HYPOTHESIS
can be elucidated from the patterns
1/7
Moreover, in the last few years a number
present as early as the preceding summer of superior computer models have been
and early autumn. These are visible in 500
produced that are based on seasonal
mb charts that are publically available. patterns in the Arctic, related North
This empirical model has particular Atlantic Oscillations (NAO) or more
An Underlying Predictability
in Winter Weather Patterns in
the North Atlantic Basin
David Sinclair Stevenson1*
Please cite this article as:
David Sinclair Stevenson. An Underlying
Predictability in Winter Weather Patterns in
the North Atlantic Basin, Hypothesis 2016,
14(1): e3, doi:10.5779/hypothesis.v14i1.483
Received: 2016/05/12;
Accepted: 2016/06/13;
Posted online: 2016/08/06
1
Department of Science, Carlton le Willows
Academy, Nottinghamshire, UK.
success in years where there is neither distantly the tropical Madden Julian
an El Niño or La Niña event. However, in
some recent El Niño or La Niña years, computer model is able to predict the
the pattern remains rather predictable
© 2016 David Sinclair Stevenson. This is an
Open Access article distributed by Hypothesis
under the terms of the Creative Commons
Attribution License (http://creativecommons.
org/licenses/by/3.0/), which permits
unrestricted use, distribution, and reproduction
in any medium, provided the original work is
properly cited.
winter season weather in Western
making subsequent long term fore- Europe with a good degree of accuracy,
Illustration by Eloïse Kremer
*Correspondence:
[email protected]
Oscillation3-5. For example, the GloSea5
casting possible.
based upon the NAO in the preceding
season4. These are successfully predicABSTRACT There now exists a number INTRODUCTION Given the impact of
tive when made from late October or early
of computer models that show some severe weather on human activity, there is
November. Meanwhile, Kang et al. show
success in predicting the meteorological an obvious interest in producing accurate
that the, predecessor, GloSea4 model
conditions in the ensuing winter in the forecasts weeks or months in advance.
could predict with some accuracy the
northern hemisphere. These examine the However, the success rate is extremely
state of the AO as early as November3 . The
1
Arctic Oscillation (AO) and are successful variable . However, in recent years a
accuracy of these forecast models has
up to two months in advance of the begin- wealth of observational data has allowed
been assessed by Shi et al. 6 and found
ning of meteorological winter. Other the identification of correlations between
to vary depending on the decadal period
models, which are based upon the North autumnal meteorological patterns and
over which they are tested.
Atlantic Oscillation (NAO), also show weather in the subsequent season. For
some predictive success depending on example, Vladimir Kryjov identified a The AO is a variation in the pressure
the period over which they are tested. connection between the surface pres- gradient from the Arctic Basin and the
Here I show that the pattern of winter sure over the Taymyr Peninsula, in surrounding mid-latitudes, south to
circulation across the North Atlantic
Basin, Arctic Basin and Western Europe
north eastern Russia in October, with
20°N7,8 . When pressure is higher in the
the strength of the so-called Arctic
Arctic and lower, further towards the
Oscillation (AO) in the subsequent winter2. equator, the Oscillation is said to be
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HYPOTHESIS
An Underlying Predictability in Winter Weather Patterns in the North Atlantic Basin
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Stevenson
negative. Conversely, when pressure is
in the polar-front jet stream and blocking
its effect, but implies that the snow
lower over the Arctic, pressure is higher over Scandinavia or the Norwegian Sea. cover is a reflection of the pre-existing
over the Horse Latitudes and the index is There is a further correlation between
said to be positive. Negative AO is associ- the extent and arrival date of snow cover
ated with weak westerly winds across the
in Siberia and the severity of the winter
mid-latitudes and a stronger prevalence
in Western Europe 9 and this mirrors an
of cold easterly or northerly outbreaks
underlying change in the Arctic Oscillation
across Western Europe during the winter with a more negative Arctic Oscillation
months. These are typically associated
giving rise to earlier snow cover. Cohen
with the formation of warm-cored areas
presumes that the snow cover influences
of high pressure, known colloquially as
the pattern of airflow from Siberia to the
blocking anticyclones. Meanwhile, a
west with more snow cover strengthening
positive AO is associated with warmer cooling and thus an expansion of the
and stronger westerlies across Europe
underlying cold Siberian high pressure
and limited, if any, blocking of the west- area . However, what if this assumption
9
erly flow across Europe. Kryjov found that
is wrong and the extent of snow cover is
there was a correlation between higher instead a reflection of the overlying circupressure over the region in October and
lation pattern, which in turn is diagnostic
a more negative Arctic Oscillation in the
of a pressure pattern that has already
subsequent winter2.
been established? Instead of driving
The North Atlantic Oscillation is an extension of the Arctic Oscillation. During positive phases there is a steep gradient of
pressure between Iceland and the Azores,
which leads to strong westerly winds.
