eprints.lincoln.ac.ukeprints.lincoln.ac.uk/.../WEATHER_linkages_paper_REVISED_15feb2…  · Web viewEngland Temperature (Parker et al. 1992) and England & Wales Precipitation (Alexander

Can Arctic warming influence UK extreme weather?

MS revised for WEATHER (RMetS), 15 February 2017

Edward Hanna1, Richard J. Hall2, James E. Overland3

1School of Geography and Lincoln Centre for Water and Planetary Health, University of

Lincoln, UK2Department of Geography, University of Sheffield, UK3Pacific Marine Environmental Laboratory, National Oceanographic & Atmospheric

Administration, Seattle, USA

Abstract. We explore a possible relation between the recent Arctic amplification of global

warming and changes in North Atlantic jet stream circulation and UK extreme weather

conditions over the last decade. Such a link is supported by some tantalising clues from

recent North Atlantic atmospheric circulation changes in summer and winter but, due to

multiple factors affecting jet-stream variability, we need extended records over at least a

further decade to more reliably attribute these changes to global warming.

The last decade has seen a spate of notable extreme weather events in the United Kingdom

(UK). These include extreme rains and floods across large parts of England and Wales in the

summers of 2007 and 2012 (Hanna et al. 2008, Parry et al. 2013), the record wet and stormy

UK winter of 2013/14 which was the wettest winter in the England & Wales Precipitation

record since the record began in 1766 (Kendon & McCarthy 2015), the almost equally

disturbed 2015/16 winter – with a record mild and wet December 2015 (McCarthy et al.

2016) - and some unusually cold, snowy winter conditions in 2009/10 and 2010/11, when

December 2010 was one of the coldest UK Decembers on record. Also, March 2013 was the

coldest March month of that name since 1962 and the equal second-coldest March since 1910

(http://www.metoffice.gov.uk/climate/uk/summaries/2013/march ). Furthermore, according

to the available digitised UK Met Office records, which go back as far as 1860, the highest

2-day, 3-day, 4-day and monthly UK rainfall records for consecutive rainfall days (0900-

0900 GMT) have all occurred since 2009

(http://www.metoffice.gov.uk/public/weather/climate-extremes/#?tab=climateExtremes; Burt

2016), and were mainly linked with major flooding episodes in the Lake District. See Table 1

for a summary of recent seasonal anomalies for summer and winter based on the Central

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England Temperature (Parker et al. 1992) and England & Wales Precipitation (Alexander &

Jones 2001) records.

Of course weather is naturally chaotic to a degree and weather extremes are a normal

part of our highly variable UK weather but globally there has recently been an increase in the

incidence of high temperature and heavy precipitation extremes, which is likely to be linked

to human-induced global warming (e.g. Coumou & Rahmstorf 2012; Fischer & Knutti 2015).

It is well known that a warmer world tends to be wetter because the warmer atmosphere tends

to contain more water vapour, although the regional pattern of precipitation changes largely

depends on jet-stream and storm-track (and hence extratropical cyclone) changes, which are

far less certain going into the future.

UK seasonal weather changes in last decade

The cold winter episodes noted above, if they are externally forced (rather than internal

“noise” in the climate system), are not so intuitively linked to climate change but reflect part

of a long-term trend towards more variable North Atlantic atmospheric circulation from year

to year in winter, especially early winter (December), that has recently been noted based on

the last 50-100 years (Hanna et al. 2015) (Figure 1). This trend has culminated in the last

decade having several record negative and positive December values of the North Atlantic

Oscillation (NAO) (Table 2). The NAO is the south-north pressure difference across the

North Atlantic, and is a good measure of the position of the North Atlantic polar atmospheric

jet stream which correspondingly shifts north and south in latitude. In winter, a positive NAO

is linked with a more northward, vigorous jet and mild, wet, stormy weather over the UK,

while a negative NAO tends to be associated with a more southerly-positioned jet and

relatively cold and dry but sometimes snowy conditions. In summer the jet stream is

displaced further north, so a positive NAO is associated with warm dry weather, and a

negative NAO corresponds to wetter, cooler conditions.

