Abrupt Changes in Arctic Sea Ice

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Abrupt Changes in Arctic Sea Ice

• Marika Holland
• NCAR

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Outline

• Why sea ice?
• Present-day observed conditions and change
• An example of simulated abrupt transitions
• Future climate projections
• Application to paleo-climate conditions?
• Considerations when using models to study Arctic
  change

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Why sea ice?

• Influence of sea ice on climate
• Modifies surface energy budget
• Albedo effects
• Ice/snow albedo of 0.6-0.8 compared to ocean
  albedo of 0.1
• Insulates atmosphere from ocean
• Modifies heat and water transfer
• Affects ocean freshwater distribution
• Salinity rejected during ice growth
• Freshwater released during ice melt
• Transport of sea ice redistributes water

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Role of sea ice as an amplifier
VAvariable albedo FAfixed albedo
(From Hall, 2004)
(DJF SAT)

• Surface albedo feedback amplifies climate
  perturbations
• Ice/snow albedo of 0.6-0.8 vs ocean albedo of
  0.1

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Role of sea ice as an amplifier
SAT Difference
SST
SST
Reduced Ice
LGM
From Li et al., 2005
Insulating effect of sea ice contributes to large
atmospheric response to sea ice changes.
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• Processes Involving ice/ocean FW exchange
• In warmer climate, increased ice growth due to
  loss of insulating ice cover results in
• Increased ocean ventilation
• Ocean circulation changes
• Transient response

Change in Ice growth rates at 2XCO2
cm
Change in Ocean Circulation
Change in Ideal Age at 2XCO2
Change in Ideal age at 2XCO2
Yr 40-60
From Bitz et al., 2006
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Observed Arctic Conditions
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The observed Arctic sea ice
Fowler, 2003
Sea ice concentration
June 6, 2005
(Perovich, 2000)
(NSIDC, 2005)
Observed thickness Laxon et al., 2003
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• Observations indicate large changes in Arctic
  summer sea ice cover

Sept Ice Extent
Trend 7.7 per decade
1980
2000
From Stroeve et al., 2005
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Suggestions that ice has thinned
Ice draft change 1990s minus (1958-1976)
Rothrock et al., 1999
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Indications that Arctic Ocean is warming

• Pulse-like warming events entering and tracked
  around the Arctic
• General warming of the Atlantic layer

Polyakov et al., 2005
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North Atlantic Oscillation Positive Phase
(From University of Reading webpage)
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Timeseries of JFM NAO Index
Maybe it is not all the NAO/AO?
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Have led to suggestions that
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Future ProjectionsWhat can models tell us?
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Future climate scenarios

• Relatively gradual forcing.
• Relatively gradual response in global air
  temperature

Meehl et al, 2005
Wigley, 2000
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September Sea Ice Conditions

• Gradual forcing results in abrupt Sept ice
  transitions
• Extent from 80 to 20 coverage in 10 years.
• Winter maximum shows
• Smaller, gradual decreases
• Largely due to decreases in the north
  atlantic/pacific

Abrupt transition
Observations Simulated 5-year running mean
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Forcing of the Abrupt Change
Dynamic

• Change thermodynamically driven
• Dynamics plays a small stabilizing role

Thermodynamic
Change in ice area over melt season
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Processes contributing to abrupt change
OW formation per cm ice melt
March Arctic Avg Ice Thickness (m)

• Increased efficiency of OW production for a given
  ice melt rate
• As ice thins, vertical melting is more efficient
  at producing open water
• Relationship with ice thickness is non-linear

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Processes contributing to abrupt change Albedo
Feedback
cm/day

• Increases in basal melt occur during transitions
• Driven in part by increases in solar radiation
  absorbed in the ocean as the ice recedes

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Processes contributing to abrupt
changeIncreasing ocean heat transport to the
Arctic
Increases in ocean heat transport occur during
the abrupt transition. Contributes to increased
melting and provides a trigger for the event.
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Changes in Ocean Heat Transport
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Arctic Ocean Circulation Changes
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• Processes Involving ice/ocean FW exchange
• In warmer climate, increased ice growth due to
  loss of insulating ice cover results in
• Increased ocean ventilation
• Ocean circulation changes
• Transient response

Change in Ice growth rates at 2XCO2
cm
Change in Ocean Circulation
Change in Ideal Age at 2XCO2
Change in Ideal age at 2XCO2
Yr 40-60
From Bitz et al., 2006
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Both trend and shorter-timescale variations in
OHT appear important
OHT natural variations partially wind
driven. Correlated to an NAO-type pattern in SLP
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Mechanisms Driving Abrupt Transition

