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New metrics and evidence are presented that support a linkage between rapid Arctic warming, relative to Northern hemisphere mid-latitudes, and more frequent high-amplitude (wavy) jet-stream configurations that favor persistent weather patterns. We find robust relationships among seasonal and regional patterns of weaker poleward thickness gradients, weaker zonal upper-level winds, and a more meridional flow direction. These results suggest that as the Arctic continues to warm faster than elsewhere in response to rising greenhouse-gas concentrations, the frequency of extreme weather events caused by persistent jet-stream patterns will increase.
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Environ. Res. Lett. 10 (2015) 014005 doi:10.1088/1748-9326/10/1/014005
Evidence for a wavier jet stream in response to rapid Arctic warming
Jennifer A Francis
and Stephen J Vavrus
Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey, USA
Center for Climatic Research, University of Wisconsin-Madison, Madison, Wisconsin, USA
Keywords: jet stream, Arctic amplication, extreme weather
New metrics and evidence are presented that support a linkage between rapid Arctic warming, relative
to Northern hemisphere mid-latitudes, and more frequent high-amplitude (wavy) jet-stream cong-
urations that favor persistent weather patterns. We nd robust relationships among seasonal and
regional patterns of weaker poleward thickness gradients, weaker zonal upper-level winds, and a more
meridional ow direction. These results suggest that as the Arctic continues to warm faster than else-
where in response to rising greenhouse-gas concentrations, the frequency of extreme weather events
caused by persistent jet-stream patterns will increase.
This paper builds on the proposed linkage between
Arctic amplication (AA)dened here as the
enhanced sensitivity of Arctic temperature change
relative to mid-latitude regionsand changes in the
large-scale, upper-level ow in mid-latitudes [1,2].
Widespread Arctic change continues to intensify, as
evidenced by continued loss of Arctic sea ice [3];
decreasing mass of Greenlands ice sheet [4], rapid
decline of snow cover on Northern hemisphere
continents during early summer [5], and the contin-
ued rapid warming of the Arctic relative to mid-
latitudes. While these events are driven by AA, they
also amplify it: melting ice and snow expose the dark
surfaces beneath, which reduces the surface albedo,
further enhances the absorption of insolation, and
exacerbates melting. Expanding ice-free areas in the
Arctic Ocean also lead to additional evaporation that
augments warming and Arctic precipitation [6].
Traditionally AA is measured as the change in sur-
face air temperature in the Arctic relative to either the
Northern hemisphere or the globe [7]. It arises owing
to a variety of factors, including the loss of sea-ice and
snow, increased water vapor, a thinner and more frac-
tured ice cover, and differences between the Arctic and
lower latitudes in the behavior of lapse-rate and radia-
tive feedbacks [813]. Here we do not address the rela-
tive importance of various factors causing AA, but it is
clear from the height-latitude anomalies of air tem-
perature, geopotential, and zonal wind (gure 1) that
AA results in large part from near-surface heating,
although contributions from poleward heat transport
may also play a role [14].
Seasonal time series and trends in AA based on two
metrics and varying initial years are presented in
gure 2. The more traditional method of assessing AA
is to subtract changes in near-surface (1000 hPa) air
temperature anomalies in mid-latitudes (6030°N)
from those in the Arctic (left side of gure 2). A posi-
tive value of AA indicates that the Arctic is warming
faster than mid-latitudes. Both the time series and
progressive 15 year trends (gure 2, bottom) indicate
an increasingly positive AA in all seasons, particularly
in fall and winter, in agreement with previous analyses
[8]. Starting in the 1990s, coincident with an acceler-
ated decline in Arctic sea-ice extent [3], AA values and
trends became positive in all four seasons for the rst
time since the beginning of the modern data record in
the late 1940s, illustrating the Arctics enhanced sensi-
tivity to global warming.
The right side of gure 2presents an alternative
metric for AA based on the difference in the
1000500 hPa thickness change in the Arctic relative
to that in mid-latitudes (same zones as for the tradi-
tional method). Arguably the thickness difference is
more relevant for assessing the effects of AA on the
large-scale circulation, as it represents differences in
warming over a deeper layer of the atmosphere that
should more directly inuence winds at upper levels.
Several recent autumns have exhibited strong warm-
ing anomalies in some mid-latitude areas, contribut-
ing to the weakened positive trend after 2007. It is
4 November 2014
11 December 2014
6 January 2015
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important to note the recent emergence of the signal of
AA from the noise of natural variability: since 1995
near the surface and since 2000 in the lower tropo-
sphere. This short period presents a substantial chal-
lenge to the detection of robust signals of atmospheric
response amid the noise of natural variability [15,16].
Thus for this study we dene the period from 1995 to
2013 as the AA era.While this demarcation is con-
sistent with previous studies [17], we also investigate
the effects of choosing different commencement years
on detecting changes in the frequency of high-ampli-
tude jet-stream congurations.
Figure 1. Annual-mean anomalies in air temperature (left), geopotential (middle), and zonal winds (right) during 19952013 relative
to 19812010 for 4080°N and 1000250 hPa. Data were obtained from the NCEP/NCAR Reanalysis at
Environ. Res. Lett. 10 (2015) 014005 J A Francis and S J Vavrus
The following linkage between AA and mid-lati-
tude weather patterns has been hypothesized [1].
Increasing AA weakens the poleward temperature
gradienta fundamental driver of zonal winds in
upper levels of the atmospherewhich causes zonal
winds to decrease, following the thermal wind rela-
tionship [18]. A weaker poleward temperature gra-
dient is also a signature of the negative phase of the
so-called Arctic oscillation/Northern annular mode
(AO/NAM), in which weaker zonal winds are asso-
ciated with a tendency for a more meridional ow,
blocking, and a variety of extreme weather events in
much of the extratropics [19]. Disproportionate
Arctic warming and sea-ice loss favor a negative
AO/NAM aloft [1,2,20,21] and a Northward
migration of ridges in the upper-level ow [1], fur-
ther contributing to an increased meridional pat-
tern. As the wave amplitude and/or frequency of
amplied ow regimes increases, the incidence of
blocking becomes more likely [2], which reduces
the Eastward propagation speed of the pattern.
Consequently, the associated weather systems per-
sist longer in a particular area. Extreme weather
events caused by prolonged weather conditions
(such as cold spells, stormy periods, heat waves, and
droughts), therefore, should also become more
likely, as illustrated by recent studies linking these
events to high-amplitude planetary waves [2224].