The other key change that has been seen with the North Atlantic atmospheric

circulation in the last decade is a wavier more meandering jet stream in summer (Figure

2a,b), that was first noted by Overland et al. (2012) and was further evidenced by Francis &

Vavrus (2015). This is linked with increased Greenland high-pressure blocking in summer

(Hanna et al. 2016a), which is depicted here by positive geopotential height anomalies in

Figure 2c. There is a synchronous clustering of recent unusually low NAO values (Table 2),

including the fourth (2011), fifth (2009), eighth (2015) and ninth (2012) lowest NAO summer

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values since the start of the record in 1899. Increased jet waviness and accompanying

blocking is closely linked to greater frequencies of daily weather extremes, although there are

distinct variations depending on the region, season and type of extreme – for example in

summer a far higher number of extreme hot days is favoured over western Russia and eastern

Canada (Rӧthlisberger et al., 2016). The North Atlantic jet stream is generally somewhat

wavy in nature but during the last 10 years it has tended to take a more sinuous route,

including a more northerly-than-normal track up over the western side of Greenland and then

back south just to the east of the island (Figure 2a). The average geopotential height and

near-surface air pressure have been lower than normal over the UK region (Figure 2c),

linked with the extreme wet, flooding summer episodes mentioned above (e.g. Blackburn et

al., 2008). A broadly similar dipole pressure anomaly change is seen for winter (Figure 2f),

still with a low height/pressure anomaly over the UK region but with the Arctic high

heights/pressure anomaly shifted from Greenland into the Barents-Kara Seas off northern

Russia. Both summer and winter have approximately central high height/pressure anomalies

that are surrounded by near-neutral heights and pressures, with regional low anomalies over

the UK region, eastern US and parts of Siberia. The similar yet distinct seasonal height

anomaly patterns may be affected by different physical mechanisms, which we consider

below. Although part of the uneven seasonal NAO changes might be due to natural random

(called internal) variability, the statistically highly unusual clusterings of extreme December

NAO values and extreme high summer Greenland Blocking Index (GBI) values since 2000

(Hanna et al. 2015, 2016) suggest a more sustained, systematic change in the North Atlantic

atmospheric circulation that may be influenced by longer-term external forcing factors.

Possible causes

The most immediate potential cause that comes to mind is global warming, although - as we

shall see - other factors may also have a role to play too. Global warming in the last 1-2

decades has been especially marked in the Arctic, called Arctic Amplification, where the rate

of warming has averaged more than twice the global rate (Overland et al. 2016a) (Figure 3).

Arctic sea-ice has undergone major losses, losing almost half its extent in late summer during

the period of available complete hemispheric satellite data coverage (since 1979) (Perovich et

al. 2016). Major ongoing thinning means that about three-quarters of Arctic sea-ice volume

has been lost in late summer during this time period. Winter losses are more modest but still

significant and there has been a fundamental change in the winter sea-ice morphology from

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thicker older (multiyear, i.e. surviving more than one melt season) ice to relatively thin ice

that is less than one-year old. At the time of writing (December 2016), Arctic sea ice growth

was sluggish in its autumn recovery and trailed at record low areal coverage for the time of

year (https://nsidc.org/arcticseaicenews/). Exceptionally high surface air temperatures

occurred in the Arctic in winter 2015-16 and again in autumn 2016 (Figure 4).

