BBC: Huge snowfall caused by rare clash of weather events

BBC: Huge snowfall caused by rare clash of weather events

2 September 2010

Scientists have shown that a severe snowfall in North America and Northern Europe in the winter of 2009-2010 was caused by a rare, once in a century, collision of two weather systems.

They concluded the harsh winter and heavy snow was an example of hard to predict weather events, not a change in climate.

They analysed historical snow records.

The research was published in Geophysical Research Letters.

In the winter of 2009-2010 much of Northern Europe experienced heavy snow and temperatures were at the lowest they had been for nearly 30 years. At the same time, record snowfall hit Washington DC and other parts of America's "Mid-Atlantic states".

Some news reports took the extreme cold weather as evidence against climate change.

By analysing 60 years of snowfall measurements and satellite data, researchers concluded the anomalous weather conditions were caused by an unusual combination of an El Nino event and the rare occurrence of a strongly negative North Atlantic Oscillation (NAO).

El Nino events result from a periodic warming of the tropical Pacific Ocean coupled with changes in the atmosphere. El Ninos move storm systems in the Northern Hemisphere towards the equator. They occur every few years and can be predicted up to a few seasons in advance.

The North Atlantic Oscillation (NAO) is climatic phenomenon resulting from shifts in atmospheric pressure between two regions above the North Atlantic Ocean. Large changes can only be forecast a week or two weeks in advance.

When the NAO enters a strongly negative phase, cold air repeatedly comes down from the Arctic. This affects eastern North America, and Western Europe.

Richard Seager, a meteorologist with the Lamont Doherty Earth Observatory at Columbia University, US, is one of the lead authors of the paper.

"The NAO was probably as negative as it's ever been in the instrumental record, which goes back to the early 1800s. This was a once in a century type of event," he told BBC News.

It was this combined with the El Nino event that caused the severe snowfall in North America.

"The NAO on its own doesn't cause much precipitation in America. It just makes it cold. The El Nino makes the US wetter than normal so combined with the NAO it caused the precipitation in America to fall as snow," he told BBC News.

The researchers also believe it is unlikely this combination will occur in the near future. Data from tree rings have shown that these same conditions caused by the same combination of weather systems happened over 200 years ago in the winter of 1783 - 1784 in Northern Europe and North America.

Many people have concluded the extreme winter in 1783/84 was caused by the eruption of an Icelandic volcano. But the researchers believe it was caused by the same combination of weather events that caused last year's harsh winter.

Richard Seager also believes the research is a counter to suggestions that the cold winter is evidence against climate change.

"Weather will continue to be weather. You have to average over a lot of weather to get the climate trends. There doesn't seem to be any need to evoke anything else other than that," he told BBC News