Polar atmospheric circulation
and its dependence on sea ice
Karen Alley
Ice affecting the atmosphere: not a new
idea, but a newly-important idea
•  Concept that has been recognized for
decades, but is becoming more important
as Arctic sea ice decreases
–  Walsh and Johnson (1979) showed that the
sea ice forcing on the atmosphere can be just
as significant as atmospheric forcing on the
sea ice in some seasons
•  Could Arctic changes foreshadow future
changes in Antarctic circulation?
Affecting the atmosphere
•  The poles serve as the world’s heat sinks,
–  Heat is transported from equatorial regions with
lots of solar radiation to polar regions with very
little solar radiation
•  Increasing surface heat flux at the poles will
affect the whole transport pattern
•  Changing sea ice changes surface heat flux
–  Leads and polynas at small-scales
–  Large-scale sea ice loss has large-scale impacts
on heat flux and therefore on atmospheric
circulation
LETTERS
TURE | Vol 464 | 29 April 2010
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Temperature trends in the
Arctic caused by
decreasing sea ice extent
Figure from Screen and Simmonds
(2010)
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Largest temperature trends are at
high
latitudes
and" low " altitudes
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d
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Demonstrates effects of increased
heat flux due to sea ice loss
Heat flux has a large effect on the
atmosphere, changing temperature,
pressure, and circulation patterns
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ure 1 | Surface amplification of temperature trends, 1989–2008.
mperature trends averaged around circles of latitude for winter
cember–February; a), spring (March–May; b), summer (June–August;
nd autumn (September–November; d). The black contours indicate
re trends differ significantly from zero at the 99% (solid lines) and 95%
tted lines) confidence levels. The line graphs show trends (same units as
olour plots) averaged over the lower part of the atmosphere
0–1,000 hPa; solid lines) and over the entire atmospheric column
0–1,000 hPa; dotted lines). Red shading indicates that the lower
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Figure 2 | Temperature trends linked to changes in sea ice. Temperature
trends over the 1989–2008 period averaged around circles of latitude for
winter (a), spring (b), summer (c) and autumn (d). The trends are derived
from projections of the temperature field on the sea ice time series (Methods
Summary). The black contours indicate where the ice–temperature
regressions differ significantly from zero at the 99% (solid lines) and 95%
(dotted lines) uncertainty levels.
radiation. In the Arctic, this greenhouse effect dominates during
autumn, winter and spring (Fig. 3), in agreement with in situ observations30. In summer, the shading effect dominates in the lower-latitude
regions of the Arctic basin whereas north of 80u N the two competing
General Aspects of Arctic and
Antarctic Circulation
Sea level pressure: Arctic
Siberian High
Aleutian Low
Icelandic Low
Mean SLP between 1966 and 1993,
from Serreze et al. 1997
•  Six main “centers of
action”
•  Three semi-permanent
centers fairly prominent
(Aleutian Low, Icelandic
Low, Siberian High)
•  Three others much
more indistinct (Azores
High, Pacific High,
Asian Low)
•  All are much stronger
during winter
Sea level pressure: Antarctic
•  Dominated by a fullycircumpolar trough at
about 66˚S mean
latitude
•  Three main centers of
low pressure at
around 20˚E, 90˚E,
and 15˚W
•  High pressure over
Antarctic continent
Cyclone patterns
Arctic
•  Regions of significant
cyclogenesis in North
Atlantic Storm Track,
Arctic Frontal Zone, Arctic
Ocean
•  Complicated storm
tracking directions
Antarctic
•  Most cyclogenesis at
Polar Front
•  Storms nearly always
track south and dissipate
in the circumpolar trough
•  Can’t really get over the
continent, because the
whole thing is a huge
orographic barrier
Arctic
Polar wind regimes
•  Mainly follow pressure
contours
•  Katabatic wind regime (cold,
density-driven drainage winds
from ice sheet) important in
Greenland
Antarctic
•  Mainly katabatic, promoting
surface divergence on
continent and strengthening
continental high
•  Turn due to Coriolis to join
geostrophic polar easterlies
near coast
Polar Lows
General characteristics
•  Short-lived – typically
3-36 hours
•  Small-scale – typically
less than 500 km in
diameter
•  Maritime features,
feeding on strong
temperature gradients
and convection over open
water
Arctic vs. Antarctic
•  Much stronger in the Arctic
–  More open water, warmer
water temperature (stronger
meridional heat transport),
higher temperature
gradients, higher heat fluxes
•  Considered a winter
phenomenon in the Arctic
–  But year-round weaker
mesocyclones, as in the
Antarctic
Major modes of circulation
affecting the Arctic
North Atlantic Oscillation (NAO)
• Compares sea level
pressure in the Arctic
(Icelandic Low) and
the North Atlantic
(Azores High)
• When both are
strong, NAO is
positive
• Primarily a
meridional pattern
Leading EOF for the North Atlantic Sector, December-March.
