Winter 2010-11 part 1 -
Heaviest Mach snow since 1982:
why is it so cold?
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The
late November cold spell persisted through into December, took a quick
break and then came back mid-month with a vengeance in terms of a
24-hour plus snowfall that gave the valley its worst snow since the
Great Blizzard of January 1982. Most winters, of course, this diary has
featured scenes of feet-deep drifts, but they are above the 300m
contour among the hills that surround the town. Deep snow in
Machynlleth itself is relatively uncommon. I'll do this post in two
parts - firstly a description of events and secondly a look at the
climatological context in the light of some interesting new research
that has been published during the past three years, which raises the
possibility that we may see a greater frequency of such conditions.
Part 1:
The Severe Snowfall of 17th-18th December 2010
Early December will be remembered by a number of local residents,
though, for one thing: ice. On the night of Friday December 3rd,
fronts pushed in over Wales, bringing slightly milder air and outbreaks
of snow - but rain and sleet mostly at valley level. It fell onto
hard-frozen ground and this was the result. This is all ice - covering
all surfaces and lethal to drivers and pedestrians alike,
including yours truly who spent the 4th limping around with a
badly-bashed knee!
Freezing rain is one of the most dangerous things the weather can dish
up - it doesn't need to be excessive as rain, snow or winds do. Any
freezing rain will do the job. Fortunately the weather cheered up a bit
and my knee stopped hurting so with the forecast looking pretty dodgy
for later the next week a firewood-gathering sortie was done. With this
lot cut, split and stacked I was ready for everything!
Everything indeed began in the early hours of December 17th, following
the clearance of a surging Arctic front the previous day. The winds
following the front were northerly: however, a strong low-pressure
system that had developed off the east coast of Greenland tracked south
in the airflow - an unusual synoptic situation. As it approached the
UK, the winds over Wales backed northwesterly. This was important: in
such setups the prime driver of snowfall is convective showers that
form over the warm seas adjacent to our coasts - the warm sea-cold air
coupling creating the instability. Where the showers end up over land
is down to the wind direction: in a Northerly they affect the northern
coastal counties of Wales and also Pembrokeshire. The more west in the
wind, the more extensive the area affected. By 0900 on the 17th,
several centimetres of snow had fallen. I wandered down to Dyfi Bridge
to be met with a heavy shower of graupel:
Graupel, sometimes called soft hail, forms when snowflakes within a
cloud encounter tiny droplets of supercooled water. Its most important
property however is that if a heavy fall occurs and gets incorporated
into the snow-profile, its loose and rounded nature means that it does
not bond properly like ordinary snowflakes: what results is an unstable
layer within the snow, which can act as a focus for major slab
avalanches. Climbers and skiers who dig test-pits to check snow-slope
safety look for layers of this stuff, among other things.
In the nearby forestry, it was still and silent, with the snow stuck to
everything....
...until, out of the stillness there came a sudden gust of wind. I just
had time to compose and shoot as these tall trees shed their load of
snow, filling the air with powder:
A streamer of
snow-showers formed up that afternoon and as a consequence there was
intermittent snowfall for the rest of the day and overnight, with the
wind backing westerly as the low moved into North Wales. Early on the
morning of the 18th, this was the view in Machynlleth:
By the time the snow had died out that morning, the depth outside was
an exceptional 25cm - this had not drifted at all so it represents an
actual total:
Reports from
nearby areas inland and uphill indicated 30-45cm had fallen in places
and just out of town there was 30cm too: by their very nature, snow
showers lead to a mixture of depths over quite short distances.
Rumblings around town announced small avalanches as gravity overcame
the tensile strength of the snow on roofs:
I climbed up onto Penyrallt to get an overall view of the scene:
The town was virtually silent apart from the occasional 4x4. Down at
the railway station, a train service was kept going although that took
a bit of extra work: I guess these guys are dealing with snow and
ice-affected points...
A snowplough came through and a bit of traffic started to turn up,
slipping and sliding its way along. I stayed on foot: it seemed a
better option given the conditions:
By the afternoon of the 19th, Machynlleth had been mostly dug-out, with
high mounds of the stuff dotted about anywhere it could be tipped....
A new hazard had in the meantime emerged: heat escaping through the
tops of buildings was causing partial snowmelt but with air
temperatures around freezing this promptly refroze into huge icicles
hanging tens of feet above the pavement. These WILL break off at some
point - beware - and always look up!
Part 2: The November-December 2010 freeze
in context
A severe wintry weather outbreak on the scale of this one is guaranteed
to generate "So This Is Global Warming"-type headlines in certain
sections of the UK media. It's not an unfair question either: if you
are freezing cold, unable to get to work due to snowfall and your pipes
have just gone and burst, the notion of a planet heating up seems a bit
distant to put it mildly. In other posts I've said that I don't like
the terms "global warming" or "climate change" and prefer "climate
destabilisation" as it's more realistic. The literature all points to
more warming in some places than others, and some places even cooling,
due to changes in circulation patterns. These are things we can
certainly do without. Let's take a closer look.
