Archive for the ‘Snow’ Category

Why has this winter been so cold in Europe?

January 6, 2011

This post was chosen as an Editor's Selection for ResearchBlogging.orgI’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.

ResearchBlogging.orgV. 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

Why does it snow?

December 22, 2010

The Barometer guys have just published a new episode on snow. It’s another good one. I’m starting to really miss being involved with the show!

In this episode they talk for a bit about how snow forms and it reminded me of this really nice video showing the melting layer in a precipitating cloud (click to play):

In the higher levels of the cloud (where it’s really cold) the precipitation starts out as ice and snow, which falls much slower than rain. But if you look closely at about 2 km you can see the point where the ice and snow melts and becomes rain – this is known as the melting layer. The rains falls much quicker than the snow above.

It snows at the ground when the lower levels of the atmosphere are particularly cold and so the snow doesn’t melt. This is what’s been happening recently.