Why has this winter been so cold in Europe?

I’ve written a couple of posts recently looking at the cold UK weather in context and how snow forms.

What I haven’t done, though, is looked at why it’s been so cold over Europe this winter (as well as last year’s winter).

It just so happens that a paper came out in the Journal of Geophysical Research recently (Petoukhov and Semenov, 2010) that might hold the answer.

They looked at whether changing the sea ice concentration in a particular area of the Arctic (the  Barents-Kara sea) in a long run of a climate model changed the conditions over Europe.

Their reason for doing this was that the very cold winter of 2005/06 was accompanied by very low sea ice in this region – they don’t mention 2009/10 and 2010/11 at all, though, as they would have been writing the paper before these cold European winters occurred.

To isolate the effect of the sea ice in this one region they use a mean state (a “climatology”) for most of the planet in the model but change the amount of ice on the  Barents-Kara sea. The results are quite surprising.

The winter wind patterns over Europe change dramatically when they changed the ice concentration from 80-100% to 40-80%. You can see this in the figure below, which is from the paper but I’ve highlighted the key areas on the wind plot and removed some of the panels.

Mean surface air temperature and 850 hPa wind anomalies for February from the model runs using 2 ice scenarios.

(When they set the sea ice close to 0%, Europe goes into a different state that is similar in temperature to the 80-100% case.)

So why does Europe get cold in a model world where the Barents-Kara sea has 40-80% sea ice concentration? In this model run, the result of this level of sea ice is to set up a big anti-cyclonic (high-pressure) anomaly over the pole. In the northern hemisphere air rotates clockwise around a high so this explains the switch in wind direction that drives the change over Europe. However, the hypothesis they present as to why the sea ice change leads to a high pressure anomaly over the pole is not straightforward and probably deserves a bit more study.

So, in essence, this all seems to be saying that it’s climate change that has led to our very cold winter! I can imagine some people finding that hard to swallow but here’s a quote from the paper that sums it up better than I just have:

Our results imply that several recent severe winters do not conflict the global warming picture but rather supplement it, being in qualitative agreement with the simulated large-scale atmospheric circulation realignment.

Anyway, all interesting stuff and I look forward to seeing some more analysis, especially a better climatology of winter temperatures in Europe and Arctic sea ice to see if that fits in with this hypothesis and a better physical model for the different changes linked with different sea ice concentrations.

UPDATE (10th Jan 2011): I just read someone claiming to have “debunked” this paper by showing that sea ice concentration and European temperature don’t correlate. However, this completely misses the non-linearity of the relationship. I think it’s fine to question the findings of the paper but I suspect that to “debunk”, or verify, the findings using the actual sea ice and temperature measurements you’d have to pick apart the contributions of other factors (e.g. polar jet changes, ENSO teleconnections) and then find some way of characterising the non-linear nature of the relationship with B-K sea ice.

V. Petoukhov, & V. A. Semenov (2010). A link between reduced Barents-Kara sea ice and cold winter extremes over northern continents Journal of Geophysical Research, 115 : 10.1029/2009JD013568

Snow again.

I wrote a post last winter about how the snow doesn’t mean that climate change is over.

Well, it’s snowing again.

And the warming on a global scale still hasn’t stopped:

October 2010 temperature anomalies relative to the period 1951-1980 from the NASA GISS webpage.

Britain’s snow and climate change

NOTE: This post is from January 2010. I put a temperature anomaly plot from October 2010 here and I’ll do one for November 2010 as soon as the data is available.

I’m sure most of the Brits out there have seen this amazing NASA image of Britain covered in snow.  I love satellite images and use them a lot in my research – they really help me get a grasp of the big picture.

But what does this cold weather tell us about climate change?  Well, if we examine the whole northern hemisphere and look at how the temperatures for December compared to those from the last 30 years, then we get an interesting picture:

So, northern Europe and North America were colder than usual.  But southern Europe, Greenland, the Arctic and north Africa were all warmer than usual.  The situation for January will probably be quite similar.  So, looking at the bigger picture, the recent cold conditions in the UK don’t really tell us much about climate change – we need to look on big scales in both time and area.

Proposed industry standards for the terms “TREACHERY” and “CHAOS”

As much as I love reporters using the same terms over and over again, I feel that without any clear definition some of the power of these words may be lost upon the audience.  Therefore, I propose official industrial standards for these two terms.

Treachery

Clearly, in cases where this term is used, the conditions must not be merely and/or obviously dangerous.  The situation should initially appear tranquil – perhaps a sunny day with happy rabbits frolicking on the hard shoulder.  However, over time, the road-based conspiracy to undermine and betray the driver will become clear.  A quiet malevolent laughter will be audible underfoot.

Possible alternatives: dangerous, unsafe, difficult.

Chaos

A long line of stationary cars is clearly not chaotic; this situation is, in fact, relatively ordered.  If the term chaos is used, the following should be expected:

• At least one overturned, burning car;
• Screaming women pulling their hair out;
• Helicopters crashing on the horizon;
• Complete confusion (as opposed to the clear realisation that you should have stayed at home)

[Apologies to anyone that got trapped or had an accident in the snow.]

Snow in Manchester

I’ve not gone to work today.  There’s quite a bit of snow out there.  But why?

Well, the main reason why it’s cold here is because it is winter.  This sounds obvious but it’s worth remembering why it gets cold in winter.  Earth rotates with a tilt so, throughout the year, different parts of the planet get more sunlight.  At the moment, the UK is getting less sunlight so it’s colder.

However, the reason why it is just so cold and snowy right now is a bit more complicated.  If you look at the pressure chart below then you can see that the isobars are almost parallel  from the Arctic all the way to the north of the UK.  This means that very cold air is flowing right to our doorstep.  Brrr!  Watch out further south as the front (the region where the cold air meets slightly warmer air, which produces the precipitation) moves southwards and takes the snow with it.