There are several fundamental reasons for this strong northern warming, including the

extra absorption of heat by a darker surface as ice and snow melt away, and the Planck

radiation feedback: arising from the basic laws of radiation physics, this stipulates that a cold

surface radiates much less heat than a warmer one; the infrared radiation emitted upwards

through the Earth’s atmosphere is proportional to the fourth power of temperature (Pithan &

Mauritsen 2014). Also, under conditions of global warming, the main foci of the warming are

in the upper and lower troposphere at low and high latitudes respectively, and because heat is

more readily lost from higher up in the atmosphere, this difference in the vertical structure of

temperature changes accentuates the above effect (Pithan & Mauritsen 2014).

One of the main drivers of Arctic Amplification is the so-called ice-ocean heat flux

feedback (Francis et al. 2009, Screen & Simmonds 2010), which is where progressively

thinner and less extensive sea-ice allows heat to escape from the ocean to the atmosphere:

this effect is thought to be most prevalent between late summer and early winter, partly

because this extended autumn season is the time of year when some of the greatest sea-ice

losses have occurred, and partly because there is then a relatively large temperature

difference between the sea and air where the latter is rapidly cooling in its seasonal cycle.

Large parts of the Arctic Ocean and adjacent seas north of Alaska and the Barents and Kara

Seas in the Siberian sector that used to be mainly sea-ice covered in winter are now largely

ice-free.

This effect heats the lower troposphere (lowest few kilometres above the surface) and

raises air masses, geopotential heights and air pressures over much of the Arctic. Arctic

Amplification tends to reduce the north-south temperature gradient in the lower troposphere,

which may weaken the belt of circumpolar westerlies that mark the northern hemisphere

polar jet stream and cause the jet to meander more (Figure 5). The result may be an increased

propensity for masses of warm mid-latitude air to force their way up into the Arctic, often in

preferential locations (e.g. west of Greenland and over the Labrador Sea), while – at other

points around the hemisphere - cold Arctic airmasses plunge to much lower-than-normal

latitudes; this seems to be apparent in maps of global temperature trends since 1990 which

show coldspots over Siberia and eastern North America (e.g. Cohen et al. 2014). These

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locations are favoured by continental heating and topographic barriers (e.g. over Greenland)

and the recent pronounced sea-ice losses north of Siberia. Also, with more long waves

sometimes developing in the hemispheric jet stream, this can predispose the jet to jet-stream

waves to become slower-moving and sometimes appear nearly stationary with respect to the

Earth’s surface (e.g. Hanna et al. 2013).

Such a weakening in the jet over the North Atlantic tends to be linked with a negative

NAO-type pattern, several extreme cases of which have been observed in the past few years,

and may have the effect of increasing the duration of some extreme weather events in the

mid-latitudes. For example, the exceptionally wet UK summers of 2007 and 2012 had

notably negative NAOs, as did the cold, snowy winters of 2009/10 and 2010/11 (all more

than one standard deviation below the respective seasonal means) (Hanna et al. 2015). 2007

and 2012 were record Arctic sea-ice loss years, and 2012 also saw near-complete surface

melting of the Greenland Ice Sheet, which was unprecedented in the modern satellite record

(Nghiem et al. 2012, Tedesco et al. 2013, Hanna et al. 2014). These summers were also

characterised by unusually high Greenland blocking (Hanna et al. 2016), and there is a well-

established antiphase relation between the Greenland Blocking Index (GBI) and NAO

(Hanna et al. 2013), that highlights the connectivity of sub-Arctic (at least Greenland-region)

air pressure changes with more widespread jet-stream changes further south in the North

Atlantic.

Francis & Vavrus (2012) originally presented the hypothesis that a warming Arctic is

connected with changes in extreme weather patterns further south in the Northern hemisphere

mid-latitudes, and their study certainly garnered a great deal of interest from both the science

community and the wider public, although it left open considerable questions about how the

meteorological dynamics of such a connection might actually work, which still haven’t been

fully answered and are currently under active investigation by a veritable cottage industry of

new studies that have been spawned by their pioneering work. Updated observational

evidence to support the hypothesis was presented in Francis & Vavrus (2015) but some of the

measures used to gauge atmospheric circulation change have been questioned by other

workers, e.g. Barnes (2013); Screen & Simmonds (2013).

