What’s going on with our winter weather?
- 11 Jan 2012, 16:16
- Verity Payne
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:
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 Niño) and cooler (La Niña)
phases - a pattern called the El Niño 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.
Updated 10:21 12/01/12 - Added image