Melting sea ice plays critical role in winter whiteouts

Arctic sea ice near its smallest extent of 2011 on Sept. 9, 2011 recorded by NASA's Aqua satellite. Low sea ice cover levels are linked to snowy winters, Jiping Liu from the Georgia Institute of Technology and his colleagues have found. Credit: NASA's Scientific Visualization Studio, Goddard Space Flight Center.

The snowy winters that have wreaked havoc in the northern half of the planet over the past five years are linked to dwindling Arctic sea ice. US and Chinese scientists this week said that they have provided evidence supporting that surprising connection between our warming world and the recent unusually cold spells. “There are some uncertainties, but probably we will see more persistent snowstorms and cold conditions in the future,” explained Georgia Institute of Technology‘s Jiping Liu.

Between 1979 and 2010 the area of Arctic sea ice in September, October and November fell by over a quarter. It reached its lowest ever value in 2007 unexpectedly quickly, outpacing the changes climate models predicted the greenhouse effect would cause. In the following winters, large areas of the US, Europe, and China have seen especially heavy snowfall. 2009-10 and 2010-11 saw the second and third largest snow cover since records began respectively for the northern half of the planet. It could have been a coincidence. But despite some other researchers blaming the cold on existing climate patterns, Jiping Liu and his colleagues thought the sea ice and snowfall might be tied together somehow.

To test their idea, the scientists turned first to existing measurements. “We analysed observational data for the past thirty years,” Jiping Liu told Simple Climate. They checked satellite records of how large Arctic ice cover has been from the US National Snow and Ice Data Center. The researchers also got a similar satellite measure of snow cover from the Rutgers University Global Snow Lab, and other data including surface air temperature and pressure from the US National Oceanic and Atmospheric Administration.

Not sea ice, just sea

These maps show the differences in snow cover relative to the long-term 1979-2010 average for the winters of (left) 2009-2010 and (right) 2010-2011. During these two winters, the Northern Hemisphere measured its second and third largest snow cover levels on record. Credit: Georgia Tech/Jiping Liu

By mathematically comparing these records the scientists showed that the changes in snow cover are closely linked to variations in Arctic sea ice variability. Autumn Arctic sea ice decreasing by 1 million square kilometres is linked to winter snow cover being as much as one-tenth higher in many places. Air pressure at sea level is also linked to sea ice cover falling. Though a similar effect is seen in an already-known climate cycle called the Arctic Oscillation (AO), in this case higher pressure reaches southwards in a less regular pattern. This brings air circulation changes quite different to the AO, the scientists write in a paper published in research journal Proceedings of the National Academy of Sciences of the USA on Monday.

“Stronger westerly winds, also known as the jet stream, keep cold Arctic air confined in the polar regions,” Jiping Liu explained. “But reduced Arctic sea ice can result in a weaker and much more variable jet stream. When the jet stream is weak we instead have atmospheric blocking circulations. These let cold, Arctic air mass into lower latitudes bringing cold conditions which favour snowfall.”

The melting Arctic also lets more water into the atmosphere. “This dramatic loss of sea ice leaves more open water in the ocean, which greatly enhances the transport of moisture from the ocean to the atmosphere,” Jiping Liu said. “Also, because the polar region is warmer, its atmosphere can contain more moisture. Together with increased cold air outbreak, it’s much easier for this moisture to become snowfall. So it’s a combination of atmospheric circulation change and moisture increase.”

Snow doubt

The summer minimum area covered by Arctic sea ice fell significantly between 1979 and 2007. Credit: climatesafety/Flickr

To check this picture is what’s happening in reality the scientists repeated the process with a different set of data, producing similar results. They also compared it against simulations of sea ice loss in computer models of the atmosphere from the US National Center for Atmospheric Research. While some  snowfall sites were different in the model than actual measurements, many places did see heavier than normal snowfall.

But Jiping Liu cautiously underlined that more work is needed before they can be sure the effect is directly down to climate change. “We need more observational data,” he said. “So far we only have snow cover. We don’t have snow depths and the frequency of snowstorms.” His team may also be able to benefit from the world’s climate modellers’ current activity enhancing their tools for the Intergovernmental Panel on Climate Change’s fifth assessment report. “We can analyse their model prediction outputs to see if this relationship is robust,” the scientist said.

If the process the scientists describe is found to be right, that could give us tools that will help prepare for bad winter weather. “We think maybe we can develop a big physical model to forecast the winter snow over Europe, the northern US, and east Asia, using sea ice,” Jiping Liu said. “This will provide information for policy makers and governments.”

And as the Arctic continues to melt, the evidence they want should soon emerge. “We know that the sea ice will continue to decrease,” Jiping Liu noted. “This means that probably the sea ice effect will become more distinct in the future. If year after year we still see this robust relationship, this will be due to climate change and not to natural variability.”

• Also this week, NASA has published findings that the thickest Arctic sea ice is disappearing faster than younger, thinner ice. You can read their press release on the research, which contains a video and picture that powerfully demonstrate how rapidly the ice has melted, at the following link.