Negative phases are associated with a
weakening of these winds, a frequent split
the observed change, which is generally
the assumption, the early winter snow
cover is an indicator of the pattern of
circulation in the overlying atmosphere
during the autumn. This does not negate
pressure pattern.
presented here: by adopting a purely
empirical approach, it is observed that
the pressure pattern in the summer and
early autumn relates to the pressure
pattern in the subsequent winter. While
this pattern is not an exclusive influence
over the pressure in the ensuing season,
the mean pressure pattern during the
summer and early autumn months does
appear to possess a considerable influence on the pressure pattern over
the Arctic Basin and Western Europe
several months later, during the winter.
Subsequently, this affects the severity
of the winter weather in Western Europe.
DERIVING THE MODEL Meteorologists
the
atmosphere
Motivated by this weak correlation, I then
the corresponding pressure at a lower examined the 500 mb charts that were
altitude. The mean surface level pres- commercially available during the late
In essence this is the hypothesis
examine
the pressure level, while colder air has
in
slices.
Whereas at the surface pressure varies
with the passage of various features,
such as frontal storms, meteorologists
look at pressure levels and the heights
at which these occur. Warmer air raises
sure is 1,000 millibars (1,000 mb). Half
2000s12,13 . The 500 mb pattern across
this value (500 mb) is found at altitudes
the North Atlantic gave a much clearer
of around 5,000 meters, while the polar correlation and this agrees with the
front jet stream, at the top of the tropo- recent computer models of Scaiff et al4.
sphere is found at 200-300 mb, depen- However, it was not until the 500 millibar
ding on the season. For the purposes
(5,000 metres) level was examined for the
of this paper we will look at the 500 mb
whole northern hemisphere that a much
level. The reasoning for this choice is
stronger correlation was noted and was,
explained below.
therefore, further examined. At the 500 mb
Driven by curiosity, I developed a series
level various atmospheric waves, known
of predictive models based purely on
as Rossby waves, are clearly apparent.
observation: i.e. an empirical approach. It is well established that the pattern of
Initially, these looked at surface level
Rossby waves relates to the conditions
pressure patterns over the North Atlantic
at the surface as they reflect the path
through to Western Russia. However, it
of the polar front jet stream above and,
soon became clear that the observed
therefore, the passage of frontal systems
patterns in the autumn did not provide
below. When the Arctic Oscillation is
a very good match to the pattern in the
positive (negative) these waves have a
subsequent season. However, there were
low (high) amplitude, thus making them
hints that the presence of autumnal
a useful discriminator between different
blocking over Western and Northern
phases of the AO 7,8 . The broad pattern of
Central Russia might link to conditions in
these waves reflects the mobility of the
Western Europe later in the year10,11.
circumpolar circulation and thus forms
the basis of this proposition. In essence
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An Underlying Predictability in Winter Weather Patterns in the North Atlantic Basin
HYPOTHESIS
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Stevenson
this is an empirical analysis of the Arctic
OBSERVATIONS AND SUPPORTING
Oscillation, North Atlantic Oscillations
ARGUMENTS The pressure over the
and related oscillations in the North
Arctic Basin varies from year to year and
Pacific.
is coupled strongly to pressure over the
HYPOTHESIS The observed 500 mb
pressure pattern over the Arctic Basin
and Northern Atlantic Basin in the winter
correlates strongly with the observed
pressure pattern over and surrounding
the Arctic Basin at 500 mb in the
preceding two seasons. This leads to a
strong predictability of the overall manner
in which weather will unfold during that
winter. The pressure pattern is modified
by the phase and strength of the El Niño
Southern Oscillation (ENSO), but is not
completely abrogated by it. While other
computer models extend predictions for
December-February to the beginning of
November3,4, I suggest here that examination of the Arctic Oscillation – and the
North Atlantic in all seasons. During the
summers of 2009 and 2010 surface level
pressure and pressure at 500 mb was
predominantly high over the Arctic Basin,
with a generally weak circulation within
the North Polar Basin (FIGURE 1). Warmcored blocking anticyclones were evident
across much of the basin, particularly in
July. In both years the pressure pattern
was largely maintained throughout the
autumn and into December with pressure
remaining generally higher than normal
across the basin indicating a negative AO.