Because AA is strongest in fall and winter
(gure 2), the atmospheric response is expected to be
largest and observed rst in these seasons. Results cor-
roborate this expectation [1], showing a marked
reduction in the poleward thickness gradient and
weaker zonal winds at 500 hPa during fall (OND) and
winter (JFM) since 1979 over the North America/
North Atlantic study region. Others nd statistically
signicant decreases in zonal-mean zonal winds in the
fall but not in winter [15].
Marked spatial and seasonal variability in the
changing poleward thickness gradient dictates pat-
terns of change in zonal winds. While hemisphere-
mean, mid-latitude, zonal winds at 500 hPa have
decreased by about 10% since 1979 during fall [23], no
robust hemispheric trends are apparent in other sea-
sons owing to the spatial variability of the AA signal.
The objectives of the present study are to examine
regional and seasonal expressions of AA that produce
changes in poleward thickness gradients, correspond-
ing effects on zonal wind speeds, and the hypothesized
increase in highly amplied jet-stream regimes.
Recent studies have presented a mixed picture regard-
ing this atmospheric response to AA. Some observa-
tional analyses nd evidence of increased wave
amplitude in certain locations and seasons, but statis-
tical signicance is often lacking [1,15,16], likely
owing to the recent emergence of AA from natural
Figure 2. Arctic amplication seasonal time series (a), (b) and trends (c), (d) based on two metrics: (left) differences in 1000 hPa
temperature anomalies (relative to 19482013 mean, °C) between the Arctic (7090°N) and mid-latitudes (3060°N); (right)
differences in 1000500 hPathickness anomalies (percentof meanfor eachzone) betweenthe twozones. Contoured colorsin c,d
indicate moving 15 year trends (C/decade for T1000 and percent of mean per decade) for each season. Thex-axis indicatesthe ending
year of each 15 year period between 1949 and 2013, and asterisks indicate ending year of periods with signicant trends (condence
>95%, assessed using an F-test goodness of t). Data were obtained from the NCEP/NCAR Reanalysis at
Environ. Res. Lett. 10 (2015) 014005 J A Francis and S J Vavrus
variability. Analyses based on climate model simula-
tions are challenged by the sometimes unrealistic
representations of complex Arctic physics and non-
linear atmospheric dynamics. Nevertheless, they, too,
suggest a more meridional ow (often resembling the
negative phase of the AO/NAM) in response to sea-ice
loss [25], and none suggests that the ow will become
more zonal or that planetary waves will decrease in
amplitude. Measuring changes in the strength of the
zonal wind is straightforward, whereas quantifying the
wavinessof the circulation is not. We therefore aim
to shed further light on this critical aspect of the link-
age by using new techniques to measure the waviness
of the upper-level ow, and we also comment on the
results of previous efforts to diagnose changes in wave
Seasons are dened as follows: winter (JFM),
spring (AMJ), summer (JAS), and fall (OND). These
denitions are selected to coincide with the summer
minimum and winter maximum of Arctic sea-ice
extent, as well as the onset times of freeze and melt. All
data are from the NCEP/NCAR Reanalysis (NRA)
[26] obtained at
Analysis of 500 hPa height contours
A simple new method was introduced to assess the
daily meridional amplitude of waves in the upper-level
ow [1]. A single contour in the 500 hPa height eld
was selected based on its climatological position within
the strongest gradient, thus representing the path of
the polar jet stream on any individual day. The
planetary wave locations and shapes depicted by height
elds at 500 hPa and those at typical heights of the jet
stream maximum (250 hPa) are very similar. The
selected heights of individual contours vary slightly
with season to match climatological jet-stream loca-
tions: within 50 m of 5600 m during the cool/cold
seasons (JFM, AMJ, OND), and 5700 m during
summer months (JAS). Daily height contours are
subsetted from daily mean 500 hPa elds from the
NRA. Correlations of 500 hPa height anomalies
between NRA and either the NCEP Climate Forecast
System Reanalysis or the European Centre for Med-
ium-Range Forecasts Interim values are over 0.99 in all
seasons (not shown), suggesting that mid-tropo-
spheric height elds in NRA are nearly identical to
those of other reanalyses. Small differences in blocking
statistics among various reanalyses have also been
reported [27].
The selection of particular 500 hPa height con-
tours used for analysis of wave amplitude [1] has been
questioned by assertions that the proper contours to
use should be those exhibiting the greatest degree of
waviness [15]. This study reproduced the increased
wave amplitude in the 5600 m contour from 1979 to
2010, but the same analysis based on the contour iden-
tied as the waviest (5300 m) exhibited no increase in
amplitude. As illustrated in gures 3(a) and (b), how-
ever, the mean latitude of the 5300 m contour during
fall (19802011) is nearly 15° of latitude farther North
than the mean latitude of the 5600 m contour. More-
over, its more Northerly position is far from the core
of strongest upper-level winds and thus its location
differs substantially from the path of the jet stream.
Winds well North of the jet are substantially weaker
(gures 3(a) and (c)), consequently is it not surprising
that the ow is wavier. Arguably, the analysis of the
more Northerly 5300 m contour does not capture the
location and evolution of the polar jet stream, while
the 5600 m contour more closely tracks the shape of
planetary waves in the strongest upper-level ow.
Note that the mean latitude of the strongest zonal
500 hPa winds is nearly identical in both the AA era
(gure 3(a)) and in earlier years (gure 3(c)), suggest-
ing that contours used here and previously [1] repre-
sent the jet-stream location throughout the satellite
record (since 1979).
Meridional circulation index (MCI)
A key outstanding question in the proposed linkage
between AA and jet-stream behavior is whether
weakened poleward thickness gradients are causing
the upper-level ow to become wavier. One measure
of ow waviness is the ratio of the meridional (North/
South) wind component to the total wind speed. We
propose a simple metric to assess this characteristic of
the ow: the MCI:
MCI *,
where uand vare the zonal and meridional compo-
nents of the wind. When MCI = 0, the wind is purely
zonal, and when MCI = 1 (1), the ow is from the
South (North). We note that a more meridional ow
can result from either a stronger vand/or weaker u
wind component through simple vector geometry.
Whatever the cause, an increase in |MCI|indicates a
wind vector aligned more NorthSouth and reects a
changed ow direction. The speed of the meridional
(v) wind may not change, as it is associated with East
West temperature gradients, but if the total wind
vector becomes more meridional, then the ow is by
denition wavier. For example, a Northwesterly wind
could shift to a NorthNorthwesterly wind solely
through a reduction of the Westerly wind component.
