The big freeze in the US was part of an Arctic weather system that is being displaced more and more frequently because of global warming, European researchers believe. As it re-centres itself over the pole, it should bring colder, icier conditions back to Europe.
‘This is very likely,’ said Dr Chantal Claud, a climate scientist who works for the EU-funded NACLIM project, which is looking at the relationship between sea ice and weather.
‘We will come back (to normal winter temperatures),’ said Dr Claud. ‘It doesn’t mean that it’s going to be extremely cold.’
The US saw freezing temperatures that grounded thousands of flights and forced businesses and schools to close in early January 2014, while Europe experienced an unusually rainy Christmas period characterised by mild temperatures and high winds.
The mild temperatures are likely to be because the cold polar weather shifted over the US and away from Europe, due to an unusual southward movement in the stratospheric vortex. The vortex is a huge swirl of air circulating eastward around the pole. It forms in the higher atmosphere during the polar winter when, in the absence of sunlight, the air cools to very a low temperature.
Now the vortex is likely to return to its normal configuration, which is centred over the pole and slightly displaced towards Europe.
The unusual positioning of the vortex is an increasingly common phenomenon that scientists say is likely a hallmark of global warming.
‘It is very likely that this is related (to climate change) but it is always difficult to relate one extreme event to global change,’ said Dr Claud.
‘In the last decade, the number of occurrences of such situations has increased dramatically in the stratosphere, and we don’t know why.’
Dr Chantal Claud, a climate scientist who works for the EU-funded NACLIM project
Since researchers started observing the stratospheric vortex using satellites just over 30 years ago, they have seen a marked increase in the number of times it has been perturbed. There were five winters where there was an unusual displacement of the vortex between 2000 and 2010. That is more than in the two previous decades combined.
A displacement of the stratospheric vortex helped cause the extreme cold experienced in Europe in February and March 2013.
‘In the last decade, the number of occurrences of such situations has increased dramatically in the stratosphere, and we don’t know why,’ said Dr Claud.
While scientists don’t yet fully understand the complex relationship between the sea ice, the stratospheric vortex, and the high-speed jet stream air current, they know they are all closely interconnected.
‘We have a partial knowledge, we know that if sea ice is melting, for example, we have energy which is stored by the ocean … and then released into the atmosphere,’ said Dr Claud, adding that more research was required.
The NACLIM project has already discovered that the Arctic sea ice cover in October can be used to predict the average temperatures over the winter in eastern North America and Europe, due to the energy passing from the ocean into the atmosphere.
‘If we have a strong sea ice loss over the eastern Arctic … we can expect colder-than-average conditions over northern Europe and warmer-than-average over southern Europe,’ said Dr Javier García-Serrano, based at the Pierre and Marie Curie University in Paris, who works on the project.
If the displacement of the stratospheric vortex – and the extreme cold weather it can cause – are made more likely by a decline in the Arctic sea ice, as researchers suspect, then the situation could get worse.
‘The extent is reduced but also the depth, we now have thinner sea ice so it’s more vulnerable, it’s easier to melt,’ said Dr Claud. ‘It’s much easier now to melt the ice than it used to be a few years ago, and I’m not sure that people realise this.’
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