Villain of the winter's tale lies in Greenland

IT MIGHT be referred to as "upside-down weather". In Scotland, temperatures are well below the seasonal norm. Yet in Canada it is positively balmy, with Goose Bay in Newfoundland barely getting below zero, whereas the average minimum for January is -23C.

• Picture: TSPL

The answer to the question of why Britain has been wrapped in snow is to be found far out into the Atlantic. In the middle of December, the water temperature dropped and an unusual layer of cold water spread over the ocean, cooling the air above and developing into a vast area of high pressure centred over Greenland.

If there is a villain in this wintry tale it is this giant mass, which is acting like an ugly bouncer, blocking the normal mild weather from the west and ushering into Britain ice queens from the north and east.

As Met Office forecaster Robin Thwaytes explained, the block is saying to our ordinary winter weather: "You are not coming this way, thank you very much" and instead our weather is being forced down towards Spain and the Mediterranean, where it has been unseasonably wet.

This cold water in the Atlantic is being blamed for the wave of cold currently enveloping North America, Europe and Asia. It is the butterfly effect. Yet it is unclear why the North Atlantic seas are so cold, but the cold water and the block of high pressure it has triggered have created a phenomenon known as the North Atlantic Oscillation. This is where high pressure in Greenland is balanced by low pressure towards the sub-tropical Azores islands. Like a seesaw rocking backwards and forwards, these two vast pressure systems can swing from one extreme to another.

North-east Asia is enduring the worst winter weather for 60 years, with more than ten inches of snow covering Seoul, the South Korean capital, while in China, Beijing and the nearby port city of Tianjin had the deepest snow since 1951. In China's far north, the temperatures fell to -32C.

The block has meant that the wind is travelling in a different direction. A band of cold is now blowing east to west from Britain, across the Atlantic to the American north-east, where in Burlington, Vermont, a record 33in of snow fell at the weekend.

Yet while we shiver, many parts of the northern hemisphere are considerably warmer than usual. Alaska and much of northern Canada is unseasonably warm for instance, with temperatures 5C to 10C warmer than expected, although it is still -30C. North Africa and the Mediterranean basin are also warmer than average, by up to 10C, while across northern Europe, temperatures are coming in 5C or so colder than average.

Yet can the gnarled human hand of global warming be detected in the current weather? Isn't this just the type of weather forecast by the apocalyptic Hollywood drama The Day After Tomorrow, in which the gulf stream, the conveyor belt that brings warm water around Britain, shudders to a halt? No so, according to the Met Office, which insisted there was no evidence of any change to the gulf stream and that a high pressure block of such magnitude and duration can appear every 30 years or so. "High pressure blocks act like heavy rocks in a stream, in the way that water has to flow around them," explained a spokesman.

Professor James Curran, who works for the Scottish Environment Protection Agency (Sepa), has gone into greater detail about the cause of the big freeze, which he calls the Omega Block, because its shape resembles the last letter in the Greek alphabet. He said: "Everyone has heard of the jet stream and it is the jet stream, high up, about where planes fly, that drives all of our weather systems. Normally it drives storms in from the Atlantic – the mild conditions and heavy rain to us which is the type of winter we normally experience in Scotland.

"Around about the second week in December, that jet stream developed a kink in the shape of the Greek letter omega – a bend – and if that kink in the jet stream is big enough, roughly the size of the Atlantic Ocean, then it locks into place and becomes very stable and can sit there for a very long time.

"At the moment there is one of these kinks in the jet stream over North America and there is one over the North Atlantic. This stops the mild winters coming in from the Atlantic. If they do come they are driven south, and at the moment they are getting driven down through Spain and through the Mediterranean."

The question of how long the block will remain in place and our weather be dictated by the Arctic is unclear. Prof Curran says the current scenario is similar to the winter of 1963, when the cold spell began on Christmas Day and lasted for a further nine weeks.

Mr Thwaytes of the Met Office said: "At the moment we can see ten to 14 days ahead, and there is no sign of it moving. If the block moves at all it will be from Greenland to Scandinavia, but that is only going to transfer the wind from north to east, and there will be no impact on temperature. It will just move around a bit. It's going to be staying cold for the next two weeks at least."

What began with cold water in the north could yet be prolonged by warm water in the south – another factor that could lead to a prolonged winter is El Nio, the growing turmoil in the tropical oceans that shunts warm water towards South America, where the sea, in some places, is 3C above normal, the warmest for 12 years.

The turbulence at the other end of the world affects the jet stream winds that carry our weather.

"We can show a robust link between European climate and El Nio in late winter, with cold conditions over Europe and mild conditions in southern Europe," said Dr Adam Sciafe, of the Met Office seasonal forecasting group.

Just a touch of frost in future due to warming

WINTER snow is deeply rooted in popular culture, particularly at Christmas, but it is likely to become increasingly rare.

During the period 1550-1850, Britain was gripped by a little Ice Age leading to persistent and severe winter weather. In 1813-4, such was the cold in far south-east of Britain, that the Thames froze over, leading to London's last recorded Frost Fair.

According to Professor James Curran of the Scottish Environment Protection Agency, Scottish winters are bound to become less and less severe as a consequence of global warming.

He said: "Climate change predictions are that you will get fewer cold winters in the future.

"The steely winter we maybe get once every five years – not as cold as this, but a typical winter that we would get once every five years – will fade and by the end of the century we will experience that once every 20 years.

"Already the average winter temperature in Scotland has increased by 1.5C in the past 40 years.

"Our winters are much, much milder than they used to be particularly over the past decade and will continue to become more mild but that does not mean to say that on increasingly rare occasions we won't get severe cold spells like this.

"The climate predictions are that we will generally get warmer but in that the weather will become more variable and we will get more extremes so we will certainly get much more heatwaves and lots more floods but we will still, occasionally, get intense cold spells like this but they are not going to disappear."