Arctic warming taking the edge off extreme winters
15 June 2014, by Alex Peel
Cold winter temperatures have become less extreme through large swathes of Europe and North America as global warming has taken hold, according to new research.
The study, published in Nature Climate Change, says this trend is likely to continue. It means that punishing winters, like the deep freeze which struck the US in early 2014, could be less frequent in years to come.
There have been fears that rapid warming in the Arctic, where temperatures have risen faster than anywhere else on Earth, could trigger a shift in winter weather patterns bringing polar air further south.
But this new research, carried out by NERC fellow Dr James Screen at the University of Exeter, suggests those fears may be misplaced.
He looked at detailed climate records for the last few decades and found that autumn and winter temperatures across temperate and polar regions of the Northern Hemisphere have fluctuated less as the Arctic has warmed. This trend is mainly down to a substantial drop in the number of severely cold days.
In January 2014, the mercury tumbled to record lows across a number of US states. Temperatures dropped to -37°C in Minnesota and at one point even tropical Florida fell below freezing. In all, 190 million people are thought to have been affected.
Meteorologists blamed the extreme cold on a weakening of the jet stream - the narrow torrent of fast-flowing air which meanders throughout the temperate zone of the Northern Hemisphere, casting a dominant influence over our weather.
This, they said, had allowed air from the Arctic to sink further south, establishing a vast system of icy conditions over North America.
The jet stream winds (arrows) follow the boundary between cold polar air (blue) and warm tropical air (red).
The jet stream is driven by the difference in temperature between the Arctic and the tropics. The faster pace of warming in the Arctic over recent decades has seen that gap begin to shrink.
This trend is set to continue for decades to come, and many feared that this could bring more extreme winters in the future. But Screen's research suggests the warming of the Arctic will instead take the edge off cold northerly winds.
'Autumn and winter days are becoming warmer on average, and less variable from day to day,' he says. 'Both factors reduce the chance of extremely cold days.'
'Cold days tend to occur when the wind is blowing from the north, bringing Arctic air south into the mid-latitudes. Because the Arctic air is warming so rapidly these cold days are now less cold than they were in the past,' he adds.
Using a mathematical computer simulation of the climate, Screen attempts to predict how this trend will continue into the future. His analysis suggests that temperatures will continue to become less variable in all seasons except summer.
James Screen, 'Arctic amplification decreases temperature variance in northern mid-to-high-latitudes', Nature Climate Change, 2014.
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