using ccsm3 to investigate future abrupt arctic sea ice change
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Using CCSM3 to investigate future abrupt Arctic sea ice change
Marika Holland
NCAR
“A mechanism that might lead to abrupt climate change would need to have the following characteristics:
• A trigger or, alternatively, a chaotic perturbation, with either one causing a threshold crossing (something that initiates the event).
• An amplifier and globalizer to intensify and spread the influence of small or local changes.
• A source of persistence, allowing the altered climate state to last ...”
From Abrupt Climate Change: Inevitable Surprises (2002)
“an abrupt climate change occurs when the climate system is forced to cross some threshold, triggering a transition to a new state at a rate … faster than the cause.”
Role of sea ice as an “amplifier”
• Surface albedo feedback amplifies climate perturbations• Models have been used to explore/quantify these effects.
(From Hall, 2004)
VA=variable albedoFA=fixed albedo
(DJF SAT)
Observed Arctic Conditions
Sea ice concentration
(NSIDC, 2005)
(Perovich, 2000)
Fowler, 2003
Observed thicknessLaxon et al., 2003
The observed Arctic sea ice
June 6, 2005
Observations indicate large changes in Arctic summer
sea ice cover
From Stroeve et al., 2005
2002 2003 2004
1980 2000
Sept Ice Extent
Trend = 7.7% per decade
Suggestions that ice has thinned…
Rothrock et al., 1999
Ice draft change 1990s minus (1958-1976)
Indications that Arctic Ocean is warming
Polyakov et al., 2005 1900 2000
Atlantic Layer Temperature
• “Pulse-like” warming events entering and tracked around the Arctic
• General warming of the Atlantic layer
North Atlantic Oscillation Positive Phase
From University of
Reading webpage
JFM NAO Index1950-1992
Timeseries of JFM NAO Index
Maybe it is not all the NAO/AO?
Have led to suggestions that:
“Researchers estimate that in as little as 15 years, the Arctic could be ice free in the summer”
J Climate, 2005
Overpeck et al., 2005
“There is no paleoclimate evidence for a seasonally ice free Arctic during the last 800 millennia”
Overpeck et al.
Future ProjectionsWhat can models tell us?
Future climate scenarios
Meehl et al, 2005Wigley, 2000
• Relatively gradual forcing. • Relatively gradual response in global air temperature
September Sea Ice Conditions
ObservationsSimulated5-year running mean
• 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
Forcing of the Abrupt Change
• Change thermodynamically driven
• Dynamics plays a small stabilizing role
Change in ice area over melt season
Thermodynamic
Dynamic
• Ice melt rates directly modify the ice thickness
• Ice thickness shows large drop associated w/abrupt event
• However, change is not “remarkable”
MarchIce Thickness
Processes contributing to abrupt change
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
% O
W f
orm
atio
n pe
r cm
ice
mel
t
March Arctic Avg Ice Thickness (m)
Basal Melt
Surface Melt
Total Melt
Processes contributing to abrupt change
Albedo Feedback
• Increases in basal melt occur during transitions
• Driven in part by increases in solar radiation absorbed in the ocean as the ice recedes
cm/d
ayW
m-2
SW Absorbed in OML5 Year Running Mean
Processes contributing to abrupt changeIncreasing ocean heat transport to the Arctic
Ocean Heat Transport to
Arctic
Increases in ocean heat transport occur during the abrupt transition.
Contributes to increased melting and provides a “trigger” for the event.
FramStrait
Sib
eria
n S
helf
Arctic
Arctic Ocean Circulation Changes
2040-2049Minus
1980-1999
Sib
eria
n S
helf
FramStrait Arctic
Evidence that ocean circulation changes are related to changing ice/ocean freshwater exchange
(Bitz et al., 2006)
Both trend and shorter-timescale variations in OHT appear important
OHT “natural” variations partially wind driven.
Correlated to an NAO-type pattern in SLP
Ocean Heat Transport to
Arctic
Mechanisms Driving Abrupt Transition
1. Transition to a more vulnerable state
• thinning of the ice cover
2. A Trigger
• rapid increases in ocean heat transport.
• Other “natural” variations could potentially play the same “triggering” role?
3. Positive feedbacks that accelerate the retreat
• Surface albedo feedback
• OHT feedbacks associated with changing ice conditions
Effects of transition on atmospheric conditions
• Winter air temperature increases rapidly during abrupt ice change, with a >5C warming in 10 yrs
• Precipitation shows general increasing trend with largest rate of change over abrupt ice event
Projections of Near-surface Permafrost
Courtesy of Dave Lawrence, NCAR
(Lawrence and Slater, 2005)
Ice E
xte
nt
10
6 k
m2
Permafrost (CCSM)Sept. sea-ice (CCSM)Sept. sea-ice (Observed)
Some Cautions in Using Models to Examine these (and other) issues…
Biases in simulated control state can affect feedback strength
Uncertainties in model physics/response
Acknowledgement that model physics matters for simulated feedbacks
Models provide a powerful tool for examining climate feedbacks, mechanisms, etc but…
“Ethical Considerations”
ITD Influence on Albedo Feedback
• 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
ITD (5 cat)1 cat.
