Wild card named La Nina to blame for bad weather

PUBLISHED : Thursday, 31 January, 2008, 12:00am
UPDATED : Thursday, 31 January, 2008, 12:00am

Mainland meteorologists have suggested that the cold and snowy weather sweeping over the nation this month was caused by a slight drop in surface sea temperatures in the eastern Pacific Ocean in August, a phenomenon known as La Nina.

For more than 20 days, the central and eastern part of China has been hit by the heaviest and longest snowfall since the 1960s.

Though much of the calamity has not been explained, one thing is certain: the Chinese Meteorology Administration failed to send out a warning despite being aware of a discrepancy for months.

Ren Fumin, deputy director of the administration's climate diagnosis and forecast department, said on Monday it had forecast some snow but had vastly underestimated the scope and duration.

'Signs of some abnormal weather activities first emerged last summer when the global extreme weather monitoring network found the average sea-surface temperatures in the eastern Pacific suddenly declined by 0.6 degrees Celsius from the normal level. This persisted for quite a few months,' Mr Ren said.

'The range and duration of the cooling was more severe than we had expected, but few people took a serious look at the problem because La Nina was commonly regarded as benign rather than destructive.

'It triggered record high rainfalls in some regions in autumn, and we held some meetings to discuss its future impact, but the La Nina phenomenon still failed to raise most people's interest.'

A lasting and powerful La Ni?a presence disrupted some important circulatory cells that were working as gears to maintain the regular, seasonal balance in the atmosphere.

The consequences have been disastrous. Extremely cold winds from the Arctic and extremely wet fronts from the south met little resistance and collided over the most heavily populated parts of the mainland. The extreme weather persists because many circulatory cells, significantly weakened by La Ni?a, have no power to bring back the former equilibrium.

On the mainland, scientists had focused almost exclusively on El Nino - a rise of sea-surface temperatures at the equator - which was thought to have caused massive flooding in 1998. As a result, understanding of La Ni?a is still limited.

Wei Fengying, a professor at the Chinese Academy of Meteorological Sciences, said: 'You can forgive the meteorology administration, because the terrible weather went against almost every past experience and has toppled some doctrines of weather forecasting.

'It is not just Chinese scientists who made a mistake; the entire international meteorological community was taken by surprise.'

She said extreme weather prediction was complex and difficult because there were many factors to consider.

'We were also caught off guard because most of us are getting used to the idea of global warming and a snow-free, mild winter.'

The phenomenon

Many experts are blaming the inclement weather on La Nina, the cyclical cooling of Pacific Ocean waters. Extremely cold winds from the North Pole and extremely wet fronts from the south met little resistance and collided over the most populated areas on the mainland

Deep freeze

This image, captured by Nasa's Earth Observatory on Tuesday, shows how extensive the unusual snow cover is. The image encompasses most of eastern China, apart from the northernmost provinces and the cloud-covered south. From north to south, nearly the entire country is covered in snow - marked in red-orange, while clouds are white and peach

How it happens

1A Air pressure falls below normal

1B Air pressure rises above normal

2 Trade winds blow west

3 Warm sea water flows west

4 Clouds form above cooler Western Pacific