For this analysis, MCI is calculated from daily 500 hPa
wind components between 20°N and 80°N at each
gridpoint in NCEP Reanalysis elds.
Coincident anomalies in thickness, zonal
winds, and MCI
In an effort to assess the effects of AA on waviness of
upper-level winds, we compare coincident seasonal
Environ. Res. Lett. 10 (2015) 014005 J A Francis and S J Vavrus
anomalies during the AA era relative to the period
from 19812010. Anomalies in the 1000500 hPa
thicknesses are presented in the top panels of gures
47. During fall (OND, gure 7(a)), when sea-ice loss
exerts its largest direct impact, the pattern of AA
extends across much of the Central Arctic, while
during spring (AMJ, gure 5(a)) and summer (JAS,
gure 6(a)) the areas of positive thickness differences
occur primarily over high-latitude land, likely in
response to earlier snow melt [5]. In all seasons,
positive thickness differences are evident in the North-
west Atlantic. This substantial regional and seasonal
variability illustrates the challenge in detecting robust
hemispheric-mean atmospheric responses to AA,
resulting in the low statistical signicance reported in
some previous studies [15,16,28].
The middle panels of gures 47present anoma-
lies in zonal wind speeds at 500 hPa corresponding to
anomalies in the poleward gradient of 1000500 hPa
thicknesses (top panels). Anomalies in |MCI|are
shown in the bottom panels. Immediately obvious is
the close association between the spatial patterns of
weakened poleward gradients (regions where positive
anomalies occur Northward of weaker or negative
anomalies) and areas where zonal winds are weaker.
During winter and autumn (gures 4and 7)a
broad area of substantially weakened poleward
gradient is evident across much of the Northern hemi-
sphere mid-latitudes, particularly in the N. Atlantic
and Northern Eurasia. These areas are closely matched
by the spatial pattern of slower zonal winds, as would
be expected according to the thermal wind relation-
ship. Widespread positive anomalies in |MCI|also cor-
respond to these regions. Changes in the meridional
wind speed, however, are not correlated with either
the changes in poleward gradient or zonal winds, sug-
gesting that changes in |MCI|arise mostly because of
changes in the zonal wind speed. These ndings sup-
port the hypothesis that AA causes a more meridional
character to the upper-level wind ow, but this change
is achieved primarily via a reduction in Westerly winds
rather than through an increase in meridional wind
Relationships between these variables on a grid-
point-by-gridpoint basis are illustrated in scatterplots
(gure 8). Gridpoints with weaker (stronger) pole-
ward gradients tend to have larger (smaller) |MCI|
values (red scatter-plots), particularly for gridpoints
with the strongest (top decile) total winds, indicative
of the jet stream. In all seasons, robust relationships
between anomalies in the poleward gradient, zonal
winds, and |MCI|are evident. Moreover, in spring,
summer, and fall, the anomaly in 500 hPa zonal winds
accounts for a much larger fraction of the variance in
Figure 3. Zonal-mean zonal winds for fall (OND) from 1995 to 2013 (a) and from 1979 to 1995 (c). Corresponding zonal-mean
geopotential heights for 19792013 are shown in (b). Dotted horizontal lines highlight the 500 hPa level, white dashed vertical lines
indicate the latitude of maximum mean zonal winds at 500 hPa, and yellow dashed lines are the latitude of the waviest 500 hPa
contour, as identied in [15].
Environ. Res. Lett. 10 (2015) 014005 J A Francis and S J Vavrus
|MCI|than does the anomaly in the meridional wind
component (variance explained by U500 in JFM, AMJ,
JAS, OND = 0.42, 0.33, 0.61, 0.38; by V500 = 0.59,
0.02, 0.07, 0.001), suggesting that weakened zonal
winds due to AA are the main factor driving the more
meridional ow in these seasons. We also note that
correlations between differences in 500 hPa mer-
idional wind speeds with either the differences in pole-
ward thickness gradient or zonal wind speed were
small and insignicant, suggesting that changes in the |
MCI|arise primarily because of changes in zonal wind
Extreme wave frequency
One aspect of the proposed linkage [1] that heretofore
has been difcult to assess is whether the amplitude of
planetary waves is increasing in response to strength-
ening AA. An alternative metric that we pursue here is
the frequency of highly amplied jet-stream cong-
urations. Single contours of daily mean 500 hPa height
elds are used to identify extreme wavesin the jet
stream. Representative contours (5600 m ± 50 m,
except 5700 m ± 50 m in JAS) are selected to represent
the streamline of the strongest 500 hPa winds as
discussed previously, and we note that the selected
contours shift little in latitude with time (gure 3).
Data are analyzed in various longitude zones to
identify days in which the difference between the
maximum and minimum latitudes (ridges and
troughs) of the contour within a region exceeds 35° of
latitude. The threshold of 35° was selected to achieve a
frequency of approximately 20 days per season
(20%). Note that individual high-amplitude events,
Figure 4. Anomalies in winter (JFM) (a) 1000500 hPa thickness (m), (b) zonal wind at 500 hPa (m s
), and (c) the absolute value of
the MCI during 19952013 relative to 19812010. Data were obtained from NOAA/ESRL at
Environ. Res. Lett. 10 (2015) 014005 J A Francis and S J Vavrus
such as blocks and cut-off lows, often persist for
several days, thus the frequency of events < frequency
of high-amplitude days.
The frequency of occurrence of high-amplitude
days is assessed in each season and for the AA era
(19952013) relative to the pre-AA period
(19791994). We repeat the analysis using two addi-
tional denitions of the AA era19902013 and
20002013to determine the sensitivity of differ-
ences in high-amplitude days to the time period selec-
ted for the AA era. The mean differences in frequency
between these periods for each season and in selected
regions are presented in table 1. Changes in frequency
are expressed as a percentage relative to the pre-AA
period. We also assess the choice of comparative years
by randomly selecting 100 sets of a number of years
from the pre-AA period corresponding to the length of
each AA era, then calculating the standard deviation of
the extreme-wave frequency in each set. Changes in
frequency from the pre-AA period to the AA era that
exceed one (two) standard deviation(s) are indicated
by an underscore (asterisk).