In contrast to summer, and as already noted, a couple of recent winters have had an

unusually strong North Atlantic jet stream and highly positive NAO conditions (Kendon &

McCarthy 2015, McCarthy et al. 2016). This may reflect different physical mechanisms

affecting the jet stream in winter compared with summer (see next section). However,

summer 2013 also had a highly positive NAO value (Table 2), and it should also be stressed

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that the observational record is too short to identify any significant changes in jet stream

behaviour against a background of considerable internal variability and at present physical

mechanisms behind Arctic-midlatitude linkages are unclear.

Jet-stream drivers

The North Atlantic jet shows large interannual variability on seasonal timescales (e.g.

Woollings et al., 2010). It can vary by up to 10 of latitude from one year to the next.

However, no significant sustained trends in jet latitude or speed are evident over the period

1870-2012 and while shorter periods of significant trends are evident, these appear to be part

of internal atmospheric variability (Hall et al. 2016a). However, as with the NAO, there is

evidence for a trend of increased latitude variability in winter. This trend towards increased

variability can be identified from the 1950s onwards, which predates the recent significant

increase in Arctic amplification, so cannot be solely attributed to Arctic warming, which has

become evident over the last 20 years (Figure 3).

At this point, we would like to re-emphasize that the jet stream is notoriously variable.

It is important to note that complexities of the underlying meteorological theory mean that the

role of the Arctic in forcing the jet stream is far from resolved, and the period of

observational record of modern Arctic Amplification is too short to define a clear cause of the

observed changes identified above; at least several decades of data are needed to clearly

separate any climate signal from short-term atmospheric noise (Hoskins & Woollings 2015,

Barnes & Screen 2015). Moreover, there are a number of external forcing factors acting on

the jet stream and NAO, of which Arctic Amplification is just one possible influence (Figure

6). Tropical sea-surface temperatures and El Nino-Southern Oscillation (ENSO)

teleconnections are very likely to have an effect (Hall et al. 2015; 2016a,b) and it is also

possible that tropical Pacific sea-surface temperature changes may act on the jet stream via an

indirect polar route (Ding et al. 2014).

In winter, there can be a significant stratospheric influence on weather in the

midlatitudes. During winter, a system of circumpolar westerlies (the stratospheric jet)

develops in the stratosphere, known as the Stratospheric Polar Vortex (SPV). Cold air with

lower geopotential heights is contained polewards of the westerlies. However, the strength of

the SPV is variable, and when winds are weaker, this tends to lead to a southward

displacement of the tropospheric jet, as the troposphere and stratosphere are dynamically

coupled in winter. Similarly a stronger stratospheric jet tends to lead to a more positive NAO

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(Kidston et al., 2015). Positive and negative stratospheric geopotential anomalies can appear

to propagate downwards into the troposphere, so a weaker SPV is often associated with a

warmer Arctic at lower levels, and a more negative winter NAO, and vice versa. The SPV

can also break down over very short timescales, where polar stratospheric temperatures can

increase by up to 80degC over a few days (Gerber et al., 2012) and the westerly winds may

even reverse. Such events are known as stratospheric sudden warmings, and circulation

anomalies can propagate downwards over a period of around a week and may persist in the

troposphere for up to two months (Baldwin & Dunkerton, 2001). A number of factors are

capable of influencing the strength of the SPV, such as solar variability (e.g Ineson et al.,

2011), ENSO (e.g. Bell et al., 2009), tropical volcanic eruptions (e.g. Stenchikov et al., 2004)

and the phase of equatorial stratospheric winds known as the quasi-biennial oscillation

(QBO), which has a period of around 27-28 months (e.g. Anstey & Shepherd, 2014). There

is also some evidence to suggest that reduced Arctic sea-ice extent may itself weaken the

SPV later in winter (e.g. Kim et al., 2014) and to further complicate matters, some of these

influences on the SPV, such as solar and QBO variability, appear to interact non-linearly (e.g.

