Monday, April 8, 2013

Jet stream changes take rap for Britain's icy weather Save

Britain's weather is excelling itself.

It produced an Easter Sunday that was the coldest on record in the UK.

Temperatures stuck below zero in many regions, freezing conditions continued to disrupt transport and experts warned of increasing threats to animals and birds already struggling to survive loss of habitat and climate change.

The start of British Summer Time was marked in Braemar, Scotland, by temperatures that fell to minus 11C. For good measure, an unappetising April for the UK looks likely to follow.

The persistence of the northern spring's grim weather is striking, for it comes after a series of other extreme meteorological events in recent years. Last winter, a severe drought triggered stern warnings by the UK's Environment Agency that water rationing and hosepipe bans would soon have to be introduced - until several months of torrential rain produced widespread flooding.

Britain's weather is now fluctuating on a grand scale.

"There is no doubt that the recent weather has been highly changeable - on both sides of the Atlantic," said meteorologist Nicholas Klingaman, of Reading University.

The problem lies with the jet stream, a narrow band of strong winds that sweep around the planet between the tropics and the Arctic.

"Its behaviour has changed dramatically in the past few years and has produced these lengthy bouts of extreme weather.

"The real question, of course, is an obvious one: why has the jet stream changed its behaviour?"

The jet stream gets its name because it circulates at an altitude of around 10km to 15km, the height at which most jets fly. It runs from west to east, which can give aircraft significant boosts on eastbound flights across the Atlantic and Pacific.

However, this giant river of air has begun to meander and to slow down, trapping regions of high or low pressure over the same part of the globe, including the freezing air that has hung over Britain for the past six weeks. As to the reason for this change, more meteorologists point to global warming. In particular, they pinpoint the warming of the Arctic as the most likely culprit for the destabilising weather patterns.

"The Arctic is warming faster than any other place on Earth," said meteorologist Jennifer Francis, of Rutgers University, New Jersey.

"Arctic temperatures have increased at more than twice the global rate. You can see this in the sea ice in summer there. In just the three decades, it has declined by 40 per cent. More to the point, that warming is now changing weather patterns across the Northern Hemisphere."

How the warming of the Arctic affects our weather has much to do with the origins of the jet stream.

Air in the tropics is warmer than the Arctic and it rises. As a result, the atmosphere there is higher than it is over the Arctic. "A gradient is created and air slides down this atmospheric hill towards the Arctic," said Francis.

"This flow of air, high up in the atmosphere, from the tropics to the Arctic, is the crucial ingredient in the creation of the jet stream.

"The world rotates from west to east, however, and that rotation whips up this northward flow of air that descends over higher latitudes and sends it flying east round the globe as the jet stream.

"Earth rotates from west to east and that is what drives the jet stream in the same direction."

Until recently, this air flew around the planet in a slightly wavy path, between 30 and 60 degrees north.

"The trouble is that the gradient between the atmosphere in the lower latitudes and in the Arctic is being disrupted by global warming," said Francis. "It is becoming less of a stream and is behaving more like a sluggish estuary that is meandering across the upper atmosphere at middle latitudes." More