The changes in frequency are predominantly posi-
tive, indicating more frequent occurrences of highly
amplied jet-stream congurations in the AA era. Sea-
sonal and regional variations are generally consistent
with the spatial patterns of anomalies in poleward
thickness gradients shown in gures 47, particularly
the most robust positive trends in extreme waves over
the Atlantic and North American regions. We nd a
statistically signicant negative correlation (Spear-
mans correlation = 0.30, >90% condence)
between the seasonal, regional-mean change in thick-
ness gradient and the change in extreme-wave fre-
quency. The autumn particularly stands out in table 1,
with increases in extreme waves in all of the categories
representing the post-AA period (19952013 and
20002013), as would be expected because fall exhibits
the largest and most regionally consistent signal of AA.
The Atlantic and North American regions also stand
Figure 5. Same as gure 4but for spring (AMJ).
Environ. Res. Lett. 10 (2015) 014005 J A Francis and S J Vavrus
out, with increased frequencies in all post-AA cate-
gories. Decreased frequencies during Asian summer
are consistent with recent cooling in North-Central
Asia (gure 6), which strengthens the poleward gra-
dient, drives stronger zonal winds, and results in a
decreased |MCI|. Overall, the pattern of frequency
change is consistent with expectations of a more
amplied jet stream in response to rapid Arctic warm-
ing. Amplied jet-stream patterns are associated with
a variety of extreme weather events (i.e., persistent
heat, cold, wet, and dry) [22], thus an increase in
amplied patterns suggests that these types of extreme
events will become more frequent in the future as AA
continues to intensify in all seasons. These results may
also provide a mechanism to explain observed associa-
tions between sea-ice loss and continental heat waves
[23,29], cold spells [24,30,31], heavy snowfall [2],
and anomalous summer precipitation patterns in Eur-
ope [32].
Discussion and conclusions
The Arctic has warmed at approximately twice the rate
of the Northern mid-latitudes since the 1990s owing to
a variety of positive feedbacks that amplify green-
house-gas-induced global warming. This dispropor-
tionate temperature rise is expected to inuence the
large-scale circulation, perhaps with far-reaching
effects. The North/South temperature gradient is an
important driver of the polar jet stream, thus as rapid
Arctic warming continues, one anticipated effect is a
slowing of upper-level zonal winds. It has been
Figure 6. Same as gure 4but for summer (JAS).
Environ. Res. Lett. 10 (2015) 014005 J A Francis and S J Vavrus
hypothesized that these weakened winds would cause
the path of the jet stream to become more meandering,
leading to slower Eastward progression of ridges and
troughs, which increases the likelihood of persistent
weather patterns and, consequently, extreme events
[1]. While weaker zonal winds have been observed in
response to reduced poleward temperature gradients,
the link to a wavier upper-level ow has not yet been
conrmed [31,33], although recent studies provide
strong support of a mechanism linking sea-ice loss in
the Barents/Kara Sea with amplied patterns over
Eurasia during winter [24,34] and summer [23]. We
also note that the annual-mean NOAA-tabulated
climate extreme index for the US [35] has increased by
approximately one-third in the AA era relative to pre-
AA years, though it is presently unknown whether
rapid Arctic warming is a contributing factor.
Here we provide evidence demonstrating that in
areas and seasons in which poleward gradients have
weakened in response to AA, the upper-level ow has
become more meridional, or wavier. Moreover, the
frequency of days with high-amplitude jet-stream
congurations has increased during recent years.
These high-amplitude patterns are known to produce
persistent weather patterns that can lead to extreme
weather events [22,23]. Notable examples of these
types of events include cold, snowy winters in Eastern
North America during winters of 2009/10, 2010/11,
and 2013/14; record-breaking snowfalls in Japan and
SE Alaska during winter 2011/12; and Middle-East
oods in winter 2012/2013, to name only a few.
We assess anomalies in the poleward
1000500 hPa thickness gradient during the AA era
(since the mid-1990s) relative to climatology
(19812010), along with corresponding changes in the
Figure 7. Same as gure 4but for fall (OND).
Environ. Res. Lett. 10 (2015) 014005 J A Francis and S J Vavrus
zonal winds at 500 hPa and the waviness of the
500 hPa ow (|MCI|). While these time periods are
short and certainly include effects of other natural
uctuations in the climate system, the conspicuous
emergence of AA since the mid-1990s dictates this
focused temporal analysis to identify responses of the
large-scale circulation to this newforcing. Future
work will analyze climate model projections of a future
with greater global warming and intensied AA. A
recent study [36] documents a reduction in the fre-
quency of atmospheric fronts under strong green-
house forcing, particularly in high latitudes where the
meridional temperature gradient relaxes the most,
suggestive of more persistent weather patterns.
We nd that in all seasons, the regions in which the
poleward gradient weakens also exhibit weaker zonal
winds (as expected via the thermal wind relationship)
and consequently a more meridional, or wavier, ow
character. This localized response is corroborated by
seasonally varying, regional-scale increases in the fre-
quency of amplied jet-stream congurations. The
strongest response occurs during fall, when sea-ice
loss and increased atmospheric water vapor augment
Arctic warming, and a robust response is also evident
during summer over North America and the Atlantic
sectors, when the observed rapid decline of early-sum-
mer snow cover and the lower heat capacity of land
promote a drying and warming of high-latitude land
areas. Signicant increases are observed in winter and
spring, as well. These results reinforce the hypothesis
that a rapidly warming Arctic promotes amplied jet-
stream trajectories, which are known to favor persis-
tent weather patterns and a higher likelihood of
extreme weather events. Based on these results, we
conclude that further strengthening and expansion of
AA in all seasons, as a result of unabated increases in
greenhouse gas emissions, will contribute to an
increasingly wavy character in the upper-level winds,
and consequently, an increase in extreme weather
events that arise from prolonged atmospheric
Figure 8. Seasonal scatterplots relating individual gridpoint values of anomalies during 19952013 for zonal and meridional winds at
500 hPa (U500, V500; m s
), poleward gradients in 1000500 hPa thickness (m deg
), and |MCI|(unitless). Black asterisks indicate
gridpoints corresponding to the highest 10% of total wind speed during 19952013 at 500 hPa, a proxy for the jet stream. Red asterisks
indicate correlations between black points that exceed 50% and p.001.
