Uncharted Territory

December 27, 2010

Call this a Cold Winter? Maybe…

Filed under: AMO, Global warming, Science, UK climate trends — Tim Joslin @ 6:29 pm

If you want publicity for a scientific paper, global warming is definitely the topic to go for. Especially if you manage to feed our collective snow madness at the same time!  The Independent’s baby brother newspaper the 20p “i” even used the recent findings of Petoukhov and Semenov as the basis for its Christmas Eve front-page lead.  Basically, as far as I can glean without actually seeing their paper – it’s shameful that we’re expected to make policy on the basis of data that’s not open access – P&S have done a bit of very specific computer modelling showing that less sea-ice in the Russian Arctic can change weather patterns in such a way as to bring cold weather to Western Europe.  Pretty much what I’ve been wittering on about for quite some time, as have others, with more specific academic credentials, such as a Dr Overland.

Essentially, the lack of ice allows heat to escape, lowering the air pressure over the relevant part of the Arctic and therefore strengthening the continental highs, those over Greenland and Scandinavia being most relevant to the phenomenon of interest, namely those cold European winters as manifested in the UK in particular.  Strangely, the Independent writes that:

“Their [P&S’s] models found that, as the ice cap over the ocean disappeared, this allowed the heat of the relatively warm seawater to escape into the much colder atmosphere above, creating an area of high pressure surrounded by clockwise-moving winds that sweep down from the polar region over Europe and the British Isles.” [my stress]

which is a bit confused to say the least, and doesn’t appear to have come from P&S themselves, at least judging by their press release.  The heat would create low pressure in the first instance.

A more reflective (obscure pun intended) source is a piece by George Monbiot who explained the effects on atmospheric pressure thus:

“Sea ice in the Arctic has two main effects on the weather. Because it’s white, it bounces back heat from the sun, preventing it from entering the sea. It also creates a barrier between the water and the atmosphere, reducing the amount of heat that escapes from the sea into the air. In the autumns of 2009 and 2010 the coverage of Arctic sea ice was much lower than the long-term average: the second smallest, last month, of any recorded November. The open sea, being darker, absorbed more heat from the sun in the warmer, light months. As it remained clear for longer than usual it also bled more heat into the Arctic atmosphere. This caused higher air pressures, reducing the gradient between the Iceland low and the Azores high.” [my stress again]

Maybe the Indy cribbed from George.  As every schoolboy knows, its always a giveaway when you copy your classmate’s errors.

What was George’s source?  Well it may have been Realclimate, where Rasmus wrote:

“One interesting question is how the Barents-Kara sea-ice affects the winter temperatures over the northern continents. By removing the sea-ice, the atmosphere above feels a stronger heating from the ocean, resulting in anomalous warm conditions over the Barent-Kara seas. The local warming gives rise to altered temperature profiles (temperature gradients) along the vertical and horizontal dimensions.

Changes in the temperature profiles, in turn, affect the circulation, triggering a development of a local blocking structure when the sea-ice extent is reduced from 80% to 40%. But Petoukhov and Semenov also found that it brings a different response when the sea-ice is reduced from 100% to 80% or from 40% to1%, and hence a non-linear response. The most intriguing side to this study was the changing character of the atmospheric response to the sea-ice reduction: from a local cyclonic to anti-cyclonic, and back to cyclonic pattern again. These cyclonic and anti-cyclonic patterns bear some resemblance to the positive and negative NAO phases.”

which doesn’t actually say that high pressure is caused by warmer air.  What Rasmus means by “local cyclonic” and “anti-cyclonic” patterns is anyone’s guess – I venture that he may not have been referring specifically to the air pressure over the Barents and Kara Seas.  Rather, he seems to be referring to the well-known positive (“cyclonic”) and negative (“anticyclonic”) NAO “patterns”. I can see a trip ahead to the British Library to access P&S’s original paper…

All I actually want to establish in this post – it’s Chrimbo after all, not a time to do anything resembling work – is that there is indeed a phenomenon to explain.

