ‘normalised’ to fit on the same scale, and therefore the indices have no units. Cyclone intensity has tended to decline over recent decades. Anticyclone intensity tended to decline until the 1990s, and has intensified since. That has driven an intensification in the Siberian High. Credit: IOP Publishing via Creative Commons License, see citation below

Journal reference:

Zhang, X., Lu, C., & Guan, Z. (2012). Weakened cyclones, intensified anticyclones and recent extreme cold winter weather events in Eurasia Environmental Research Letters, 7 (4) DOI: 10.1088/1748-9326/7/4/044044