Environ. Res. Lett. 10 (2015) 014005 J A Francis and S J Vavrus
The authors are grateful for funding provided by the
National Science Foundations Arctic System Science
Program (NSF/ARCSS 1304097), to programming
assistance from R Kyle Zahn, and for helpful sugges-
tions from Dr John Walsh and two anonymous
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Table 1. Percentage change in seasonal frequency of high-amplitude days from the pre-AA period to the AA-era assessed using three differ-
ent initial years to dene the AA-era: 19902013 (left column under each seasonal heading), 19952013 (middle column), and 20002013
(right column). High-amplitude days are identied when the difference between the maximum and minimum latitude of the selected daily
500 hPa height contour within a specied region exceeds 35° latitude (see text for contour selections). Underlined values indicate changes
that exceed one standard deviation of wave frequencies during the pre-AA period as determined from 100 sets of randomly selected groups
of years of the same number as the corresponding AA era (i.e., 24, 19, and 14 years beginning in 1979). Underlined and asterisked values
exceed two standard deviations.
Environ. Res. Lett. 10 (2015) 014005 J A Francis and S J Vavrus
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Environ. Res. Lett. 10 (2015) 014005 J A Francis and S J Vavrus
... Firstly, as the troposphere above the Arctic warms it decreases the temperature differential between the colder air in the Arctic and the warmer air in the mid-latitudes. Since this difference drives the strength of the polar jet stream, its decline may lead to a weakened jet stream (Francis & Vavrus, 2015). A weak jet stream is more easily pushed from its typical path straight from west to east, instead becoming 'wavier' and creating north-south waves, known as Rossby waves, where high-pressure ridges move warm air north into the Arctic and force cooler, low-pressure troughs to the south (see Figure 1) (McSweeney, 2019;Lindsey, 2021). ...
... A weak jet stream is more easily pushed from its typical path straight from west to east, instead becoming 'wavier' and creating north-south waves, known as Rossby waves, where high-pressure ridges move warm air north into the Arctic and force cooler, low-pressure troughs to the south (see Figure 1) (McSweeney, 2019;Lindsey, 2021). These larger waves also contribute to more persistent, lingering weather patterns, as it takes longer for weather systems to move from west to east (Francis & Vavrus, 2015). ...
Technical Report
Full-text available
The powerful cold wave that swept over much of North America in February 2021 was a rare but not unprecedented event. In fact, similar cold waves occurred in 1989 and 2011, and the United States’ state of Texas had encountered similar problems then. Still, when the cold wave hit in 2021, it found Texas poorly prepared. Texas is the only state of mainland United States that has its own electrical grid, which intentionally does not connect to the country’s other power grids, to avoid federal regulations. This has resulted in the Texas energy supply being both isolated and largely deregulated. During previous winter storms, many generators failed, but in the Texas deregulated, market-based system, energy producers had barely any incentive to invest in cold weather protection; after all, in the average year they are much more affected by heat than by cold. In a state that prides itself on being independent and having freedom of choice, the free market and deregulation were prioritized over more disaster-resilient infrastructure.
... Isopleths were selected following Hassanzadeh et al. (2014), and we additionally performed the calculation on the isopleth used to evaluate sinuosity. 3. Meridional circulation index (MCI, Francis & Vavrus, 2015): the ratio of the square of the meridional component of the wind to the square of the total windspeed at each grid cell. 4. Local wave activity (LWA, Chen et al., 2015;Huang & Nakamura, 2016), evaluated at each latitude and longitude by iterating over latitude and (a) finding the Z500 isopleth whose equivalent latitude matches the selected latitude, then (b) integrating the geopotential difference from this Z500 value over the area between the Z500 isopleth's actual and equivalent latitudes, including cut-off highs and lows. ...
... For a given metric, the left column shows the time-means for each simulation, while the right column shows density histograms for the control (black), HKF80 (thin blue), HKF40 (thick blue), OS400-2 (thin orange), and OS400-4 (thick orange). From top to bottom, rows show: sinuosity (Cattiaux et al., 2016), wave amplitude (Hassanzadeh et al., 2014), MCI (Francis & Vavrus, 2015), LWA (Chen et al., 2015), and the LWA-E. Values of MCI, LWA, and LWA-E are averaged between 40° and 60° North and South. ...
Full-text available
Arctic amplification has been proposed to promote temperature extremes by slowing the midlatitude jet and increasing the amplitude of its meanders. Observational and modeling studies have used a variety of metrics to quantify jet waviness. These studies show conflicting changes in jet waviness depending on the metric used and period examined. Here, we evaluate common metrics for dry idealized model simulations in which we apply polar warming of varying depth and meridional extent. In all simulations, polar warming increases the spatial extent of jet meanders, but reduces the magnitudes of ridges and troughs within the wave. As a result, geometric and anomaly‐amplitude measures of jet waviness can yield opposing responses. This contrast between metrics is particularly evident when warming extends into the midlatitudes. In all simulations, midlatitude temperature anomalies weaken as the poles warm, suggesting that a wavier jet need not imply stronger temperature extremes.
... In recent decades, the warming in the Arctic region is much faster than in the other regions of the world and is termed Arctic amplification (e.g., Serreze et al. 2011;Cohen et al. 2014;Rantanen et al. 2022). It has been studied that the Arctic amplification decreases the near-surface meridional temperature gradient, hence reducing the strength of the jet stream aloft (Francis and Vavrus 2015;Cvijanovic and Caldeira 2015;Deser et al. 2015). Such conditions are conducive to atmospheric blocking formation (Francis and Vavrus 2012) which has consequences for the weather and climate extremes. ...
Full-text available
The prolonged positive phase of the Arctic Oscillation (AO) and associated quasi-stationary equivalent barotropic Euro-Atlantic blocking high (EABH) and the Siberian high (SH) existed during January 2020. The presence of the persistent quasi-stationary barotropic high resulted in higher (lower) than normal surface temperatures in most parts of northern (southern) Eurasia. The large-scale analysis suggests the detouring of the mid-latitudinal westerlies from EABH and the formation of the west–east trough from East Atlantic to Northwest Pacific across North Africa, the Middle East, North India and China. The convergence of the moisture and positive convection anomalies along the region of the trough is perceived from the analysis. In the backdrop of this large-scale circulation anomalies, higher than normal precipitation was received in most of the central and north Indian regions with thunderstorms/hailstorm events. The variations in the AO index (AOI) and EABH were found to be in concurrence with the precipitation anomalies over the Indian region. The detailed analysis of a selected thunderstorm/hailstorm case suggests that the lowering of the 0 °C isotherm due to the intrusion of mid-latitudinal westerlies and the development of atmospheric instability with moisture supply from the adjacent seas facilitated the occurrence of the thunderstorms/Hailstorm events during January 2020.