I’m prompted by a comment Rasmus made in his piece:

“I admit, last winter felt quite cold, but still it wasn’t so cold when put into longer historical perspective. This is because I remember the most recent winters more vividly than those of my childhood – which would be considered to be really frosty by today’s standards. But such recollections can be very subjective, and more objective measurements show that the winters in Europe have in general become warmer in the long run…”

I’m tempted to start with my own contrary anecdotal evidence, but let’s consider the data first.

The Beeb were reporting on all media (lead on News 24 and radio bulletins) on Christmas morning that this December is set to be the coldest since records began in 1890, sorry 1910 (from mid-morning – the online article presumably reflects this correction).  Totally confused and can’t be trusted.  In fact, earlier in the week they had me wondering what happened in 1910 – there’s a big difference between “since 1910” and “since records began in 1910”.

Why there’s a Year Zero in 1910 is beyond me.  I’ll let you know when I find out.  Presumably someone has decided that records are unreliable before that point, despite the tens of thousands of hours of effort that have gone into constructing the Central England Temperature (CET) record which goes back to 1659.  I can believe that the monthly averages are off by 0.1 or 0.2C, but they’re going to be good enough for the purposes of comparison. Regular readers will be aware that I have imported the CET data into Excel.

The facts are as follows:

Mean December temperature

1. No “records began” in 1890 either.  December that year is the coldest in the entire CET at -0.8C.  There are only 5 other Decembers with mean temperatures below zero: 1676 at -0.5C; 1788, 1796 and 1878 at -0.3C  and 1874 with a pathetic -0.2C.

2. Only one December since 1890 has averaged below 1C – 1981 at 0.3C.

3. The CET for December 2010 up to and including 26th is -1.0C!  OK there are 6 days to go when the weather is expected to be a little milder.  Each of these could knock 0.1 or so off the monthly average.  Even so, it’s odds on that December 2010 will be only the 7th in the entire CET since 1659 averaging a temperature below 0C.

4. It might be worth pointing out that the first cold snap began in November, so the 30 or 31 days up to Boxing Day may be even more exceptional – although this may have happened in previous years as well.

5. December isn’t usually the coldest month.  In fact the last month averaging below 0C in the CET was nearly a quarter of a century ago (though it seems like yesterday, sigh!) – January 1986 at -1.1C.  Before that, not surprisingly was January 1979, the Winter of Discontent at -0.4.   Before that, we have to look to January and February 1963 at -2.1C and -0.7C respectively.  Postwar that only leaves February 1956 at -0.2C and February 1947 at -1.9C.

6. 2010 as a whole will average no more than 8.9C in the CET, so will be the coldest since 1986 at 8.74C (though there’s no chance of it being the coldest since 1963 as suggested at Real Science).

Record daily minima

Another way of assessing a spell of severe weather is by the number of exceptional days, in this case exceptionally cold days.  Ideally we’d ask how many days this year have been in (say) the 10 coldest on record, but I only have data as to the very coldest days, courtesy of The Wrong Kind of Snow, by Antony Woodward and Robert Penn (“W&P”).  This limitation introduces a little more randomness into the exercise than I’d ideally like.  You could have an exceptionally cold day corresponding just by chance to another one on the same date in the past.  In fact, this has happened several times this year:

– 2nd December 2010 was -20.9C at Altnaharra, but failed to beat the -21.1C at Kelso during the Great Frost of 1879.

– similarly the -20.4C recorded at Braemar on 3rd December 2010 is trounced by the -26.7C at Kelso in 1879.

– and there’s a bit of a pattern here as the same thing also happened on 6th and 7th December, when the cold didn’t quite match 1879.

– later in the month, the exceptionally cold Christmas and Boxing Days this year didn’t quite match those in 1878 and 1981 respectively.

The records in W&P go back well over a century, so on average you’d expect no more than 3 over December to be broken per decade.  Let’s make it tough for ourselves and set a benchmark of 5 over November and December per decade.

Now that I’ve built up the suspense how many record low daily minima have occurred so far this winter?