Each of these seasons was followed by a
colder than normal winter, but with some
Figure 1 | 500 mb charts from July 16th 2009 (left) and July 16th 2010 (right). Warm-cored high
notable differences that will be discussed
pressures areas (H) are prevalent across the Arctic Basin. While, this is a snap-shot from the middle
later.
of the meteorological summer, it is a consistent pattern. Note, that the geographical region identi-
overriding state of the circumpolar circu- Conversely, during the summer months
fied by Kryjov and centred on the Taymyr Peninsula 2 is under one such blocking high in both years
lation – makes it possible to success- of 2013 the 500 mb level was depressed
(arrowed). Charts from Colorado State University.
fully predict the state of the wintertime
with a stronger, colder circulation and
AO as far back as the preceding summer smaller (lower amplitude) Rossby waves
(FIGURE 1).
surrounding the circumpolar region
(FIGURE 2) . This is indicative of a positive
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HYPOTHESIS
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Stevenson
Figure 2 | The growth of the cold circulation over the Arctic during the late summer and autumn
of 2013. While many of the preceding summers and early autumns had evidence of warm-cored
blocking anticyclones, the summer and early autumn showed a stronger colder circulation (blue
and purple in these charts from www.netweather.tv ). This circulation grew in strength with only
minimal incursions of warmer air leading to my prediction that the winter would be mild and
wet. Charts are for 17th August left); 6th September (middle) and 17th October (right). Charts
courtesy of www.netweather.tv
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An Underlying Predictability in Winter Weather Patterns in the North Atlantic Basin
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Stevenson
AO. This pattern continued to strengthen
Western Europe and the North Atlantic Taymyr Peninsula in agreement with the
principally on the extent and orientation
During the subsequent winter of 2014-
during the autumn leading to Britain’s
Basin. This idea was then further tested
observations of Kryjov . However, more
of the polar vortex. This in turn, depends
2015
wettest and warmest winter on record
by retrospective analysis of the pres- generally pressure remains higher than
on a number of other factors, such as the
temperatures close to the seasonal
(at the time of writing). During the late
sure patterns in other non El Niño or La
phases of the El Niño Southern Oscillation
average with a good mixture of cold and
summer and autumn of 2014 the circu- Niña years. Data available from Colorado
2
during episodes of positive AO across the
Western
Europe
experienced
central Arctic Basin, as well. Conversely, or the (North Atlantic Oscillation). Thus, mild conditions. North America again
lation was mixed with a predominantly
State University14 allowed the same
during July and August, when the AO
although the polar vortex may extend
cold circulation, but with high amplitude
hypothesis to be tested in various years
is positive, there is a preference for the
across much of the continental land- there was no direct correlation between
Rossby waves evident circulating around
from the year 2000. Further data was
axis of the cold-cored circulation to lie
mass during periods of negative AO as
the North Pole. Interestingly, while the AO
available, extending back to 1899 12,13,14. over northern Canada and Greenland. was seen during the winter of 2014-2015, in Western Europe. This lack of correla-
is negative during the summer months, it
However, this was restricted to analysis
In this situation warm-cored blocking
it may also be present when the Arctic
tion is exemplified by further analysis
is evident that the circulation at 500 mb
of the North Atlantic Basin and thus of
over northern Siberia in the summer and
Basin is experiencing a positive AO as
going back to 1950s (9). Looking at data
above the basin is as strong as it was in the
more limited use. Commercially avail- autumn is far more likely to favour the
in the winter of 2013-2014. During the
for New York on the Atlantic seaboard,
summer of 2013. Therefore, while there
able numerical data was also examined
latter winter the principle driver of this
the majority of the coldest winters from
development of a strong surface high
suffered significant snowfall events. Thus,
the weather over North America and that
were significant cold circulations across
from National Oceanic and Atmospheric
pressure area than an early snow cover. cold incursion was a strongly modified 1950 to present day are associated with
Asia and the northern Pacific, this was
Administration (NOAA)15.
This is by virtue of the blocks ability to
jet stream with a large amplitude crest
divert the jet stream north on the western
(Rossby wave) over the Pacific and corre- This is readily understood because a posi-
offset by the relatively strong circulation around the pole. Thus overall, the
pressure pattern during the autumn of
2014 was transitional between the two
patterns seen in the autumn of 2009
and 2013.
At this point it seemed apparent that
There was a concern that the observed
correlation was either a statistical fluke
or one that was related to a particular but
non-obvious pattern in the AO that was
specific to the last seven years. To test
this hypothesis, the summer-winter AO
patterns were examined in the years 1995,
flank and south with cold, moisture – sponding trough over North America at
laden winds along its eastern flank. From
lation pattern surrounding the Arctic
Basin during the summer season and the
weather in the subsequent winter across
Within this broad pattern you can discern,
in most cases, a prevalence for warm
cored blocking anticyclones over the
tive AO will mean lower amplitude and
the 500 mb level. During this phase, longer wavelength Rossby waves. With
here positive feedback can further inten- Western Europe remained predominantly
a predominantly westerly wind direction,
sify subsidence at the surface and an
very mild and wet, with a strong circum- cold air is directed from the continental
increase in the intensity of the otherwise
polar circulation driving repeated storms
interior towards the eastern seaboard.
across the region. The location of the jet
However, a negative AO, with large ampli-
cold-cored Siberian high.
there was a straightforward correla- 1998 and 2004: the same correlation held. EXCEPTIONS Predictability of the winter
tion between the AO and overall circu-
a positive AO, rather than a negative one.
weather for Western Europe depends
on the phase of the Arctic Oscillation.