... Due to the latitudinal temperature gradient the Intertropical Convergence Zone (Prell & Kutzbach, 1992;Sirocko et al., 1993;Y. J. Wang et al., 2001) was pushed southward (equatorward) and Mid-latitude Westerly jet stream was strengthened (Archer & Caldeira, 2008;Williams & Bryan, 2006), bringing moisture via storm tracks (Davis & Brewer, 2009;Francis & Vavrus, 2015;Y. Li, Song, et al., 2016;Routson et al., 2019;Semmler et al., 2016;Shaw et al., 2016). ...
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Terrestrial cosmogenic nuclide (TCN) dating has emerged as one of the most useful techniques in the last two decades for quantifying geomorphological processes and building the chronology of late Quaternary glacial advances/retreats. The chronology based on TCN and optically stimulated luminescence (OSL) dating of glacial landforms from the northwestern (NW) Himalaya suggests that glaciers responded to a complex interaction between temperature and moisture essentially derived from either of the climate systems, the Indian Summer Monsoon (ISM) and the Mid‐latitude Westerlies (MLW). The discrepancies between the TCN ages obtained on moraine boulders/bedrock surfaces, and the OSL ages on the stratigraphically equivalent deposits, highlighted the need for a detailed investigation. The present study attempts to build the chronology of Quaternary glaciation events in the Karakoram and Ladakh Ranges using TCN dating of stratigraphically constrained moraine boulders and striated bedrock surfaces. The TCN ages from glacially eroded surfaces (GES) having prominent striations are narrowly clustered around the Marine Isotopic Stage‐2 (MIS‐2). Agreement between GES TCN ages and OSL ages on the stratigraphically equivalent moraines suggests negligible geological inheritance. The glacial advance during MIS‐2 can be attributed to the combined effect of reduction in north hemispheric insolation and enhanced westerly precipitation. However, relict non‐glacial surfaces and moraine boulders with minimal ice flow modifications yield wide age distributions, most likely suggesting denudational events (interglacials) and/or contribution from tributary valley flanks.
... A typical argument is that climate change might weaken the north-south temperature gradient on the surface leading to the jet stream meandering more actively, leading to more extremes Herein et al. 10.3389/feart.2023.1267473 (Francis and Stephen, 2015), while complicated dynamical mechanisms contribute to the mid-latitude wave amplification (Shaw et al., 2016;Sun et al., 2022). In the upper levels of the troposphere, however, the north-south temperature contrast increases, which would imply less extreme events. ...
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In view of the growing importance of climate ensemble simulations, we propose an ensemble approach for following the dynamics of extremes in the presence of climate change. A strict analog of extreme events, a concept based on single time series and local observations, cannot be found. To study nevertheless typical properties over an ensemble, in particular if global variables are of interest, a novel, statistical approach is used, based on a zooming in into the ensemble. To this end, additional, small sub-ensembles are generated, small in the sense that the initial separation between the members is very small in the investigated variables. Plume diagrams initiated on the same day of a year are generated from these sub-ensembles. The trajectories within the plume diagram strongly deviate on the time scale of a few weeks. By defining the extreme deviation as the difference between the maximum and minimum values of a quantity in a plume diagram, i.e., in a sub-ensemble, a growth rate for the extreme deviation can be extracted. An average of these taken over the original ensemble (i.e., over all sub-ensembles) characterizes the typical, exponential growth rate of extremes, and the reciprocal of this can be considered the characteristic time of the emergence of extremes. Using a climate model of intermediate complexity, these are found to be on the order of a few days, with some difference between the global mean surface temperature and pressure. Measuring the extreme emergence time in several years along the last century, results for the temperature turn out to be roughly constant, while a pronounced decaying trend is found in the last decades for the pressure.
... Furthermore, recent observational analysis based on a homogenized radiosonde dataset showed that the subtropical westerly jets on both hemispheres strengthened and moved poleward by a few degrees from 1979-2012 (Sherwood and Nishant [39] ). In the lower troposphere, by comparison, the Arctic warms faster than other mid-and low-latitude regions due to global warming, which tends to make the meridional temperature gradient much weaker and thus slow down the wind speed (Francis and Vavrus [2] ; Haarsma and Oldenborgh [41] ; Francis and Vavrus [42] ). ...
The long-term height-resolved wind trend in China under global warming still needs to be discovered. To fill this gap, in this paper we examined the climatology and long-term (1979-2021) trends of the wintertime wind speed at the near-surface and upper atmosphere in China based on long-term radiosonde measurements. At 700, 500, and 400 hPa, much higher wind speed was found over eastern China, compared with western China. At 300, 200, and 100 hPa, maximum wind speed was observed in the latitude zone of around 25-35°N. Furthermore, westerly winds dominated most parts of China between 20°N and 50°N at altitudes from 700 hPa to 100 hPa. A stilling was revealed for the near-surface wind from 1979-2003. From 2004 onward, the near-surface wind speed reversed from decreasing to increasing. This could be largely due to the joint impact of reduced surface roughness length, aerosol optical depth (AOD), and increased sensible heat flux in the ground surface. The decrease of AOD tended to reduce aerosol radiative forcing, thereby destabilizing the planetary boundary layer (PBL). By comparison, the wintertime wind in the upper atmosphere exhibited a significant monotonic upward trend, albeit with varying magnitude for different altitudes. In the upper troposphere, the wintertime maximum wind was observed along a westerly jet stream, with a pronounced upward trend within the zone approximately bounded by latitudes of 25-50°N, particularly above 500 hPa. This accelerating wind observed in the upper troposphere and lower stratosphere could be closely associated with the large planetary-scale meridional temperature trend gradient. Besides, the direction for the wind at the near-surface and lower troposphere (925 and 850 hPa) exhibited a larger variance over the period 1979-2021, which could be associated with the strong turbulence of PBL caused by the heterogeneous land surface. For those pressure levels higher than 850 hPa, large wind directional variance was merely found to the south of 25°N. The findings from long-term radiosonde measurements in winter over China shed light on the changes in wind speed on the ground and upper atmosphere under global warming from an observational perspective.
