Wet UK summers: melting Arctic sea ice may be partly to blame
Between 2007 and 2012, northern Europe experienced a run of wetter than average summers. In England, the summer of 2012 was the wettest for 100 years. Now a new study says rapidly diminishing Arctic sea ice may be partly responsible.
In the last few years, scientists looking into the consequences of melting Arctic sea ice have suggested a link to colder winters in the UK.
The new study, just published in the journal Environmental Research Letters, is the first to find evidence that changes at the North Pole may be affecting our summer weather too.
Soggy summers
For six consecutive summers, northern Europe experienced unusually wet weather. The amount of rain that fell each year was higher than average, but having such a long run of wet summers is what really surprised scientists.
Dr. James Screen, author of the new study and research fellow at the University of Exeter, tells us:
“Taken together the six summers 2007 to 2012 were around 15 to 25 per cent wetter than average over much of Northern Europe.”
In England and Wales, the summer of 2012 was the wettest since 1912. The summer of 2007 was the second wettest. Screen adds:
“During the wettest summer, 2012, northern Europe experienced 80 per cent more rainfall than normal.”
Arctic influences
The new research suggests this unusual run of wet summers could be linked to the long term decline in Arctic sea ice. Arctic sea ice has been declining by about four per cent per decade, with the seasonal low at the end of summer shrinking particularly quickly.
Image - Met Office _Sea Ice Decline (note)
The decline in monthly area covered by sea ice in the Arctic (blue) and summer minimum in September (brown). Source: Met Office
The research suggests that the amount of sea ice can affect the position of the jet stream – a river of fast-flowing air high up in the atmosphere.
The position of the jet stream is important, because it steers weather systems towards or away from the UK. Scientists know that wet summer months in northern Europe tend to occur when the jet stream sits further south than usual.
An explanation of the Jet Stream and how it affects weather. From the Met Office.
By studying computer models of the atmosphere, scientists have suggested the jet stream responds to a change in the temperature difference between the poles and the equator – the temperature gradient.
As sea ice retreats, sunlight that would have been reflected is absorbed by open water instead. It’s largely for this reason that the Arctic region is warming twice as fast as the rest of the planet, a phenomena known as Arctic amplification.
This means the temperature difference between the frozen north and the more temperate mid-latitudes is getting smaller, especially in summer when sea ice is at its lowest. According to the research, the summer jet stream over Europe is shifting southwards towards the equator in response.
Co-conspirators
The weather we get in the UK and northern Europe is already influenced by a number of factors. So if the loss of Arctic sea ice is increasing the chances of wet summers here in the UK, the effect is likely to be small against the natural swings in rainfall from one year to the next.
On top of normal year-to-year fluctuations, a slow natural cycle of ocean warming and cooling called the Atlantic Multidecadal Oscillation (AMO) can also affect rainfall patterns on decade-long timescales. Screen tells us:
“Since about the late-1990s the North Atlantic has been in the warm phase of that cycle, which is believed to increase the risk of wet summers.”
Back in June, a number of newspapers reported that this ocean circulation could lead to a ” decade of wet summers” – a claim we looked at closely at the time. The headlines were the result of a Met Office meeting where scientists discussed the UK’s run of unusual seasons.
According to the new research, it’s likely that a combination of natural climate fluctuations and the loss of Arctic sea ice were behind the UK’s rainy summers recently. But Screen says it’s not yet clear whether one single factor is playing a bigger role than the others.
Cold winters
This study isn’t the first time the Arctic has been linked to bad weather in the UK and northern Europe. Over the last few years, emerging research has made the link between the loss of Arctic sea ice and recent UK cold winters, although it’s still early days for the research.
The mechanism behind the recent cold winters and wet summers is similar, Screen tells us. Both are a result of the jet stream sitting further south than usual, one reason for which could be low sea ice cover in the Arctic.
But the changes that take place in the atmosphere as a result are different. They could be triggered by ice loss in different parts of the Arctic, Screen explains:
“Cold winters have been linked to sea ice changes in the Barents and Kara Seas – off the coasts of Siberia and north-east Scandinavia – whereas the wet summers appear to be related to sea ice loss off the coast of northern Canada and possibly in the western Pacific.”
Looking ahead
As this is the first study to link Britain’s wet summers to changes in the Arctic, it’s worth being somewhat cautious about its findings. Screen says more work needs to be done to understand the role of Arctic sea ice loss in northern hemisphere weather.
What does this all mean for future summers? Given that sea ice isn’t the only factor influencing the UK weather, it’s hard to say whether we’ll have washout summers for the foreseeable future. The study doesn’t make any specific predictions, but if sea ice continues to decline as predicted, the risk of wet summers could well increase.