The following list isn’t necessarily complete, I could have missed some (I’ve jotted them in the margins of my copy of :

– 28th November: -18.0C in Llysdinam (Powys).

– 1st December: -21.1C at Altnaharra.

– 8th December: -18.3C at Tyndrum (finally beating one of those 1879 records).

– 19th December: -19.6C at Shawbury (removing one of those 1981 records).

– 20th December: -18.7C at Pershore.

– 21st December: -17.8C at Katesbridge.

– 22nd December: -20.2C at Altnaharra.

– 23rd December: -18.6C at Castlederg.

– and 24th December: -17.4C, also at Castlederg.

I make that 9 daily records.  On this basis, not just Decembers, but early winters (November and December) in the 2010s are after just one year notably cold!

There are a few comparable cold years, of course.  1919 has the coldest days from 13th-16th November (4 daily records), including -23.3 at Braemar on 14th.

1879 has lost 8th December to 2010, but still holds the records for the six days 2nd-7th December inclusive.  It must have been more intensely cold back then than in 2010 as trees were reportedly killed.  This happens somewhere below -20C when the sap can freeze and the tree splits with a loud crack.  W&P’s entry for 4th December reports the same phenomenon in 1855.  That hasn’t happened this year.  Yet.

More recently, 1981 has lost 19th to 2010 but still holds the daily records for 7 days: 11th-14th, 17th-18th and 26th December.  And the four 1995 daily records for 27th-30th December, including -27.2C at Altnaharra on 30th, don’t look under threat this time round.

So putting our global warming expectations to one side for a minute, on the basis of daily minima extremes, 2010 is up there with the 4 or 5 other most notable early winter cold snaps in the last century and a third.

Anecdotal Evidence

I mentioned earlier the very few postwar months averaging below 0C.  These occurred in 5 winters, three of which – 1947, 1962-3 and 1978-9 – feature in Frozen in Time by Ian McCaskill and Paul Hudson (“M&H”).  Of these, I only remember 1978-9.  And that is mostly for a single snow event between Christmas and New Year.  The dry powder snow was more severe than anything this year, but I’d say the 2010 winter weather has already been more sustained in Southern England, at least.

The exceptionally cold January 1986 isn’t covered in M&H.  I suppose it wasn’t photogenic and provides little to write about because there was very little snow.  It was a thoroughly miserable month.  I remember day after day of an unceasing easterly wind bringing grey, bitterly cold, but dry weather.  I was working onsite in a poorly heated office.  If my memory isn’t playing tricks, we eventually used a thermometer to back up our complaints about the conditions.   I also remember the cold 1985 well as the Year of Crossing Frozen Car-parks.  These winters do seem to occur in runs (though I haven’t yet been able to demonstrate any persuasive statistics).

1956, with its cold February, is occasionally mentioned as a severe winter, but largely forgotten.

Will the winter of 2010(-11) be one of those that is remembered decades hence?   Much depends on the social significance.  The industrial action during 1978-9 has become the stuff of legend.  It hardly deserves its place in the Big Three on meteorological grounds alone.  The impact of 1947 was exacerbated by the continuance of wartime rationing – 1940 was also severe, but not reported to the same degree apparently because of government restrictions enacted for reasons of morale and propaganda.   1962-3 was simply exceptionally severe and prolonged.


Can we draw any conclusions?  Whilst I certainly can’t remember a December as persistently cold, and the records suggest there hasn’t been one since the 19th century – I haven’t even discussed the 10 or so days of lying snow we’ve had in Southern England this December, compared to an average here of only one or two – objectivity and a look at the history books is called for.  Taking my various measures in the round, in terms of the early winter, I’d judge 2010 to be around a once in 30 years event, perhaps 50 if we’re feeling generous, and only 100 if we weigh duration a lot more than severity.  But, and it’s a big “but”, given global warming (and the absence of recent cooling volcanic events), and the perhaps unwise predictions of less frequent cold winters that have frequently been made, there is indeed a phenomenon requiring scientific explanation.

