What connects this weekend’s glorious summer weather with the record heatwave in Death Valley California, the devastating floods this spring in Germany, the miserably cold March in Britain, unusually warm temperatures in Alaska this winter and many other examples of extreme weather around the world?
The answer according to some scientists is a high-altitude ribbon of fast-moving air in the northern hemisphere called the jet stream which appears to have changed from travelling in a relatively straight direction from west to east to a path that meanders widely between north and south.
A growing body of evidence suggests that something has happened to the jet stream, a river of wind which circumnavigates the globe at an altitude of between 5 and 7 miles and at speeds of up to 200 mph. Over the past few years scientists have noticed that it increasingly becomes “locked” in one position, sometimes for weeks at a time, bringing extreme heat or cold as well as droughts or floods.
This spring was a prime example. Instead of following its usual path to the north of Scotland, the jet stream shifted south for several weeks, bringing the coldest March in 100 years to Britain, as well as devastating rainfall and floods to central Europe and record high temperatures to Finland and western Russia.
Over the past week or so it has shifted north again, flipping to its more usual position over the top of Scotland, allowing a mass of warm air and sunshine to move over most of the British Isles from the Azores further south – bringing joy to Wimbledon’s centre court.
“The key question is what is causing the jet stream to shift in this way?” asked Professor Stephen Belcher, head of the Met Office Hadley Centre last month when he helped to organise a workshop of 26 experts to discuss the recent run of unusual seasons in Europe.
One possible answer lies in what is happening in the Arctic, which has seen temperature increases some two or three times higher than elsewhere in the world, and leading to dramatic and unprecedented loss of sea ice and melting of the massive Greenland ice sheet.
Scientists know that the jet stream is driven by temperature differences between the Arctic and latitudes further south. The smaller this temperature difference, the weaker the jet stream, and the weaker the jet stream the more likely it is to meander as travels around the northern hemisphere, said Jennifer Francis of Rutgers University in New Jersey.
“Certainly it all fits together. We are seeing big fluctuations in the path of the jet stream and where it gets into a meandering the north-south waves tend to stand still in one place, bringing extreme weather because whatever weather you are getting tends to hang around for a long time,” Dr Francis said.
“We see the west to east winds of the jet stream that are weaker and these winds are driven by the temperature differences between higher latitudes to the north and lower latitudes to the south. We know these temperature differences are becoming less marked,” she said.
“The evidence is piling up and at some time the circumstantial evidence will pile up enough to prove the case. The problem is that we don’t yet have enough long-term data from the real world to verify the connection, but the climate models seem to be telling the same story,” she added.
It is not just over Europe where the jet stream is playing up. It has meandered widely this summer over North America, producing a sharp trough of low pressure over central United States and equally sharp ridges of high pressure over the western and eastern states.
The jet stream usually acts as a barrier between the cold mass of air to the north and warm air to the south, which means that the meandering path of the stream is bring colder than usual temperatures to places such as Waco in Texas which saw the coldest July temperature on record, and higher temperatures to places such as Death Valley, which recorded a head-splitting 54C.
What seems to be a feature of a meandering jet stream is that the wider it wanders, the more likely it is to become locked in one position for days or weeks on end. Whatever extremes it brings, they are likely to remain in place for longer than usual.
Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research has produced mathematical evidence to support the idea that the jet stream is becoming locked in global “planetary waves” where the high-altitude wind meanders widely from its usual west-east path and becomes locked for long periods in one position.
He and his colleague Vladimir Petoukhov have shown that Arctic warming could be behind many climate extremes seen over the past few years, including the Moscow heatwave of 2010 and the Indus river flood in Pakistan of the same year, as well as the heatwave in the US in 2011.
“These so-called planetary waves are well known and a normal part of atmospheric flow. However, during several recent weather events these planetary waves almost froze in the tracks for weeks,” Dr Rahmstorf said.
“So instead of bringing cool air after having brought warm air before, the heat just stays, and stays and stays. Two or three days of 30C are no problem, but 20 or more days lead to extreme heat stress,” he said.
Meanwhile, the outlook in Britain is for the current sunny weather to last at least for another week – and possibly longer.
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