Sunday, 28 September 2014

Jennifer Francis on the Jet Stream

Crazy weather traced to Arctic's impact on jet stream

Sticky weather: the normal gentle curve of the jet stream is forced into becoming steeper and slower, causing hot or cold weather to "stick" for longer periods than normal

26 September, 2014

The rapid retreat of Arctic sea ice caused by climate change may be to blame for more frequent prolonged spells of extreme weather in Europe, Asia and North America, such as heat waves, freezing temperatures or storms.
These are relatively short-term periods of bizarre weather, like the cold snap that paralysed North America earlier this year, rather than longer-term rises in temperature.
They are related to "stuck" weather patterns, Jennifer Francis of Rutgers University in New Brunswick, New Jersey, told a conference on Arctic sea ice reduction in London on 23 September. "Is it global warming? I think it's safe to answer yes," she told the meeting.
Francis said a growing number of studies, including her own, suggest that the melting Arctic is having knock-on effects on the jet stream, the river of air that snakes around the northern hemisphere at an altitude of around 5 to 6 kilometres, and which has a profound impact on the world's weather.
The jet stream is driven by the flow of air between the cold Arctic pole and warmer air that moves upwards from nearer the equator. As the warmer air advances polewards, it is swung eastwards by the Coriolis force which comes from Earth's spin, creating a snake-like stream. "It's a fast-moving river of air, a very messy creature," says Francis.
The strength of the jet stream depends on the temperature gradient between the regions of cold and warm air – the wider the difference, the faster and stronger the jet stream.
Twice as fast
The Arctic is warming twice as fast as the rest of the planet, an effect enhanced when the sea ice that normally cools the Arctic air melts away. Because of this, the air currents that come from that region are getting disproportionately warmer too, narrowing the temperature difference between the Arctic and southerly winds, and thereby weakening the jet stream itself. "The winds have weakened by 10 per cent over the past three decades in the west-to-east wind of the jet stream," says Francis.

Francis thinks that, as the cool air of the Arctic becomes warmer, the jet stream is slowing down, almost to the point of stopping trapping weather systems in one place for prolonged periods. Instead of swirling round the world, winds reverberate back and forth in the same place, creating what she calls "extreme waves".
Her research shows that these extreme waves are becoming much more common, helping to explain the increase in prolonged extreme weather events. Between 1995 and 2013 – the period when the Arctic began warming disproportionately fast – extreme waves over North America became 49 per cent more common during autumn and 41 per cent more common in the winter than they were between 1979 and 1994, before the disproportionate Arctic warming.
Between 1980 and 2010, extreme weather events doubled from about 400 to 800 a year, according to the insurance firm Munich RE.
Variable history
But some researchers say the link between the ice retreat and the weakened jet stream requires more evidence. "The direct connection between the 'waviness' and the weather extremes is fine," says Gavin Schmidt of the NASA Goddard Institute for Space Studies in New York. "The discussion is whether you can establish a causal relationship between the sea ice changes and these patterns." Schmidt says that over decadal timescales or longer, the jet stream is very variable, so the correlations drawn with ice cover might yet be down to chance, he says.
Last week, on 17 September, the Arctic sea ice reached its lowest annual extent for 2014, at 5.02 million square kilometres, the sixth-lowest extent ever recorded ,The US National Snow and Ice Data Center at the University of Colorado in Boulder says that the annual sea ice extent has been declining by 4.52 per cent per decade, or 50,000 square kilometres of ice per year.
Jennifer Francis - Understanding the Jetstream

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