Northern Europe and northeast Asia have experienced brutal snowstorms and freezing temperatures, and cities as far south as Seville in Spain and Miami in Florida have been plunged into the deepest chill in decades.

The reason is a natural phenomenon known as the Arctic Oscillation, said Barry Gromett of Britain's Met Office.

"It's a relatively abnormal pattern but it's not unprecedented at all, it's something that happens every 10 years or so," he said in an interview with AFP.

Also called the Northern Hemisphere Annular Mode or the North Atlantic Oscillation, the anomaly consists of a massive system of high-pressure air that sits over Greenland.

"It's like a great big boulder in the stream. It cuts off Europe's supply of mild, moist Atlantic air. Instead, we get Arctic winds that feed in clockwise, which means we get the cold stuff off Scandinavia and the Arctic regions."

Frigid air streams are also deflected around the "boulder" into southerly North America, he said. And another consequence is to strengthen the grip of the so-called Siberian high-pressure system, intensifying wintery weather in northern China and other parts of northeast Asia.

Just as some parts of the world are experiencing extreme lows in temperature, others are having unusual highs, as warmer winds are directed to unusual places, said Gromett.

Parts of Canada and Alaska have been experiencing temperatures that are five to 10 degrees Celsius (nine to 18 degrees Fahrenheit) above the norm, while parts of North Africa and the Mediterranean basin have also been significantly warm.

"In fact, in the first week of January, Crete recorded a temperature of more than 30 C (86 F)," he said.

Michel Daloz with Meteo France, the French national weathercaster, said the northern hemisphere's cold spell in 2010 was relatively balmy compared with previous episodes.

"The natural variability of the climate means that there are troughs of cold from time to time," he said.

In 1956, 1963 and 1985, "there were temperatures of between -25 and -15 C (-13 F to 5 F) across France," Daloz said.

Nor did it undermine data showing a relentless warming, he said.

"In fact, in early December, our main focus was on the clement weather," he said.

According to the Met Office, 2009 is provisionally the fifth warmest on record, and 2010 could be the warmest ever, driven by the man-made greenhouse effect and by a return to El Nino, a natural warming phenomenon caused by a buildup of warm water in the western Pacific.

El Nino reappeared in June 2009 and is likely to persist through the first quarter of 2010, according to the UN's World Meteorological Organisation (WMO).

In Geneva, WMO expert Omar Baddour said the present Arctic Oscillation was probably the severest in 30 to 50 years, but predicted it would soon end.

"Generally it lasts a few weeks or a month, a month and a half. It started in December, so we are nearing the end of the episode," Baddour said.

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