Why is the UK cold? The NAO is stuck.

December 26, 2010

Positive NAO

Negative NAO

Science journalist Fred Pearce has a piece in the Daily Mail explaining part of the UK deep freeze puzzle.

The NAO is a see-saw in weather systems in the North Atlantic. It has two states – positive and negative. They bring very different weather. The positive phase happens when air pressure is low over Iceland, but high down south over the Azores islands off West Africa.

The resulting pressure difference drives strong westerly winds and weather fronts that crash into us, whipping up storms and sometimes causing floods. That’s what happened a decade ago.

But when the pressure difference is low, like now, the weather systems slow down and get blocked by what meteorologists call ‘blocking highs’ – big zones of high pressure that spread down from Greenland in winter or up from the Azores in summer, and stick around for weeks at a time. That’s the negative phase – and that’s what we’ve got now.

A big high centred on Greenland – one of the most intense ever, say meteorologists – has spread south and is blocking westerly winds from crossing the Atlantic. Instead, it is dragging bitterly cold winds out of the Arctic down over Europe. And it seems to have become stuck.


What about global warming? Doesn’t this all make those theories about warming look a bit stupid? Well, no. The NAO is only cooling a small area round northern Europe. Elsewhere in the world, 2010 has seen near-record warm temperatures.

In November, northern Europe was a most unusual 4C colder than normal. But according to Nasa, last month was the warmest-ever November worldwide. Right now, there are record temperatures west of Greenland and into Canada, where they are wondering when winter will start.

We seem to have got the weather they usually have around Hudson Bay in Canada – and they have got ours. Blame that on the NAO.

Some researchers, like Jim Overland, NOAA’s Pacific Marine Environmental Laboratory, one of the world’s experts on Arctic weather and climate, believe that Global Warming may be responsible for the NAO extreme negatives of the last two years, that reduced sea ice in the arctic is changing the air flows that have historically dominated and allowing cold arctic air to flow into North America and Eurasia, while sucking warm air into the polar reaches. (where temps are now 5 to 20 degrees c above normal..)

During the positive phase of the North Atlantic Oscillation (NAO), a large pressure gradient across the North Atlantic creates strong winds that drive winter storms across the Atlantic and into Northern Europe. During the negative phase, there is only a small pressure gradient. Southern Europe and Africa receive weak winter storms while Northern Europe and the eastern United States are cold and dry.Click for larger image from UCAR

3 Responses to “Why is the UK cold? The NAO is stuck.”

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