Abrupt Changes in Arctic Sea Ice

Abrupt Changes in Arctic Sea Ice

Marika Holland

NCAR

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

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

Role of sea ice as an “amplifier”

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

(From Hall, 2004)

VA=variable albedoFA=fixed albedo

(DJF SAT)

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.

SST SST

LGM ReducedIce

SAT Difference

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

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)

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.

Changes in Ocean Heat Transport

Ocean Heat Transport to Arctic

FramStrait

Sib

eria

n S

helf

Arctic

Arctic Ocean Circulation Changes

2040-2049Minus

1980-1999

Sib

eria

n S

helf

FramStrait Arctic

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

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 of ice to a more vulnerable state

• thinning of the ice

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

Impacts of Abrupt Ice Transitions on Other Aspects of the Climate

System

Using the model to assess

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

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

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)

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

-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)

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 wonderful tool for examining climate feedbacks, mechanisms, etc BUT…

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

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

(From Holland and Bitz, 2003)

SAT Change at end of 21st century

From A1B scenario

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 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

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

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

Processes involving ice/ocean FW exchange

Change in poleward ocean heat transport at 2XCO2 conditions

Yr 40-60

V’T

VT’

(Bitz et al., 2006)

Change in Arctic OHT

By end of 21st century

•SAT consistently warms

•SLP changes are evident

Meehl et al, 2005

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)

Sea ice representation in GCMs

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

parameterized

SW

Basal heat flux

Changes in sea ice model representation over last 5 Years

0

10

20

30

40

50

60

70

80

90

ThermoOnly

FreeDrift

Rheo-logy

ITD

IPCC TARIPCC AR4

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