Contours indicate amplitude, at 0.5 hPa intervals.
From Hurrell and Deser (2009)
Arctic Oscillation (AO)/Northern
Annular Mode (NAM)
• Compares sea level
pressure in the Arctic
(Icelandic Low) with
lower-latitude centers
(Azores High and
North Pacific High)
• When all are strong,
AO is positive
• Primarily a zonal
pattern
Leading EOF for the Northern Hemisphere, December-March.
Contours indicate amplitude, at 0.5 hPa intervals.
From Hurrell and Deser (2009)
Trends in the NAO/AO
•  Both were strongly positive from
late-1970s through mid-1990s
•  Many attributed this to greenhouse gas
forcings and atmospheric warming
•  Switched to a neutral or slightly negative
mode since mid-1990s
•  Underscores lack of understanding of
NAO/AO forcings
Arctic Dipole Anomaly (DA)
•  Positive phase has anomalous high pressure
over Canadian Arctic and anomalous low
over Siberian Arctic
•  Primarily meridional pattern
•  Recent decades have seen persistent
positive phase
•  May help explain faster-than-expected sea
ice loss in the Arctic
–  Shown to be more important than a positive AO
for exporting ice from the Arctic
Relative importance of DA and AO
for exporting sea ice
•  A comparison of
the AO (a and c)
and DA (b and d)
for winter (top) and
summer (bottom)
•  Black arrows
indicate wind
anomaly, red
dashed arrows
show the Transpolar Drift Stream
•  Units are hPa
From Wang et al. (2009)
Major modes of circulation
affecting the Antarctic
Southern Annular Mode (SAM)
•  Leading empirical orthogonal function (EOF)
accounting for geopotential height changes at
several levels in the atmosphere
•  Annular (ring) structure, almost symmetrical
because Antarctica is almost circular
•  Recently in a persistent positive phase
because of stratospheric cooling from ozone
loss
Trends in the SAM
Figure from Thompson and Solomon (2002)
Southern Annular Mode vs.
Northern Annular Mode
Figure from Thompson and Wallace (2000)
El Niño/Southern Oscillation
(ENSO)
•  Tropical Pacific phenomenon with worldwide consequences
•  Oceanic component (El Niño) and
atmospheric component (Southern
Oscillation)
•  El Niño is an increase in SST near Peru
•  Southern Oscillation refers to strength of
Easterlies and Walker Circulation
From Cane (2005)
From Cane (2005)
ENSO and the poles
•  Largest source of climate variability in the
world
•  ENSO signals have been identified in both
Antarctic and Arctic sea ice fluctuations
•  Particularly important in Antarctica
–  With SAM, influences temperatures across
the continent, as well as sea ice extent
Arctic vs. Antarctic Sea Ice
Arctic sea ice seasonal trend
Historically, the winter-summer difference was not this big, and the Arctic
contained a large percentage of multi-year ice
Antarctic sea ice seasonal trend
The Antarctic has a tendency to lose most of its ice in the summer, so it is
generally dominated by first-year ice
Arctic sea ice multi-year trend
Major decreases attributed to a variety of sources, such as direct greenhouse
gas forcings, delayed response to positive AO/NAO, and positive DA
Antarctic sea ice multi-year trend
Subtle increases attributed mainly to decreases in stratospheric ozone and
consequently positive SAM
Sea ice concentration trends
1979-2010
(month of greatest change)
Arctic decrease is spatially consistent, while Antarctic increase is highly
regional. Antarctic sea ice is projected to decrease as the ozone hole recovers.