Labitzke & van Loon, 1988). As an illustration of the influence of the SPV, years with low

solar activity (and fewer sunspots), have been associated with negative NAO-type circulation

patterns via a weaker SPV and a southward displacement of the jet stream. This tends to

favour easterly wind anomalies with cold, snowy winter weather over the UK (Lockwood et

al. 2010, Ineson et al. 2011). There is a cycle of solar ultraviolet heating of the stratosphere,

via photochemical production of ozone, which when the Sun is less active, results in less

heating of the tropical stratosphere and reduces the stratospheric temperature gradient with

latitude, which tends to slow down the westerly wind circulation and can lead to polar

stratospheric warming.

In summer recently increased Greenland blocking (associated with negative NAO)

has been closely linked to Arctic sea-ice reduction and enhanced Greenland Ice Sheet surface

melt rather than natural variability in the initial atmospheric state (Liu et al. 2016). Also the

recent clustering of negative NAO summer values may have been influenced by the current

positive phase of the Atlantic Multidecadal Oscillation (AMO, = area-averaged North

Atlantic sea-surface temperatures) (Sutton & Dong 2012).

Many of these factors are non-linearly coupled, and any potential relation between

Arctic warming and jet-stream flow is itinerant, intermittent and state-dependent (Overland et

al. 2016b). Itinerant means that the atmosphere can suddenly and apparently randomly shift

between different circulation states, for example a relatively uniform, zonal jet stream and a

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much waviermore meandering jet stream. Such shifts may be amplified by external factors,

making it useful to search for signs of changes in jet-stream variability - although such

studies are so far few and inconclusive and more work is urgently needed. Intermittent

means that a certain jet-stream pattern may not always arise due to the same (e.g. Arctic)

forcing factors, or that different combinations of forcings (not necessarily just Arctic) may

give rise to the same jet pattern. State-dependence is a particular type of intermittency where

jet-stream and blocking patterns are influenced by pre-existing surface heat flux anomalies

that develop in response to regional sea-ice, snow cover and/or ocean conditions; the prior

state of the stratospheric polar vortex, which strongly affects couplings through wave

interaction between the troposphere and stratosphere, is another important factor here. This

emphasizes the importance of considering all these Arctic and wider environmental factors

together when assessing the apparent response of the jet stream to lower tropospheric

temperature changes. Well-designed modelling studies are essential to unravel the

complexities of the situation, since with observations cause and effect cannot be attributed

with confidence due to the wide range of potential explanatory factors.

Predicting jet-stream changes

Until recently it has proven very difficult to make seasonal forecasts of winter weather in the

mid latitudes. However, over the last few years there has been significant progress using the

latest generation of numerical weather prediction models. Scaife et al. (2014) have

demonstrated significant skill in forecasting the winter NAO. Others have also shown

increased skill in northern hemisphere winter forecasts (e.g. Riddle et al., 2013; Kang et al.,

2014). However, these forecasts do not indicate the sources of this increased predictability.

Through work with statistical forecast models, it is possible to identify some of the likely

driving factors. Hall et al. (in review) find that decreased November sea ice extent in the

Barents-Kara sea is associated with a more negative winter NAO, in agreement with other

authors (e.g. Kretschmer et al., 2016). However, there are further influences from the tropics

and Atlantic SSTs. Such driving factors can be in opposition to each other and their effects

may be masked by internal atmospheric variability. Thus reduced sea ice alone in a particular

year may not be a reliable indicator of a cold winter to come. Others find that significant

predictability also comes from the state of the stratospheric polar vortex (e.g. Stockdale et al.,

2014). The identification of such sources of predictability can in turn help to improve

8

numerical weather prediction by ensuring that model initialisation conditions more accurately

represent reality. At present there is little skill in summer seasonal forecasts.

