Wet UK summers linked to Arctic sea ice retreat
29 October 2013 by Alex Peel
The UK's remarkable recent run of wet summers may have been partly down to increased melting of Arctic sea ice.
A new study, published in Environmental Research Letters, says sea-ice loss in the Arctic may be tugging wet weather systems further south during summer months, bringing them into a collision course with the UK and Northern Europe.
"There's probably no one single reason why we've had these wet summers in recent years," says Dr James Screen - a NERC research fellow at the University of Exeter - who carried out the research.
"But our computer models show that the loss of Arctic sea ice is one potential reason why these weather systems have been sitting south of their usual position."
The six summers from 2007 to 2012 saw unusually high rainfall across Britain and Northern Europe.
In England and Wales, it was the longest run of wet summers since at least 1900 when records began. The washed-out summers of 2007 and 2012 were the two wettest since 1912, and brought widespread flooding across the UK.
According to weather experts, it's all down to the jet stream - a narrow, meandering torrent of fast-flowing air in the upper atmosphere.
In winter, the jet stream tends to sit right across the UK, bringing wave after wave of wind and rain. As spring turns to summer, it normally shifts northwards, leaving more settled conditions in its wake.
But for the six wet summers, the jet stream remained stubbornly south of its normal track, sustaining a soggy assault on Britain's barbeques.
I don't think we'll ever get to a position where we can link a specific event like the 2012 floods to climate change
- Dr James Screen, University of Exter
Scientists have offered a number of explanations, with most blaming warmer sea surface temperatures in the North Atlantic.
The extra ocean heat is thought to disturb weather patterns above, effectively steering the jet stream and its wet weather systems directly into the UK.
But wet summers also tend to occur when Arctic sea ice is low. Screen wanted to test whether this is mere coincidence, or whether increasing summer sea-ice melt is also influencing our weather.
He used computer simulations to compare the position of the jet stream when sea ice is high to its track when sea ice is low, keeping ocean temperatures the same throughout.
From 1997 to 2009, summers in Northern Europe have become 11 per cent wetter. According to the study, the retreat of Arctic sea ice could be responsible for around a third of that overall trend.
Screen suggests it may be to do with disturbances to local weather patterns as far away as the North Pacific and Hudson Bay.
"When the sea ice melts, it's effectively like lifting a lid on the heat stored up in the ocean," he explains. "This can have an influence on local weather patterns."
"The jet stream is like a meandering wave going right around the mid-latitudes - when you disturb one part of it, it can have knock-on effects thousands of miles away."
But Screen is keen to point out that sea-ice loss is probably not the only factor affecting the position of the jet stream.
"For the run of wet summers that we've had, the loss of sea ice and warm ocean temperatures were both acting in the same direction - pulling the jet stream south."
"We expect that sea surface temperatures will return to a cooler phase of their natural cycle - reducing the risk of wetter summers - but Arctic sea ice is expected to continue to melt and it is unclear how these competing effects will influence weather over the next decade or so."
The red areas indicate the position of the jet stream in a wet year (left), and a dry year (right).
2013's melt of Arctic sea ice was not as dramatic as in recent years. According to the Met Office, we've also had the hottest, driest and sunniest summer since 2006.
Screen says this is likely to be down to the normal year on year variations in the climate system, but that it's difficult to interpret one individual year.
His research will now focus on how declining sea ice might affect summer weather into the future. He also wants to look at whether changes in the Arctic have been a factor in the cold winters of recent years.
But he remains cautious about linking specific weather events to broad changes in the climate.
"I don't think we'll ever get to a position where we can link a specific event like the 2012 floods to climate change. But we might be able to say that these changes are tipping the balance in favour of particular weather regimes, and this research is a step in that direction."
'Influence of Arctic sea ice on European summer precipitation,' JA Screen, Environmental Research Letters, 2013.