JAPAN: Heavier snowfalls in Japan due to big chill blowing from the Arctic

2010/03/17

BY KOSUKE SO, THE ASAHI SHIMBUN

Bitterly cold air blowing south from the Arctic Circle triggered highly unusual weather fluctuations that brought heavier snowfalls to areas of Japan this winter.

A committee of experts under the Japan Meteorological Agency said the highly unpredictable weather is an anomaly that may occur "only once in 30 years."

A key factor was a dramatic change in what meteorologists call the Arctic oscillation. When the oscillation is positive, atmospheric pressure in the Arctic Circle is lower than in regions to the south and warm air flows north into the Arctic.

This year, however, the oscillation turned strongly negative, meaning pressure in the Arctic Circle was markedly higher than in mid-latitude regions and blasts of Arctic cold flowed south to Japan and other parts of the Northern Hemisphere.

The difference in the atmospheric pressure between the Arctic Circle and mid-latitude regions this year was the biggest since records began in 1979.

Japan experienced wide fluctuations in temperatures as a result. Although average temperatures for the country as a whole were higher than in an average year, the central Tokyo area had snowfalls on 10 days this winter for the first time in 26 years.

The northern Tohoku region and the Sea of Japan coast of eastern Japan recorded more than 1 meter of snow in December, for the first time since 2005.

Tokamachi in Niigata Prefecture had 3 meters of snow in January for the first time in four years.

Extreme weather conditions were seen across the Northern Hemisphere. Unusually low temperatures claimed numerous lives in Europe.

"We seldom make a conclusive statement, but we can say we are having abnormal weather this year," said Masahide Kimoto, the University of Tokyo professor of meteorology who heads the meteorological agency's experts committee.

"Finding out about the mechanism of the Arctic oscillation will lead to more accurate weather forecasts for the winter," Kimoto told a news conference on March 3.

"We hope to improve the precision of weather forecasts by analyzing in detail this winter's weather phenomena."

The meteorological agency says two negative Arctic oscillations have occurred this winter--one between early December and early January and one from late January to the present. The effect is currently weakening.

Three phenomena contributed to the unusual pattern. First, dramatic deviations of the prevailing westerly winds over Alaska in early December and over the North Atlantic immediately afterward caused high atmospheric pressure to develop near Alaska and Greenland.

This helped induce the negative Arctic oscillation.

Second, there was "sudden warming" in the stratosphere, which raised the temperature over eastern Siberia by more than 30 degrees in December and over the North Pole by a similar amount in January.

Finally, the El Nino climate pattern in the Pacific Ocean may have strengthened the Arctic oscillation.

But it also had a contrary effect of warming Japan, contributing to the higher average temperatures recorded by the Japan Meteorological Agency.

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