• Transition of ice to a more vulnerable state
• thinning of the ice
• A Trigger - rapid increases in ocean heat
  transport.
• Other natural variations could potentially play
  the same triggering role?
• Positive feedbacks that accelerate the retreat
• Surface albedo feedback
• OHT feedbacks associated with changing ice
  conditions

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Impacts of Abrupt Ice Transitions on Other
Aspects of the Climate System
Using the model to assess
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Associated atmospheric conditions
Winter air temperature increases rapidly during
abrupt ice change Arctic region warms 5C in 10
years in December Changes are particularly large
along the Eurasian coast
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Precipitation Changes
2040-2049 minus 1990-1999

• Precipitation generally increases over the
  20th-21st centuries
• Rate of increase is largest during the abrupt sea
  ice transition

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Projections of Near-surface Permafrost
Courtesy of Dave Lawrence, NCAR (Lawrence and
Slater, 2005)
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How common are abrupt transitions?
September Ice Extent
Obs
Simulated 5yr running mean
Abrupt transition
Transitions defined as years when ice loss
exceeds 0.5 million km2 in a year
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How common are forcing mechanisms?
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How common are effects?
Lagged composites relative to initiation of
abrupt sea-ice retreat event
Arctic Land Area
Courtesy of David Lawrence, NCAR
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Role of sea ice for abrupt transitions in a
paleoclimate context?
GISP2, Greenland
-30
Temperature (?C)
-40
d18O (per mil SMOW)
-50
-60
x1000 years ago
(slide courtesy of Carrie Morrill)
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Simulated abrupt transitions in sea ice
abrupt forcing (freshwater hosing) can result in
abrupt ice changes
Sea ice change
SAT Change
(From Vellinga and Wood, 2002 Vellinga et al,
2002)

• Sea ice changes amplify climate response
• Global teleconnections can result
• Longevity of these changes are an issue

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Some Cautions in Using Models to Examine these
(and other) issues
Models provide a wonderful tool for examining
climate feedbacks, mechanisms, etc BUT
Biases in simulated control state can affect
feedback strength Uncertainties in model
physics/response Acknowledgement that model
physics matters for simulated feedbacks
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ITD Influence on Albedo Feedback
ITD (5 cat) 1 cat. 1cat tuned
Strength of albedo feedback in climate change
runs
(Holland et al., 2006)

• Model physics influences simulated feedbacks
• Getting the processes by which sea ice amplifies
  a climate signal right can be important for our
  ability to simulate abrupt change

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Feedbacks contribute to Arctic amplification
But, that amplification varies considerably among
models
(Holland and Bitz, 2003)
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Sea ice in fully coupled GCMs
IPCC AR4 1980-1999 ice thickness Red line marks
observed extent
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Importance of sea ice state for the magnitude of
polar amplification
(From Holland and Bitz, 2003)

• Magnitude of polar amplification is related to
  initial ice thickness
• With thinner initial ice, melting translates more
  directly into open water formation and consequent
  albedo changes

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SAT Change at end of 21st century
From A1B scenario
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Aspects of the Models Internal Variability
Model Standard Deviation
Model 1 1.93
Model 2 1.90
Model 3 1.72
Model 4 1.68
Model 5 0.42
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Summary

• Sea ice plays an important role in the climate
  system and is an effective amplifier of climate
  perturbations
• due to surface albedo changes
• due to ice/ocean/atm exchange processes, OHT
  changes
• Observations indicate recent changes in the
  Arctic system
• Climate models indicate continued change into the
  foreseeable future and suggest abrupt reductions
  in the Arctic ice cover
• These studies have possible implications for
  paleo-climate transitions
• Climate models are a useful tool for exploring
  the mechanisms that may contribute to rapid
  climate transitions, but need to be used with
  some caution

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Importance of sea ice state for location of
warming

• Models with more extensive ice cover obtain
  warming at lower latitudes
• The location of warming can modify the influence
  of changes on remote locations

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• Increased Arctic Ocean heat transport occurs even
  while the Atlantic MOC weakens

20th Century
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Do other models have abrupt transitions?
Some do
Data from IPCC AR4 Archive at PCMDI
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Processes involving ice/ocean FW exchange
Change in poleward ocean heat transport at 2XCO2
conditions
Change in Arctic OHT
(Bitz et al., 2006)
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• By end of 21st century
• SAT consistently warms
• SLP changes are evident

Meehl et al, 2005
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OHT and polar amplification
Change in poleward ocean heat transport at 2XCO2
conditions
DOHT
Both control state and change in OHT are
correlated to polar amplification
(From Holland and Bitz, 2003)
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Sea ice representation in GCMs
SW
Basal heat flux

• Motionless Slab of uniform thickness
• Slab of uniform thickness in motion
• A thickness distribution of slabs in motion,
  ridging/rafting parameterized

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Changes in sea ice model representation over last
5 Years