1cat tuned“Strength” of albedo
feedback in climate
change runs
(Holland et al., 2006)
Feedbacks contribute to Arctic amplification
But, that amplification varies considerably
among models
(Holland and Bitz, 2003)
Sea ice in fully coupled GCMs
IPCC AR4
1980-1999 ice
thickness
Red line marks
observed extent
Aspects of the Model’s Internal Variability
ModelStandard Deviation
Model 1 1.93
Model 2 1.90
Model 3 1.72
Model 4 1.68
Model 5 0.42
Summary• Sea ice is an effective amplifier of climate perturbations:
• due to surface albedo changes• due to ice/ocean/atm exchange processes
• CCSM3 simulates abrupt transitions in the future ice cover• preconditioning (thinning) • trigger (ocean heat transport changes)• positive feedbacks (surface albedo; oht changes)
• Models provide a useful tool for exploring the mechanisms that result in simulated rapid climate transitions
• Never completely trust the tool • comparisons to other models; sensitivity tests; “digging” into the feedbacks, etc. can increase confidence in simulated processes
Role of sea ice as an “amplifier”
From Li et al., 2005
Insulating effect of sea ice contributes to large atmospheric response to sea ice changes.
Models are a useful tool to quantify these impacts.
SST SST
LGM ReducedIce
SAT Difference
-44
-40
-36
-32
0 10 20 30 40 50 60 70
x1000 years ago
18O
(p
er
mil
SM
OW
)
Heinrich events
Dansgaard/Oeschger oscillations
Younger Dryas
8.2 k event
-30
-40
-50
-60
Te
mp
era
ture
(C
)
GISP2, Greenland
Role of sea ice for abrupt transitions in a paleoclimate context?
(slide courtesy of Carrie Morrill)
Simulated abrupt transitions in sea iceabrupt forcing (freshwater hosing) can result in abrupt ice changes
• Sea ice changes amplify climate response• Global teleconnections can result• Longevity of these changes are an issue
Sea ice change SAT Change
(From Vellinga and Wood, 2002; Vellinga et al, 2002)
SAT Change at end of 21st century
From A1B scenario
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
Change in Ideal age at 2XCO2
From Bitz et al., 2006
Change in Ocean Circulation
Yr: 40-60
Change in Ideal Age at 2XCO2
cm
How common are abrupt transitions?
Transitions defined as years when ice loss exceeds 0.5 million km2 in a year
ObsSimulated5yr running mean
September Ice Extent
“Abrupt”transition
How common are forcing mechanisms?
How common are effects?Lagged composites relative to initiation of abrupt sea-ice retreat event
Courtesy of David Lawrence, NCAR
Arctic Land Area
20th Century
21st Century
• Increased Arctic Ocean heat transport occurs even while the Atlantic MOC weakens
Do other models have abrupt transitions?Some do…
Data from IPCC AR4 Archive at PCMDI
Climate models as a useful tool for addressing ACC
As a tool to flesh out/test hypotheses or processes
• How is a climate signal amplified by sea ice interactions
• What processes influence threshold behavior in the sea ice
• How does the control climate state modify the persistence of anomalies
• How are teleconnections between high latitudes and tropics realized
Precipitation Changes
• Precipitation generally increases over the 20th-21st centuries
• Rate of increase is largest during the abrupt sea ice transition
2040-2049 minus 1990-1999
OHT and polar amplification
Change in poleward ocean heat transport at 2XCO2 conditions
Both control state and change in OHT are correlated to polar
amplification
OHT
(From Holland and Bitz, 2003)
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
Importance of sea ice state for the magnitude of polar amplification
• 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
Complicates paleoclimate issues since “control state” not as well known
(From Holland and Bitz, 2003)
Does it matter?•Sea ice is an important “amplifier” in the system•When a change is made in a coupled model, often the most dramatic response is in the ice covered regions•This often occurs for changes that are not polar specific - e.g. diurnal cycle stuff.•Getting the processes by which sea ice amplifies a climate signal “right” can be quite important for our ability to simulate abrupt change•These will likely include ice/ocean and ice/atmosphere interactions (ice growth/brine rejection - how it changes - seasonally, etc.; how changes influence the ocean; •Threshold behavior of the ice cover - examples: on/off of the initial ice growth (freezing temp); perennial to seasonal ice cover; perennial to seasonal snow cover;
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