# Presentation: 083300

## 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 albedo
- FA=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

- Reduced
- Ice

- 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

## The observed Arctic sea ice

- Sea ice concentration

- (NSIDC, 2005)

- (Perovich, 2000)

- Fowler, 2003

- Observed thickness
- Laxon et al., 2003

- June 6, 2005

## Observations indicate large changes in Arctic summer sea ice cover 

- From Stroeve et al., 2005

- 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

- “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:

## Future ProjectionsWhat can models tell us?

## Future climate scenarios

- Meehl et al, 2005

- Wigley, 2000

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

## September Sea Ice Conditions

- 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

- “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

## 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

- % OW formation per cm ice melt

- March Arctic Avg Ice Thickness (m)

## Processes contributing to abrupt change Albedo Feedback

- 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/day

**Notes:**
OHT stabilizes ice extent at beginning of 21st century.
Then contributes to increase melting/decrease growth prior to and during abrupt change. Indications from previous work that increases in OHT may be related to the changing ice cover.

## Processes contributing to abrupt changeIncreasing 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.

## Changes in Ocean Heat Transport

## Arctic Ocean Circulation Changes

## 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

## 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

## 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)

## How common are abrupt transitions?

- Transitions defined as years when ice loss exceeds 0.5 million km2 in a year

- Obs

- Simulated
- 5yr running mean

- September Ice Extent

- “Abrupt”
- transition

**Notes:**
Many runs have abrupt transitions. They typically are smaller/shorter-lived than the b.ES01 run. Stabilization of the 040b.ES01 from 2000-2020 may be responsible for this
In the different runs, different combinations of effects seem to be important. Have not identified any critical threshold (e.g. ice thickness) that the ice attains which triggers 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

## Role of sea ice for abrupt transitions in a paleoclimate context?

**x1000 years ago**

******18****O (per mil SMOW)**

**-30**

**-40**

**-50**

**-60**

**Temperature (********C)**

**GISP2, Greenland**

- (slide courtesy of Carrie Morrill)

## Simulated abrupt transitions in sea ice

- abrupt 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

| 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 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

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

- 20th Century

## 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

- (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

- %