It is still unclear what will happen to the North Atlantic jet stream and NAO with

ongoing anthropogenic climate change, even discounting other external forcing factors and

natural (internal) variability. The tropopause is highest at the equator (about 15 km above the

surface) and slopes down toward the poles (about 10 km altitude). With increasing

greenhouse gas levels, temperatures warm at the surface and in the lower troposphere while

the lower stratosphere cools (e.g. Santer et al. 2013). This effect has been well observed in

recent decades and is due to a denser blanket of greenhouse gases trapping infrared radiation

in the lower atmosphere. Although, as we have seen above, Arctic Amplification might be

expected to reduce the amount of energy available for driving the polar jet stream, this is just

a near-surface expression of global warming. Meanwhile, in the upper troposphere at low

latitudes, there is a higher specific humidity under global warming, and this raises the

tropopause and increases upper troposphere temperatures near the equator (about 15 km

above the surface) while the same altitude near the poles (i.e., well within the stratosphere at

these high latitudes) significantly cools with global warming. Therefore there is a

significantly enhanced meridional temperature gradient at this higher altitude just at the same

time that the north–south temperature gradient reduces near the surface (Harvey et al. 2015).

Thus there are two competing influences that can result in changes in mid-latitude (i.e., polar)

jet-stream dynamics under conditions of global warming. One recent model-based study

(Harvey et al. 2015) suggests that the near-surface meridional temperature gradient change is

most important for determining changes in the wintertime North Atlantic storm track, but

overall this is far from certain and more work is clearly needed (Barnes & Screen 2015).

Way forward?

This is still a relatively nascent area of research, since we are largely dealing with the period

since 2007 (although some studies take the current period of Arctic Amplification as being

from 2000 or 2001). There are some tantalising clues from recent North Atlantic atmospheric

circulation changes in winter and summer, and resulting extreme weather events in the UK in

the last decade, that suggest there may be a link with Arctic Amplification, and certainly at

least the strong summer warming and increase in blocking that has been observed over

Greenland. However, given the myriad of factors affecting jet-stream variability over the UK,

and the complex non-linear coupling of these as well as natural variability, we need extended

9

records over at least a decade more to be able to more reliably attribute these changes in

weather extremes to global warming. Early work on reconstructing jet-stream variability

based on reanalysis data and modelling the relation of jet changes with various driving factors

shows considerable promise, and an extension of this work – together with other studies

based on models and an improved understanding of the atmospheric dynamics – should help

resolve some of the current key uncertainties. It is also difficult to identify causation with

observational data as individual factors cannot be isolated. Therefore experiments with

climate models provide one way forward, although such models have well-known biases in

their representations of the jet stream and can give contradictory results, and simple planetary

model experiments (e.g. Hassanzadeh et al. 2014) – while intriguing – are too idealised to be

readily applicable to the real world and so their results are hard to interpret. Historical studies

of jet-stream behaviour and changes during the Early Twentieth Century Warm Period, a

noted period of previous Arctic sea-ice decline, should also help shed light on the problem.

Acknowledgements

We thank the Met Office Hadley Centre for the provision of CET and EWP data. The

weather charts plotted in Figs. 2& 4 were provided by the NOAA/ESRL Physical Sciences

Division, Boulder Colorado from their Web site at http://www.esrl.noaa.gov/psd/. We thank

the International Arctic Science Committee and World Climate Research Programme’s

Climate & Cryosphere project for sponsoring workshops that aided this collaboration, and

acknowledge the University of Sheffield’s Project Sunshine (now Grantham Centre). This

paper is PMEL contribution 4617.