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Under the background of rapid global warming, the hydrological regime in the Arctic river basins, where permafrost is widely developed, has changed significantly. These changes not only altered the local ecological environment, but also had far-reaching impacts on the global climate system and socio-economy. Therefore, the study of hydrological processes in Arctic river basins has become a hot-spot issue at the forefront of the international scientific community. Based on a thorough review and critical analysis of domestic and international literature, this paper systematically summarizes the research findings and latest progress on the spatial and temporal changes of the runoff of major Arctic rivers, as well as the driving mechanisms behind these variations. In addition, the patterns and spatiotemporal differences in runoff changes between Eurasia and North America were analyzed in detail. Furthermore, the direct and indirect effects of precipitation changes (e.g., precipitation amount, rain/snowfall fractions) and permafrost degradation on Arctic runoff are thoroughly examined. Despite significant progress in data accumulation and scientific understanding in current Arctic hydrological research, considerable challenges persist, such as the scarcity of ground observations and the difficulty of quantitatively assessing the interactions among climate, snow/permafrost, and hydrological processes. Thus, establishing a robust observation network in the Arctic river basins and developing cold region hydrological models with account for the Arctic specifics are fundamental for gaining in-depth insights into the rapid changes occurring in the Arctic hydrological system. This is also crucial for addressing the risks of water-related disasters and enhancing water resource management in the Arctic region.
One of the triggers of the Arctic amplification is the poleward heat transport; however, the relative contribution of its oceanic and atmospheric components remains poorly understood. In this study, we investigate the role of the Atlantic meridional oceanic and atmospheric heat fluxes to the Arctic for the emergence of the Arctic amplification. The meridional heat fluxes were computed using ORAS4 and ERA5 reanalyses for the Atlantic Water layer and over the lower troposphere. The horizontal and vertical boundaries of the sections in the ocean and atmosphere were selected to include only the areas, where the time mean heat transport is directed into the Arctic. It is found that the meridional heat fluxes and the regional Arctic amplification in the Eurasian Arctic are strongly coupled on the decadal time scales (10–15 years). We argue that the low-frequency variability of the Arctic amplification is regulated via a feedback, which elements include the oceanic heat transport, atmospheric heat transport and Arctic amplification. An increase (decrease) in the oceanic heat transport leads to a decrease (increase) in the atmospheric heat transport and to a decrease (increase) in the Arctic amplification. The atmospheric response to the ocean forcing occurs with a delay of three years and is attributed to the Bjerknes compensation mechanism. In turn, the atmospheric heat and moisture transport directly affects the magnitude of the Arctic amplification, with the latter lagging by one year. Thus, the variability of the oceanic heat transport at the southern boundary of the Nordic Seas might be a predictor of the magnitude of the Arctic amplification over the Eurasian Basin of the Arctic Ocean with a lead time of four years.
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This study investigates the stratospheric response to Arctic sea-ice loss and subsequent near-surface impacts by analyzing 200-member coupled experiments using the Whole Atmosphere Community Climate Model version 6 (WACCM6) with preindustrial, present-day, and future sea-ice conditions specified following the protocol of Polar Amplification Model Intercomparison Project. The stratospheric polar vortex weakens significantly in response to the prescribed sea-ice loss, with a larger response to greater ice loss (i.e., future minus preindustrial) than to smaller ice loss (i.e., future minus present-day). Following the weakening of the stratospheric circulation in early boreal winter, the coupled stratosphere-troposphere response to ice loss strengthens in late winter and early spring, projecting onto a negative North Atlantic Oscillation-like pattern in the lower troposphere. To investigate whether the stratospheric response to sea-ice loss and subsequent surface impacts depend on the background oceanic state, ensemble members are initialized by a combination of varying phases of Atlantic Multidecadal Variability (AMV) and Interdecadal Pacific Variability (IPV). Different AMV and IPV states combined, indeed, can modulate the stratosphere-troposphere responses to sea-ice loss, particularly in the North Atlantic sector. Similar experiments with another climate model show that, although strong sea-ice forcing also leads to tighter stratosphere-troposphere coupling than weak sea-ice forcing, the timing of the response differs from that in WACCM6. Our findings suggest that Arctic sea-ice loss can affect the stratospheric circulation and subsequent tropospheric variability on seasonal timescales, but modulation by the background oceanic state and model dependence need to be taken into account.
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A vertically nonuniform warming of the troposphere yields a lapse rate feedback by altering the infrared irradiance to space relative to that of a vertically uniform tropospheric warming. The lapse rate feedback is negative at low latitudes, as a result of moist convective processes, and positive at high latitudes, due to stable stratification conditions that effectively trap warming near the surface. It is shown that this feedback pattern leads to polar amplification of the temperature response induced by a radiative forcing. The results are obtained by suppressing the lapse rate feedback in the Community Climate System Model, version 4 (CCSM4). The lapse rate feedback accounts for 15% of the Arctic amplification and 20% of the amplification in the Antarctic region. The fraction of the amplification that can be attributed to the surface albedo feedback, associated with melting of snow and ice, is 40% in the Arctic and 65% in Antarctica. It is further found that the surface albedo and lapse rate feedbacks interact considerably at high latitudes to the extent that they cannot be considered independent feedback mechanisms at the global scale.
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The Arctic region has warmed more than twice as fast as the global average - a phenomenon known as Arctic amplification. The rapid Arctic warming has contributed to dramatic melting of Arctic sea ice and spring snow cover, at a pace greater than that simulated by climate models. These profound changes to the Arctic system have coincided with a period of ostensibly more frequent extreme weather events across the Northern Hemisphere mid-latitudes, including severe winters. The possibility of a link between Arctic change and mid-latitude weather has spurred research activities that reveal three potential dynamical pathways linking Arctic amplification to mid-latitude weather: changes in storm tracks, the jet stream, and planetary waves and their associated energy propagation. Through changes in these key atmospheric features, it is possible, in principle, for sea ice and snow cover to jointly influence mid-latitude weather. However, because of incomplete knowledge of how high-latitude climate change influences these phenomena, combined with sparse and short data records, and imperfect models, large uncertainties regarding the magnitude of such an influence remain. We conclude that improved process understanding, sustained and additional Arctic observations, and better coordinated modelling studies will be needed to advance our understanding of the influences on mid-latitude weather and extreme events.
