May 15, 2013
Using Airflow To Predict Winter Weather
Lee Rannals for redOrbit.com — Your Universe Online
A team at the National Oceanography Center uncovered the relationship between winter weather and the strength of the airflow coming in from the Atlantic, saying it is stronger in some years than others. This airflow is one of the factors used by meteorologists to predict weather. The strange behavior is making it more difficult for forecasters to determine what future weather holds.
"There are two major atmospheric pressure systems centered around Iceland and the Azores that are very influential for the weather in Europe," said co-authors Drs. JoÃ«l Hirschi and Bablu Sinha from the National Oceanography Center. "Air flows between these two systems, bringing mild air from the North Atlantic to Europe. The pressure difference between the two pressure systems — and the corresponding airflow — fluctuates and scientists call this phenomenon the 'North Atlantic Oscillation', or NAO, which is by convention positive when the pressure difference is a stronger than average and negative when it is weaker than average."
They said when there is more air coming from the Atlantic, there are milder, wetter winters in Europe. However, when the airflow is weaker and originating from Siberia and the Arctic, the weather is colder and drier. The scientists set out to determine how reliable the relationship between the pressure systems over Iceland and the Azores is and its effects on European weather.
The researchers decided to look at the strength of the NAO during each year between 1880 and 2009. They compared this data with observations of winter weather conditions over the same period. Results showed when airflow is weaker, European weather is harder to predict than when it is stronger.
In order to make things a little more understandable, Hirchi compared the weather to a guitar string.
"When there is tension in the string, it stays straight. If you slacken the tension enough, the string goes wobbly," he said. "So when we have a vigorous airflow driven by a large pressure difference, it tends to stay straight and the weather is easy to define. When weaker, the airflow meanders, leading to more complicated weather patterns. You get big differences in weather over Europe, switching between cold Arctic/Siberian air masses, and milder subtropical air masses."
Hirchi said this is why, in December 2010, the UK suffered from low temperatures and heavy snowfall, while other European regions, like the Balkans, experienced warm and wet conditions.
In February 2012, scientists wrote in the scientific journal Tellus A they found a link between Summer Arctic sea ice cover and winter in central Europe. They said the probability of cold winters with a lot of snow increases when the Arctic is covered by less sea ice in the summer.
While neither study has offered up a confident way to look into the future and predict how next winter's weather is going to look, they both show science is on the verge of tapping into that mystery.