10

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Table 1. Recent (2007-2016) seasonal anomalies of (a) Central England Temperature (CET)

(degC) and (b) England & Wales Precipitation (EWP) (mm) for summer (JJA), winter (DJF)

and December. Winter data are marked by the year of the January (so winter 2016 = Dec

2015 + Jan & Feb 2016). Anomalies are with respect to the latest (1981-2010) climatological

normal period, and are emboldened where they are at least ±1.5degC (CET) or ±25% of the

long-term mean (EWP). CET data are from Met Office at

http://www.metoffice.gov.uk/hadobs/hadcet/, and EWP data are from

http://www.metoffice.gov.uk/hadobs/hadukp/ .

(a)

Year JJA DJF December2007 -0.6 1.9 0.32008 -0.4 1.1 -1.12009 0.0 -1.0 -1.52010 0.0 -2.1 -5.32011 -1.1 -1.4 1.42012 -0.7 0.5 0.22013 0.4 -0.7 1.72014 0.0 1.5 0.62015 -0.6 0.0 5.12016 0.5 2.1 1.4

(b)

Year JJA DJF December2007 134.3 69.6 -1.92008 77.5 22.4 -34.82009 49.8 -38.3 11.72010 8.6 13.4 -63.32011 29.4 -48.1 16.22012 165.8 -44.9 77.82013 -51.5 58.3 36.82014 14.2 197.3 -20.12015 30.8 -20.6 48.52016 15.9 117.2 -56.2

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Table 2. Recent (2007-2015) summer (JJA) and December values of North Atlantic

Oscillation, based on Hurrell PC NAO data (NCAR 2016). NAO values are standardised

relative to the respective 1899-2015 average. Values greater than ±1.5 standard deviations

from the long-term mean are highlighted in bold.

Year JJA December2007 -1.15 1.132008 -1.33 0.572009 -2.04 -2.992010 -0.42 -3.592011 -2.09 2.622012 -1.59 -1.522013 1.73 1.872014 -0.85 1.442015 -1.61 1.73

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Figures

Figure 1. North Atlantic Oscillation December time series, 1899-2015, based on Hurrell PC NAO data (NCAR 2016). The faint black line shows the yearly values for December, and the bold black line shows a version smoothed using a five-year running mean.

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Figure 2. Mid-high northern latitude mean700 mb geopotential height (GPH) for summer (JJA) for (a) 2007-2016 and (b) 1981-2010, and for winter (DJF) for (d) 2007-2016 and (e) 1981-2010. GPH differences for (2007-2016)-(1981-2010) are shown for (c) summer and (f) winter. Note different scales on top two sets of plots for summer/winter. These plots were produced using NCEP/NCAR Reanalysis data (Kalnay et al. 1996).

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Figure 3. Arctic (land stations north of 60° N) and global mean annual land surface air temperature (SAT) anomalies (in °C) for the period 1900-2016 relative to the 1981-2010 mean value. Note that there were few stations in the Arctic, particularly in northern Canada, before 1940. The data are from the CRUTEM4 dataset, which is available at www.cru.uea.ac.uk/cru/data/temperature/. Reproduced from Overland et al. (2016a) Arctic Report Card, Surface air temperature section.

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Figure 4. Arctic near-surface (925 mb) air temperature anomalies for (a) winter (DJF) 2015/16 and (b) autumn (SON) 2016. These plots were produced using NCEP/NCAR Reanalysis data (Kalnay et al. 1996).

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Figure 5. Arctic Amplification-mid latitude extreme weather linkages schematic from Jennifer Francis, adapted from Overland et al. (2015). The dashed box indicates residual uncertainty regarding the causal physical mechanisms of such linkages, which are a subject of intense ongoing research (Overland et al. 2016b).

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Figure 6. Jet-stream/NAO drivers schematic for (a) winter and (b) summer, adapted and updated from Hall et al. (2015), showing drivers mentioned in the text. A red arrow indicates a positive association while a blue arrow indicates a negative association. The “+” sign indicates the combined influence of solar and QBO variability and the black arrow indicates a variable sign of association dependent on the particular combination of solar and QBO phases.

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