My feeling, though, is that we haven’t yet seen enough for 2010-11 to be ranked amongst the overall Great Winters.  The worst Januaries and Februaries are significantly colder than the worst Decembers.  And although there’s been disruption, it’s not really been unprecedented.  Both the snow events and the nights have been severe, but have not in themselves exceeded others in living memory.  There’s been nothing, for example, that you’d really describe as a blizzard and we’ve been a little way off recording the very coldest nights.  The most notable feature has been how long the cold and snow has gone on for, as evidenced by the number of record daily minima and the low mean temperature for December.

We can’t yet expect people to say in one breath, “1947, 1962-3 and 2010-11”.  But the show goes on – we’ll just have to see what happens over the next couple of months!


December 6, 2010

Hey, Who Moved Greenland?

Filed under: AMO, Global warming, Science, Uncategorized — Tim Joslin @ 4:53 pm

The temperature in London is forecast to be below freezing all day today, which is fairly unusual, especially as the last week-long cold snap only ended a couple of days ago. It’s not quite there yet, but we’re well on our way to a memorable winter, possibly even a Great Winter. Regular readers will be aware of my interest in the question as to whether winters like those of yore could occur today. I’m not expecting a Frost Fair (because we’re keeping the Thames warm) but am puzzled why – since 1947 and 1962-3 – we don’t nowadays see conditions similar to those which caused such events.

Anyway, right now it’s a northerly airstream that’s causing the bracing conditions. You can see it in this chart for midnight last Saturday/Sunday night from the Met Office:

I really like this particular map because it illustrates the effect of one of the great meteorological forces on the planet. Greenland is a massive chunk of ice, the only respectable ice-sheet in the northern hemisphere and the coldest thing this side of Siberia. Just nearby the North Atlantic (or Norwegian) current flows past the fjords. The current generates the largest temperature anomaly on the planet, that is, it makes the sea much warmer than anywhere else at that latitude. The contrast between cold Greenland ice and the warm sea has created the pressure difference that’s funnelling Arctic air down over Scotland and eventually here to London. Of course, the cold air over Scandinavia plays a part too.

But why is this year different?

Perhaps this map from the National Snow and Ice Data Center (NSIDC) gives a clue:

The salient feature is the ice on the east coast of Greenland, which in some places extends further out to sea than the median for the period 1979-2000, despite there generally being less ice because of global warming. Contrast this with the situation to the west of Greenland, in the Labrador sea, Hudson Bay and Canadian Archipelago, as well as that in the Bering Sea.

In meteorological terms, sea ice surely behaves more like land than open water. It’s cold, there’s little evaporation, so high pressure will tend to build. It’s as if Greenland has been moved east compared to years when there is less ice to the east and/or more to the west. Hence this week’s particular weather pattern.

Of course, all this is a result of the Atlantic Multi-decadal Oscillation (or AMO), about which I have previously written.

Historical records show an oscillation between periods when there is more ice to the west and more to the east of Greenland. But what could be driving this? Simple physics, that’s what. When the North Atlantic is warmer than usual this will cause a higher pressure difference between Greenland (which is always just as cold) and the ocean. The Greenland high will be more intense than usual, pumping air (and sea ice) down its east coast and warmer air up the west coast, as we’ve seen for the last couple of years. Hence more chance of cold weather in the UK. Who needs those supercomputers, eh?

The reason for the oscillation, I suggest, is that this weather pattern cools the North Atlantic. Of course, with global warming, this cooling phase may take longer and/or cause stronger winds round Greenland, affecting those of us in its vicinity.

As an addendum I show the Met Office forecast for the same time (midnight yesterday morning, Sunday 5th December) from 3 days before:

Note how much less tight the isobars are between Greenland and Norway than in the actual chart (above). Could it be that the Met Office is underestimating the forces creating the pressure difference? The current forecast is for temperatures to climb slightly to a toasty 2 or 3 degrees above freezing towards the end of this week. But if that north wind is a bit stronger than expected maybe it’ll pump down more Arctic air and we’ll get more of a freeze. Watch this space.

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