Here's an interesting chart from NASA. It shows global temperature
anomalies for November 2010, a month in which Wales broke a monthly
all-time cold temperature record.
Sources and parameters: GHCN_GISS_HR2SST_1200km
Temperature anomaly: compared to the 1951-1980 average.
Note: Grey areas signify missing data.
Note: Ocean data are not used over land nor within 100km of a reporting
land station.
A number of features stand out: for example, the relatively warm
Canadian Arctic, the cold anomaly off the west coast of South America -
an effect of La Nina - and, relevant to this piece, the very cold UK
and Scandinavia.
The UK and
Ireland generally enjoy a climate that reflects their geographical
position: islands situated off the NW tip of a continent, jutting out
into a large warm ocean and right in the path of the prevailing
south-westerly wind – the direction from which most of our weather
tends to arrive. Steering their way between quasi-permanent features
such as the Azores High and the Icelandic Low, the south-westerlies
bring warmth and rainfall in equal measure, ensuring our crops are
watered and the heating bills are kept down. Or they should. This
current outbreak of wintriness, with its plummeting temperatures and
paralysing snowfalls, has come via an alternative route - pretty much
straight over the top of the world. What's going on?
Regional-scale fluctuations in weather are often controlled, sometimes
in a cyclic fashion, by large-scale climatic patterns: thus we have El
Nino and La Nina in the Tropical Pacific, with their sometimes major
impacts on rainfall – or lack of it. Here in the North Atlantic, things
are usually controlled by the strength of the pressure-gradient between
the Azores High and the Icelandic Low and by their positions relative
to one another. If the gradient between them is strong – as would be
the case with intense high pressure over the Azores and deep low
pressure over Iceland, a strong west-to-east airflow results and
Atlantic storms trundle along one after another, powered by a strong
jetstream, bringing mild, wet and often windy conditions to our shores.
This weather-pattern is referred to as highly zonal, or mobile. It is
the North Atlantic Oscillation (NAO) in its positive phase - the chart
below shows a typical example - the red arrows show the mild winds
sweeping in from the warm ocean:
The weaker the pressure-gradient between the Azores High and the
Icelandic Low, the more negative the NAO becomes. This results in
strongly reduced zonality, to the point that instead a meridional
pattern develops – where the jetstream is relatively weak and our
weather comes from either the north or the south, bringing (depending
on the time of year and direction of airflow) heatwaves or big freezes
- as shown in the recent chart below - warm winds are red, cold ones
blue:
Now,
to complicate matters further, a new atmospheric circulation pattern
has been identified: the Arctic Dipole, which has become an
increasingly-important feature of the Arctic climate during the first
decade of the 21st Century. Arctic weather has until recently been
driven by the NAO and its close relative, the Arctic Oscillation (AO),
both of which broadly produce a circumpolar airflow from west to east.
Now, with the Dipole, they have competition and it is having some
strange affects on the climate of the Arctic and further afield.
The Arctic Dipole pattern features anomalously high and low pressure
systems – they are occurring and persisting where previously they did
not. When the Dipole is dominating things, high pressure builds over
the American side of the Arctic and low pressure forms on its Eurasian
side: this results in an extremely meridional pattern in which winds
blow south-to-north through the Bering Strait and into the Arctic from
the Pacific, importing extra heat in the process and driving Arctic
temperatures further upwards, encouraging further melting of sea-ice
well beyond that expected due to anthropogenic greenhouse gas emissions
alone. But – ironically – several studies published over the past few
years have concluded that the mechanism for its formation was triggered
by low sea-ice extent in the first place: it is an example of a
positive climate feedback pattern. It works thus: sea-ice has a high
albedo – that is, it reflects a lot of solar energy back out to space.
Over areas where that ice has melted, the energy is instead absorbed by
the open sea-water, warming it. The open water reaches its maximum
extent in mid-September: during the Autumn, the research has found, it
returns some of that heat back to the lower atmosphere, driving up air
temperatures and thereby affecting pressure and atmospheric circulation
patterns, which in return go on to cause further excessive summer
ice-loss in subsequent years. But these changes are not just affecting
the Arctic.
In their updated Arctic Report Card for 2010 (reference 1 at end of
page), Overland et al note: “Winter 2009-2010 showed a major new
connectivity between Arctic climate and mid-latitude severe weather,
compared to the past.” They show what would be considered to be a
“normal” pressure-pattern, with anticlockwise circumpolar winds, and
then explain how December 2009 saw a complete reversal of this pattern,
that essentially eliminated the normal west-to-east jet stream winds.
This allowed cold Arctic air to penetrate deeply into some southern
regions such as Europe, resulting in very low temperatures and snowy
conditions: Northern Eurasia (north of 50° latitude to the Arctic
coast) and North America (south of 55° latitude) had negative
monthly temperature anomalies of -2°C to -10°C – whilst at the
same time, Arctic regions had positive anomalies of +4°C to
+12°C. This change in atmospheric circulation has been given the
working name of the Warm Arctic-Cold Continents climate pattern. This
year a similar pattern appears to have emerged: on November 28th,
temperatures in parts of Wales fell to -18C (a November record
minimum), but at Kangerlussuaq, inside the Arctic Circle in western
Greenland, the minimum was +9C, an amazing 27C warmer.
