Posts Tagged ‘atmospheric circulation’

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

WUWT alarmism?

December 6, 2010

Watts Up With That? recently published a post about an improvement to a method developed by Steig et al. (2009). This paper aimed to identify temperature trends over the data sparse Antarctic. The improved method has been accepted for publication in Journal of Climate, which is a decent achievement.

Firstly, I think its great that this exchange of ideas is happening in the peer-reviewed literature and not only on blogs.

I say this because, as Watts demonstrates, blogs can be used to insinuate things that are not the case.

For example, there is a quote from one of the paper authors in the post:

“I would hope that our paper is not seen as a repudiation of Steig’s results, but rather as an improvement.”

Yet Watts decided to title his post “Skeptic paper on Antarctica accepted – rebuts Steig et al”. Whilst I realise the difference between “rebut” and “repudiate”, it strikes me as poor form.

There also seems to be a tone of indignation in Watts’ part of the post about how long it took to get the paper through peer review and that one of the “difficult” reviewers had probably been involved with the initial paper:

“Anyone want to bet that reviewer was a “[hockey] team” member?”

I don’t understand why Watts is surprised about this: if you contribute something novel to the literature then the peer review process assesses that work against itself; if, on the other hand, you criticise and amend other people’s work then it would be irresponsible of the journal editor not to send the paper to one of the people being questioned.

Anyway, so what is the difference between the two analyses? Here are the plots that are provided before the paper is published properly:

The striking differences in the update are the increased positive trend on the peninsula and a “new” negative trend from the South Pole to the eastern Weddell Sea. The positive trend over most of Western Antarctica has also largely gone.

I expect Real Climate will post a response once the full paper is published so I don’t want to try to pick the methods apart here.

However, it struck me as a little odd that Watts was almost celebrating the re-affirmation of a massive warming on the Antarctic Peninsula!

Sure, the atmospheric dynamics of this region are very complicated and it’s not clear exactly what the distribution of temperature changes mean. But this “victory” seemed to focus more on getting one over the “hockey team” (ugh) rather than achieving something potentially useful.

ResearchBlogging.orgRyan O’Donnell, Nicholas Lewis, Steve McIntyre, Jeff Condon (2011). Improved methods for PCA-based reconstructions: case study using the Steig et al. (2009) Antarctic temperature reconstruction Journal of Climate, in press

The Icelandic volcano and weather

April 15, 2010

If you’re in the UK then I’m sure you’ve heard about the Icelandic volcano.  Its caused a shutdown of airspace in the UK as well as in Scandanavia and Holland because volcanic ash doesn’t mix well with aircraft engines.  The satellite image below shows the plume as of 10am on 15th April:

I just saw an interesting volcanologist on BBC News 24 explaining why this volcano is producing so much ash.  Apparently it’s because this volcano is beneath a glacier and this has led to the explosive eruptions that have sent the ash high into the atmosphere (up to the stratosphere).  An eruption from another Icelandic volcano earlier this month didn’t produce any problems as it wasn’t under a glacier and resulted in lava flows.  [Update: here's a blog post about subglacial eruptions by a proper geologist.]

The interesting volcanologist also said that these eruptions can last anything from hours to years!

So it could be down to the weather to sort this one out.  The Met Office have issued a plot showing the location of the plume at 6am on 15th April.

If we take a look at the weather charts, we’re in a region of high pressure at the moment and this is drawing the ash south eastwards at the moment:

There’s not a lot of change tomorrow either so I doubt things will be much different then:

The high pressure moves eastwards a bit on Saturday so that may clear things away but I wouldn’t bank on it:

So it could be a while before the atmospheric circulation clears this ash away to make it safe for aviation.

On the plus side, this large injection of particles (aerosols) into the atmosphere could result in some really colouful sunsets – here’s an example – or even a blue moon.  These particle clouds can catch the light from the setting Sun at different heights to normal sunsets and this can be really beautiful!

Storms and climate change

March 25, 2010

I’ve been pretty distracted recently with the Institute of Physics issue. I’ll hopefully draw that chapter to a close in the next couple of weeks (it looks like the IoP are going to stick their head in the sand and wait for it to blow over) but right now I’m bringing my current project to a close so I thought I’d look at how that work has gone and where it’ll go in the future.

The work I’ve been doing at Manchester over the last few years is looking at how storms form – the kind of storms that lead to floods like the one seen in Boscastle in 2004.

These type of storms are pretty difficult to forecast because they are often smaller than the computer model grid box size that such models use to chunk up the atmosphere into a more manageable mathematical problem. I spoke about this in a recent edition of the NERC Planet Earth podcast.

I’ve been part of a couple of big projects that try and observe these storms as they form. We do this by getting big teams of scientists, around 50 or so, to go to a certain location for a few months and then use surface stations, aeroplanes, weather balloons, radar, lidar and satellite data to get a really good picture of what’s going on.

The video below shows the Chilbolton radar scanning convective clouds as they develop whilst someone launches weather balloons in the bottom left hand corner – this is pretty typical of what goes on at several locations during these campaigns, although this is the biggest dish we’ve used.