Recent atmospheric circulation
changes forced by Arctic sea ice
changes
General atmospheric changes
•  LOTS of controversy still remaining, but a few major trends
emerging
•  Arctic temperatures are increasing a lot (Arctic Amplification)
•  Significant decreases in equator-to-pole temperature
gradients decreases the strength of the westerlies, also
decreasing storm intensity
•  Storm tracks may or may not weaken in both the Atlantic and
the Pacific
•  Icelandic Low weakens and shifts to the south, while the
Aleutian Low strengthens and shifts to the East
•  General negative feedback to atmospheric perturbations in
the Atlantic, positive feedback in the Pacific
•  E.g. Murray and Simmonds (1995), Alexander et al. (2004)
Changes in modes of circulation
•  North Atlantic response may resemble negative
NAO pattern (e.g. Deser et al. 2007)
•  Overall Arctic response may be negative AO
(Cohen et al. 2005)
•  Sea ice decrease feeds
back into a positive DA
(Blüthgen et al. 2012)
•  May see a persistently
Negative NAO/AO and
a persistently negative
DA as climate warms
SLP anomalies supporting a positive DA
in the Blüthgen et al. (2012) model
Effects on mid-latitudes
•  Francis and Vavrus (2012) link Arctic sea ice
decreases to extreme weather in midlatitudes
•  Decreasing the equator-to-pole T gradients
weakens zonal winds and elongates Rossby
wave crests northward
–  These two effects slow eastward propagation of
Rossby waves
•  Since Rossby waves guide smaller-scale
weather patterns, the weather patterns move
slower more severe weather events like
droughts
Avenues for similar future research
in the Antarctic
•  A few older modeling studies have been done
(e.g. Mitchell and Hills 1986)
–  Showed that the atmospheric circulation will
change if sea ice is removed
•  Start by looking at the Antarctic Peninsula
•  Modeling studies using more modern
technology to understand teleconnections
•  SAM effects will be hard to observe until
ozone hole recovers
•  Who knows what/if anything sea ice decrease
could do to ENSO
Why do we care?
•  We don’t know much about
teleconnections
•  Teleconnections matter – change weather
and climate patterns across the globe
•  Sea ice loss can change teleconnection
patterns, so it can change our weather
•  Important factor in the quest to predict sea
level rise
Reference cited:
Alexander, M. A., Bhatt, U. S., Walsh, J. E., Timlin, M. S., Miller, J. S., & Scott, J. D. (2004). The atmospheric response to
realistic Arctic sea ice anomalies in an AGCM during winter. Journal of Climate, 17(5), 890–905.
Blüthgen, J., Gerdes, R., & Werner, M. (2012). Atmospheric response to the extreme Arctic sea ice conditions in 2007.
Geophysical Research Letters, 39(2), L02707.
Cane, M. A. (2005). The evolution of El Niño, past and future. Earth and Planetary Science Letters, 230(3-4), 227–240.
Deser, C., Tomas, R. A., & Peng, S. (2007). The Transient Atmospheric Circulation Response to North Atlantic SST and Sea Ice
Anomalies. Journal of Climate, 20(18), 4751–4767.
Francis, J. A., & Vavrus, S. J. (2012). Evidence linking Arctic amplification to extreme weather in mid-latitudes. Geophysical
Research Letters, 39(6), L06801.
Hurrell, J. W., & Deser, C. (2009). North Atlantic climate variability: The role of the North Atlantic Oscillation. Journal of Marine
Systems, 78(1), 28–41.
King, J. C., & Turner, J. (1997). Antarctic Meteorology and Climatology. Cambridge: Cambridge Univ Press.
Mitchell, J. F. B., & Hills, T. S. (1986). Sea-ice and the antarctic winter circulation: A numerical experiment. Quarterly Journal of
the Royal Meteorological Society, 112(474), 953–969.
Murray, R. J., & Simmonds, I. (1995). Responses of climate and cyclones to reductions in Arctic winter sea ice. Journal of
Geophysical Research, 100(C3), 4791–4806.
Screen, J. A., & Simmonds, I. (2010). The central role of diminishing sea ice in recent Arctic temperature amplification. Nature,
464(7293), 1334–1337.
Serreze, M. C., & Barry, R. G. (2005). The Arctic Climate System. Cambridge: Cambridge Univ Press.
Serreze, M. C., Carse, F., Barry, R. G., & Rogers, J. C. (1997). Icelandic low cyclone activity: Climatological features, linkages
with the NAO, and relationships with recent changes in the Northern Hemisphere circulation. Journal of Climate, 10(3),
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Thompson, D. W. J., & Solomon, S. (2002). Interpretation of Recent Southern Hemisphere Climate Change. Science, 296
(5569), 895–899.
Thompson, D. W. J., & Wallace, J. M. (2000). Annular Modes in the Extratropical Circulation. Part I: Month-to-Month Variability.
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record lows in Arctic summer sea ice extent? Geophysical Research Letters, 36(5), L05706.
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