Whats going on with our winter weather?
As the media reports today, this winter has so far been particularly mild in the UK, and wild flowers are blooming unusually early.
Met Office figures suggest that December 2011 was around half a degree warmer than the 1971 – 2000 average, in stark contrast to the previous two winters – December 2010, for example, saw average UK temperature around 5 °C below average.
However, as John Prior, National Climate Manager at the Met Office points out, this year’s mild temperatures are not particularly unusual:
“While it may have felt mild for many so far this December, temperatures overall have been close to what we would expect. It may be that the stark change from last year, which was the coldest December on record for the UK, has led many to think it has been unseasonably warm.”
It’s not just the UK which is having a mild winter – much of the rest of the Northern Hemisphere is experiencing higher than normal temperatures, with the US reportedly having an unusually warm and dry winter which is causing difficulties for its ski industry.
On the other hand, in the Arctic things are cooling off, and the National Snow and Ice Data Center (NSIDC) have just released data showing that air temperatures over the much of the Arctic ocean and Greenland have been lower than average throughout December.
Anyone who follows the seasonal patterns of weather-related news coverage might ask whether why the last two winters were so cold, and whether it signifies much that we’re having a mild winter this year.
Cycling the climate
Let’s start with the two very cold winters of 2009-10 and 2010-11. Not everywhere in the Northern Hemisphere was cold – while the UK, Northern Europe and the Eastern US were experiencing very cold weather, northeast Canada and Greenland were unusually mild.
Weather patterns are altered by the position of the jet stream – the current of air high in the atmosphere that steers weather systems in the Northern Hemisphere. What the jet stream is doing is determined by how big a pressure difference there is between the high pressure air at mid-latitudes (off the coast of Spain) and the low pressure air over Iceland.
When pressure differences are smaller than normal – a condition scientists call the negative phase of the North Atlantic Oscillation (NAO) – this causes the jet stream to weaken and shift south to the Mediterranean. This cuts off our supply of mild, Atlantic air and allows more cold air from continental Europe and Russia to reach us.
Both winters 2009-10 and 2010-11 saw strong negative NAO patterns, leading to cold UK, Eastern US and Northern European winters, as shown in the image below:
Image - Global average T anomaly 09/10 (note)
In contrast, so far this winter we have had a positive NAO, bringing mild weather conditions to the UK and to the Eastern US.
Temperatures and snowfall in the US are also influenced by how warm the Pacific Ocean is in the Eastern tropics. The sea temperature switches between warmer (El Ni̱o) and cooler (La Ni̱a) phases Рa pattern called the El Ni̱o Southern Oscillation (ENSO). ENSO is also thought to affect European climate, partly due to it altering the North Atlantic Oscillation, but how is not yet well understood.
Outlook for winters in a warming world
There’s nothing unusual about the past few winters, and the year-to-year variation in temperatures that we’ve recently seen in the UK is clearly down to natural atmospheric variation. But as Professor Richard Seager, Lamont-Doherty Earth Observatory of Columbia University, puts it:
“When we get this variability it’s against a background that is gradually, decade to decade, getting warmer and warmer.”
So what effect will global warming have on the large-scale climate cycles like the NAO and ENSO that cause this variability? Could a warmer atmosphere change the timing or duration of these cycles, and reform our weather patterns as a consequence?
The short answer is that we don’t yet know. A climate modelling study found that El Niño events might become more frequent as global warming progresses, but it’s not clear if and how the NAO might change in a warmer atmosphere. And there are other complicating factors, such as evidence that low phases of the sun’s 11-year activity cycle might alter the NAO, and bring colder winters to the UK.
A group of US scientists have run climate model simulations of a moderate 21st century warming scenario and noted the ‘extreme cold’ winter events that occur. Their results indicate that cold spells might become less frequent over the 21st century, but when they do occur they will be as cold and long lasting as those we currently experience, even towards the end of the century.
One thing is clear – it’s not time to throw out the cold weather gear yet.
Much of this information came from our article ” If the world is warming, why were the past two winters so cold?” in the Guardian Ultimate Climate Change FAQ series, produced in collaboration with the Guardian and checked over by the Met Office.