Transcript Simulating Sea Ice - An abrupt change perspective

Using CCSM3 to investigate future
abrupt Arctic sea ice change
Marika Holland
NCAR
“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.”
“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)
Role of sea ice as an “amplifier”
VA=variable albedo
FA=fixed albedo
(From Hall, 2004)
(DJF SAT)
• Surface albedo feedback amplifies climate perturbations
• Models have been used to explore/quantify these effects.
Observed Arctic Conditions
Sea ice concentration
Fowler, 2003
June 6, 2005
The observed Arctic sea ice
(Perovich, 2000)
(NSIDC, 2005)
Observed thickness
Laxon et al., 2003
2003
2002
2004
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
Suggestions that ice has thinned…
Ice draft change 1990s minus (1958-1976)
Rothrock et al., 1999
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
Atlantic Layer Temperature
1900
2000
North Atlantic Oscillation Positive Phase
From
University of
Reading
webpage
JFM NAO Index
1950-1992
Timeseries of JFM NAO Index
Maybe it is not all the NAO/AO?
Have led to suggestions that:
J Climate, 2005
Overpeck et al., 2005
“Researchers estimate that in as little as 15
years, the Arctic could be ice free in the
summer”
“There is no paleoclimate evidence for a seasonally
ice free Arctic during the last 800 millennia”
Overpeck et al.
Future Projections
What can models tell us?
Future climate scenarios
• Relatively gradual forcing.
• Relatively gradual response in global air temperature
Wigley, 2000
Meehl et al, 2005
September Sea Ice Conditions
“Abrupt”
transition
Observations
Simulated
5-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
Dynamic
Thermodynamic
Change in ice area
over melt season
Forcing of the
Abrupt Change
• Change thermodynamically
driven
• Dynamics plays a small
stabilizing role
• Ice melt rates directly modify
the ice thickness
March
Ice Thickness
• Ice thickness shows large drop
associated w/abrupt event
• However, change is not
“remarkable”
% OW formation per cm ice melt
Processes contributing to abrupt change
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
cm/day
Total Melt
Basal Melt
Surface Melt
SW Absorbed in OML
5 Year Running Mean
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
Processes contributing to abrupt change
Increasing 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.
Siberian Shelf
Arctic Ocean Circulation Changes
Evidence that ocean
circulation changes
are related to changing
ice/ocean freshwater
exchange
(Bitz et al., 2006)
Arctic
Siberian Shelf
Fram
Strait
2040-2049
Minus
1980-1999
Fram
Strait
Arctic
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
Ice Extent 106 km2
Courtesy of Dave
Lawrence, NCAR
(Lawrence and Slater,
2005)
Permafrost (CCSM)
Sept. sea-ice (CCSM)
Sept. sea-ice (Observed)
Some Cautions in Using Models to
Examine these (and other) issues…
Models provide a powerful 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
“Ethical Considerations”
ITD Influence on Albedo Feedback
“Strength”
of albedo
feedback in
climate
change runs
ITD (5 cat)
1 cat.
1cat tuned
(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
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
Model
Standard
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”
SAT Difference
SST
LGM
SST
Reduced
Ice
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.
Role of sea ice for abrupt transitions
in a paleoclimate context?
d18O (per mil SMOW)
-32
8.2 k
event
-30
Dansgaard/Oeschger oscillations
-36
-40
-40
-50
Younger Dryas
Heinrich events
-44
0
10
20
30
40
50
Temperature (C)
GISP2, Greenland
-60
60
70
x1000 years ago
(slide courtesy of Carrie Morrill)
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
SAT Change at end of 21st century
From A1B scenario
Change in Ice growth rates at 2XCO2
Processes Involving ice/ocean
FW exchange
In warmer climate, increased
ice growth due to loss of
cm insulating ice cover results in
• Increased ocean ventilation
• Ocean circulation changes
• Transient response
ChangeininIdeal
Idealage
Age
2XCO2
Change
at at
2XCO2
Change in Ocean Circulation
Yr: 40-60
From Bitz et al., 2006
September Ice Extent
Obs
Simulated
5yr running mean
“Abrupt”
transition
How common are
abrupt transitions?
Transitions defined as years when ice loss
exceeds 0.5 million km2 in a year
How common are forcing mechanisms?
How common are effects?
Lagged composites relative to initiation of abrupt sea-ice retreat event
Arctic Land Area
Courtesy of David Lawrence, NCAR
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
2040-2049 minus 1990-1999
• Precipitation generally increases over the 20th-21st centuries
• Rate of increase is largest during the abrupt sea ice transition
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)
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
(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
Complicates paleoclimate issues since “control state” not as well known
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;