However, this does not readily extend to
North America, where cold winters depend
stream was south of its averaged winter tude waves, will tend to display a trough
time position but a positive AO ensured
over the continental interior – an effect
that the polar front jet was active across
driven both by significant land cooling
Europe, bringing repeated rain storms
and by the Rocky Mountains. This, in
and record-breaking flooding to the UK.
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HYPOTHESIS
An Underlying Predictability in Winter Weather Patterns in the North Atlantic Basin
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Stevenson
CONFLICTS OF INTEREST
negative Arctic Oscillation and blocking
coast of the continent, thus favouring a
prevailed across Europe. This was
conspired with this La Niña preference
testable hypothesis: that the winter The author declares no conflicts of
warmer winter.
despite it driving more typical mild El
for early season blocking, producing
weather in Western and Central Europe
The impact of the phase of the ENSO is
Niño conditions in the Pacific North-West. instead enhanced blocking, but at some- is determined in large part by the pres-
more problematic, in part because strong
During the ensuing summer of 2010,
El Niños are relatively rare and thus
blocking persisted over the Arctic with
comparing the effect of the ENSO phase
a very weak circum-polar circulation
with that of the AO suffers from limited
during the summer and early autumn
data. However, some key themes emerge. (FIGURE 1). Therefore, I made a predicAnalysis was done of the winters of 1982- tion of a colder than normal winter, with
1993, 1997-1998, 2009-2010 and 2010- significant blocking. By late November
2011, with a comparison of the winter substantial blocking across northern
pattern with that of the preceding two
Europe and the North Atlantic led to the
seasons. Strong El Niño events began
coldest December on record.
in 1982, 1997 and 2015; while 2009 saw
a moderate central Pacific El Niño; and
2010 a strong La Niña.
Typically, El Nino episodes are thought to
2010 the negative AO appears to have
CONCLUSION I present a simple and
turn directs warm air up, along the east
Interestingly, there was an additional
effect of the La Niña. Although La
Niñas typically bring mild winters to
Western Europe and the North Atlantic
add additional energy to the polar circula- Basin there is an additional effect on
tion. They also establish poleward-prop- pressure patterns with the formation
agating Rossby waves that strengthen
of blocking anticyclones over central
westerlies and bring mild conditions
and Western Europe in late November
to the North Atlantic Basin, particularly
and early December. These bring cold
during the first half of any given winter. and dry conditions. This pattern tends
However, in the winter of 2009, the El Niño
to give way to predominantly westerly
failed to override the effect of the already
conditions from mid-winter onwards. In
what higher latitude than normal. This
sure pattern over or around the Arctic in
led to the UK’s coldest December since
the preceding two seasons. The pattern is
records began. Subsequently, typical
modified by strong El Niño and strong La
late winter La Niña conditions prevailed
Niña events but is otherwise discernible
with mild conditions. Contrast this with
in every winter tested. While this pattern
the winters of 2007-2008 and 1998-1999
has been identified in the late autumn as
where a positive AO and La Niña condi- being predictive, these empirical obsertions conspired to produce mild winters
vations take the period from which
across Western Europe.
prediction can be made back two seasons
The strong El Niño winters of 1982-1983,
1997-1998 all produce predominantly
mild conditions. Here, the effect of the El
Niño appears to supersede any effect of
the AO. However, in the summer of 2015
there was a negative AO. Despite the
strong influence of the El Niño, there was
still a propensity for blocking over the
Arctic and Scandinavia in late December
2015 and early January 2016. Thus,
although the El Niño had the strongest
meteorological impact, there was still
evidence of the impact of the negative
AO from the preceding seasons.
with good reproducibility in the last
two decades. Predicting general weather
patterns several months in advance has
two benefits. Immediate benefits include
better preparedness for extreme weather
events which are associated with particular weather patterns. However, the ability
to predict patterns in advance implies
that there are much stronger correlations
in weather patterns between seasons.
interest.
ABOUT THE AUTHOR
David Stevenson is a former postdoctoral academic with a background in
molecular genetics. Subsequent publications include authoring a number of
astronomy books for Springer, including
“The Exoweather Report”, which is
currently in press. Other published titles
for Springer covered supernovae, the
habitability of planets orbiting red dwarfs
and finally star clusters. A number of other
popular astronomy articles are available in Popular Astronomy, Astronomy
and Sky & Telescope magazines. I
currently work at a successful Academy
in Nottinghamshire, while writing two
further books on planetary geology and
astrobiology for Springer.
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HYPOTHESIS
An Underlying Predictability in Winter Weather Patterns in the North Atlantic Basin
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Stevenson
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