I’ve only been looking at a small part of the storm problem: how does air that descends from the stratosphere influence these storms?

I’ve shown that this upper-level air can sometimes make storms less likely by introducing thin layers into the atmosphere that can cap storms before they get going (Russell et al., 2008) and sometimes make storms more likely by changing the temperature structure of the atmosphere to make the convection more powerful (Russell et al., 2009).

That doesn’t sound much like progress, does it? Well, now that we know a lot more about these two types of effect, we can start to generalise them and use that knowledge to help forecasters improve their predictions and computer models. This is what we are working on now.

So how does climate change fit in to all this?

The next big step in this work is to try and define a “storm environment”, including what we now know about these upper-level features, and apply this to the kind of projections that climate models produce. These projections use even bigger grid boxes than weather models so this step will not be easy but if we can show that this method works using the data that is available now (and hope for better resolution soon) then we can start to think about the likely changes in this storm environment. These big storms can really affect people’s lives so this type of work is something that would influence how we start to prepare for the future.

References:

ResearchBlogging.orgRussell, A., Vaughan, G., Norton, E., Morcrette, C., Browning, K., & Blyth, A. (2008). Convective inhibition beneath an upper-level PV anomaly Quarterly Journal of the Royal Meteorological Society, 134 (631), 371-383 DOI: 10.1002/qj.214

ResearchBlogging.orgRUSSELL, A., VAUGHAN, G., NORTON, E., RICKETTS, H., MORCRETTE, C., HEWISON, T., BROWNING, K., & BLYTH, A. (2009). Convection forced by a descending dry layer and low-level moist convergence Tellus A, 61 (2), 250-263 DOI: 10.1111/j.1600-0870.2008.00382.x

Antarctic climate change – the exception that proves the rule?

March 1, 2010

Antarctica has been in the news recently because two large icebergs (one about 60 miles long and the other about 50) have broken off the continent. These “calving” events often occur naturally and these ones are probably not linked to climate change, although they might affect the global ocean circulation.

But I thought that this would be a good opportunity to have a look at the general climate situation in the South Pole region…

The clearest signal is rapid warming that has been seen on the Antarctic Peninsula (the bit that points up to South America) over the last 50 years.

The picture for the rest of the continent is not so clear, mainly because of the lack of data. For comparison, the USA has over 1000 climatological observing stations, some of which go back to the late 1800s; Antarctica currently has around 55 stations, very few of which go back to before 1957, (plus a similar number of automatic weather stations, which tend to not be maintained for long periods) and these data are used to represent a much bigger land area.

Antarctica compared to the USA[Image from NASA]

Nonetheless, there have been some high profile studies looking at Antarctic temperature trends, some finding cooling, some finding warming and nearly all being controversial.

So why is the warming on the Peninsula so clear?

The reason is that the warming is mostly driven by atmospheric circulation changes and not the increase in the greenhouse gas concentrations (although global climate change patterns forced by GHGs can include atmospheric circulation changes).

Ozone "hole"The key factor is that the ozone hole above the South Pole has changed the wind patterns – when ozone is removed from the stratosphere, less solar UV radiation is absorbed so the polar stratosphere cools. This increases the temperature change as you move away from the pole and, in turn, has changed the westerly (clockwise) winds that circle the pole – they are now further south and faster.

This wind pattern spreads down through the atmosphere towards the planet’s surface and has, therefore, brought more warm air from over the Southern Ocean to the Peninsula. This circulation change has less effect on the Antarctic interior and possibly even isolates it from the rest of the Earth system.

This climate change pattern is really interesting to study and we can even use ice core data from the Antarctic to look at how these winds have changed in the past – I’ve recently reviewed the literature on this subject (Russell and McGregor 2010).

Korhonen et al. (2010) have even found another mechanism of how these wind changes have affected the climate. As the wind speed over the ocean increases, it throws up more spray and this means that more clouds can form over the Southern Ocean and Antarctica (I’ll write a post later about how clouds form). If there are more, bright clouds around then these reflect away more incoming sunlight, which will cool the region beneath these clouds.

So, to bring all this together, if the Antarctic continent has been cooling (which isn’t clear) then this could be because the normal rules don’t apply to Antarctica. Does this mean that we can say that Antarctic climate change is the exception that proves the rule of GHG forced climate change?

Probably not, but it does highlight just how complicated the climate system is and how much more there is find out about it!

References:

ResearchBlogging.orgKorhonen, H., Carslaw, K., Forster, P., Mikkonen, S., Gordon, N., & Kokkola, H. (2010). Aerosol climate feedback due to decadal increases in Southern Hemisphere wind speeds Geophysical Research Letters, 37 (2) DOI: 10.1029/2009GL041320

ResearchBlogging.orgRussell, A., & McGregor, G. (2009). Southern hemisphere atmospheric circulation: impacts on Antarctic climate and reconstructions from Antarctic ice core data Climatic Change DOI: 10.1007/s10584-009-9673-4


Follow

Get every new post delivered to your Inbox.

Join 260 other followers