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We examine the evolution of sea-ice extent (SIE) over both polar regions for 35 years from November 1978 to December 2013, as well as for the global total ice (Arctic plus Antarctic). Our examination confirms the ongoing loss of Arctic sea ice, and we find significant (p< 0.001) negative trends in all months, seasons and in the annual mean. The greatest rate of decrease occurs in September, and corresponds to a loss of 3 × 106km2 over 35 years. The Antarctic shows positive trends in all seasons and for the annual mean (p<0.01), with summer attaining a reduced significance (p<0.10). Based on our longer record (which includes the remarkable year 2013) the positive Antarctic ice trends can no longer be considered 'small', and the positive trend in the annual mean of (15.29 ± 3.85) × 103 km2 a-1 is almost one-third of the magnitude of the Arctic annual mean decrease. The global annual mean SIE series exhibits a trend of (-35.29±5.75) × 103km2a-1 (p<0.01). Finally we offer some thoughts as to why the SIE trends in the Coupled Model Intercomparison Phase 5 (CMIP5) simulations differ from the observed Antarctic increases.
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Successive cold winters of severely low temperatures in recent years have had critical social and economic impacts on the mid-latitude continents in the Northern Hemisphere. Although these cold winters are thought to be partly driven by dramatic losses of Arctic sea-ice, the mechanism that links sea-ice loss to cold winters remains a subject of debate. Here, by conducting observational analyses and model experiments, we show how Arctic sea-ice loss and cold winters in extra-polar regions are dynamically connected through the polar stratosphere. We find that decreased sea-ice cover during early winter months (November-December), especially over the Barents-Kara seas, enhances the upward propagation of planetary-scale waves with wavenumbers of 1 and 2, subsequently weakening the stratospheric polar vortex in mid-winter (January-February). The weakened polar vortex preferentially induces a negative phase of Arctic Oscillation at the surface, resulting in low temperatures in mid-latitudes.
Severe winters have occurred frequently in mid-latitude Eurasia during the past decade. Simulations with a 100-member ensemble of an atmospheric model detect an influence of declining Arctic sea-ice cover.
There has been an ostensibly large number of extreme weather events in the Northern Hemisphere mid-latitudes during the past decade(1). An open question that is critically important for scientists and policy makers is whether any such increase in weather extremes is natural or anthropogenic in origin(2-13). One mechanism proposed to explain the increased frequency of extreme weather events is the amplification of mid-latitude atmospheric planetary waves(14-17). Disproportionately large warming in the northern polar regions compared with mid-latitudes-and associated weakening of the north-south temperature gradient-may favour larger amplitude planetary waves(14-17), although observational evidence for this remains inconclusive(18-21). A better understanding of the role of planetary waves in causing mid-latitude weather extremes is essential for assessing the potential environmental and socioeconomic impacts of future planetary wave changes. Here we show that months of extreme weather over mid-latitudes are commonly accompanied by significantly amplified quasi-stationary mid-tropospheric planetary waves. Conversely, months of near-average weather over mid-latitudes are often accompanied by significantly attenuated waves. Depending on geographical region, certain types of extreme weather (for example, hot, cold, wet, dry) are more strongly related to wave amplitude changes than others. The findings suggest that amplification of quasi-stationary waves preferentially increases the probabilities of heat waves in western North America and central Asia, cold outbreaks in eastern North America, droughts in central North America, Europe and central Asia, and wet spells in western Asia.
This study uses cluster analysis to investigate the interdecadal poleward shift of the subtropical and eddy-driven jets and its relationship to intraseasonal teleconnections. For this purpose, self-organizing map (SOM) analysis is applied to the ECMWF Interim Re-Analysis (ERA-Interim) zonal-mean zonal wind. The resulting SOM patterns have time scales of 4.8–5.7 days and undergo notable interdecadal trends in their frequency of occurrence. The sum of these trends closely resembles the observed interdecadal trend of the subtropical and eddy-driven jets, indicating that much of the interdecadal climate forcing is manifested through changes in the frequency of intraseasonal teleconnection patterns. Two classes of jet cluster patterns are identified. The first class of SOM pattern is preceded by anomalies in convection over the warm pool followed by changes in the poleward wave activity flux. The second class of patterns is preceded by sea ice and stratospheric polar vortex anomalies; when the Arctic sea ice area is reduced, the subsequent planetary wave anomalies destructively interfere with the climatological stationary waves. This is followed by a decrease in the vertical wave activity flux and a strengthening of the stratospheric polar vortex. An increase in sea ice area leads to the opposite chain of events. Analysis suggests that the positive trend in the Arctic Oscillation (AO) up until the early 1990s might be attributed to increased warm pool tropical convection, while the subsequent reversal in its trend may be due to the influence of tropical convection being overshadowed by the accelerated loss of Arctic sea ice.
Over the past decade, severe winters occurred frequently in mid-latitude Eurasia, despite increasing global- and annual-mean surface air temperatures. Observations suggest that these cold Eurasian winters could have been instigated by Arctic sea-ice decline, through excitation of circulation anomalies similar to the Arctic Oscillation. In climate simulations, however, a robust atmospheric response to sea-ice decline has not been found, perhaps owing to energetic internal fluctuations in the atmospheric circulation. Here we use a 100-member ensemble of simulations with an atmospheric general circulation model driven by observation-based sea-ice concentration anomalies to show that as a result of sea-ice reduction in the Barents–Kara Sea, the probability of severe winters has more than doubled in central Eurasia. In our simulations, the atmospheric response to sea-ice decline is approximately independent of the Arctic Oscillation. Both reanalysis data and our simulations suggest that sea-ice decline leads to more frequent Eurasian blocking situations, which in turn favour cold-air advection to Eurasia and hence severe winters. Based on a further analysis of simulations from 22 climate models we conclude that the sea-ice-driven cold winters are unlikely to dominate in a warming future climate, although uncertainty remains, due in part to an insufficient ensemble size.
Atmospheric fronts are important for the day-to-day variability of weather in the midlatitudes. It is therefore vital to know how their distribution and frequency will change in a projected warmer climate. Here we apply an objective front identification method, based on a thermal front parameter, to 6-hourly data from models participating in CMIP5. The historical simulations are evaluated against ERA-Interim and found to produce a similar frequency of fronts and with similar front strength. The models show some biases in the location of the front frequency maxima. Future changes are estimated using the high emissions scenario simulations (RCP8.5). Projections show an overall decrease in front frequency in the Northern Hemisphere, with a poleward shift of the maxima of front frequency and a strong decrease at high latitudes where the temperature gradient is decreased. The Southern Hemisphere shows a poleward shift of the frequency maximum, consistent with previous storm-track studies.