The graph below shows Arctic sea-ice extent on December 18th 2010.
Although ice volume is a better measure, as it demonstrates how much
multi-year ice is left (not a lot), extent gives a good idea as to how
the seasonal re-freeze is going. It got off to a good start in
September 2010, but has since slowed, so that for parts of both
November and December it has been the lowest on record.
In the case of the Warm Arctic-Cold Continents pattern, one explanation
with respect to effects further afield in Europe may be this observed
anomalously low late Autumn ice coverage – and
accompanying high heat-flow from sea to atmosphere - in the Barents and
Kara seas, marginal parts of the Arctic Ocean situated to the north of
Scandinavia and Russia (location map below). This potential influence –
the “B-K Effect” -
has been analysed using a global atmospheric circulation model by
Vladimir Petoukhov of the Potsdam Institute for Climate Impact Research
and Vladimir Semenov of the Leibniz Institute of Marine Sciences at
Kiel University in a study submitted in November 2009 and recently
published in the Journal of Geophysical Research (reference 2).
Petoukhov and Seminov found that the model responded in a non-linear
fashion: rather than resulting in a warming over adjacent continents as
might have been expected, a strong regional cooling was generated
within a certain range of sea-ice cover. In the abstract, they go so
far as to state: “Here we show that anomalous decrease of wintertime
sea-ice concentration in the Barents-Kara (B-K) seas could bring about
extreme cold events like winter 2005-2006.” The paper, and indeed the
whole subject of cold European winters and how they relate to the
overall global climate, has been the subject of much lively discussion,
in the context of that particular winter and others past and present.
Writing on the Realclimate blog on December 14th 2010 (reference 3),
Rasmus Benestad of the Norwegian Meteorological Institute noted that,
while Petoukhov and Seminov's findings sound plausible, things may not
be so straightforward. With respect to using global circulation models
to determine regional effects, he commented: “There is a limit to what
they are able to describe in terms of local regional details, and it it
reasonable to ask whether the response to changes in regional sea-ice
cover is beyond the limitation of the global model.”
However, talking about non-straightforwardness, it is also apparent
that, whilst important, the NAO alone is not an absolute way to
determine how a particular cold winter compares to others. NAO data are
available online from the National Center for Atmospheric Research at
Boulder, Colorado (reference 4). The data show that the cold winter of
2009-10 featured the lowest ever December-February (DJF) NAO index of
-5.1. However, whilst 2009-10 caused major problems in parts of the UK,
it was not as cold as the historic 1962-63 winter, with a higher NAO
index of -4.0 but, in the UK, having a Central England Temperature
(CET) of -0.3C for the DJF period. The CET for the equivalent period in
2009-10 was 2.4C. If the NAO was the absolute control-mechanism with
respect to the severity of our winters, then by rights 2009-10 should
have been colder than 1962-3.
So: has recent research found an added influence on our European
winters? The Arctic has become an area of substantial positive
temperature anomalies: this has been directly observed. The sea-ice
anomalies and lower atmosphere heating over open Arctic water in Autumn
have been directly observed. The relatively new Arctic pressure and
atmospheric circulation patterns have been directly observed. Papers
have been published citing evidence for these features to have a
potential effect on the climate further afield, including an increased
severity of European winters. Although climate trends are multidecadal
affairs and the research discussed above is relatively recent,
the influence of open sea water in the Arctic, where at one time there
was extensive sea-ice, is clearly one to watch in the coming years. As
with most matters of science, the truth will come out in the wash in
due course. This may not be the black-or-white conclusion that
policymakers seem to expect these days, but that's how the world works:
they might like to stop and reflect that the decisions that led to the
Allies' victory in World War 2 were not made in the light of absolute
certainty of outcomes.
Acknowledgments:
Thanks to James Overland of NOAA for sending me reprints of papers
discussing the matters described above, and to the Climate Rapid
Response Team at http://www.climaterapidresponse.org/
for their useful help with my research.
References & further reading:
1) http://www.arctic.noaa.gov/reportcard/atmosphere.html
2) Petoukhov,
V., and V. Semenov (2010): A link between reduced Barents-Kara sea ice
and cold winter extremes over northern continents. J. Geophys.
Res.-Atmos., ISSN 0148-0227.
3) http://www.realclimate.org/index.php/archives/2010/12/cold-winter-in-a-world-of-warming/
4) http://www.cgd.ucar.edu/cas/jhurrell/indices.data.html#naostatseas
Budikova, D. (2009): Role of Arctic sea ice in global atmospheric
circulation: A review. Global Planet. Change, 68(3), 149–163.
Honda, M., J. Inoue, and S. Yamane (2009): Influence of low Arctic
sea-ice minima on anomalously cold Eurasian winters. Geophys. Res.
Lett., 36, L08707, doi:10.1029/2008GL037079.
Overland, J.E., and M. Wang (2010): Large-scale atmospheric circulation
changes associated with the recent loss of Arctic sea ice. Tellus, 62A,
1–9.
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