Polar Amplification of Polar Amplification of ClimateClimate

Polar Amplification of Climate Polar Amplification of Climate Change in Coupled ModelsChange in Coupled Models

Holland, M. M., Bitz, C. M. Climate Dynamics, Vol. 21, Issue 3-4, pp.221-232, Holland, M. M., Bitz, C. M. Climate Dynamics, Vol. 21, Issue 3-4, pp.221-232,

Sept 2003.Sept 2003. Analysis of northern hemisphere Analysis of northern hemisphere

polar amplification of climate by polar amplification of climate by means of comparison amongst a means of comparison amongst a group of climate modelsgroup of climate models

Surface warming in the arctic Surface warming in the arctic MagnitudeMagnitude DistributionDistribution Seasonality Seasonality

Sea-ice physics for models with Sea-ice physics for models with high polar amplificationhigh polar amplification

Whats going on with Whats going on with BRMC?BRMC?

VariablesVariables

Sea iceSea ice ExtentExtent ThicknessThickness

Snow coverage on landSnow coverage on land Cloud coverageCloud coverage Ocean heat transportOcean heat transport

Sea Ice ExtentSea Ice Extent

Ice extent and latitude Ice extent and latitude of maximum warming of maximum warming are significantly are significantly correlated, R=-.80correlated, R=-.80

Models with larger ice Models with larger ice extents have a extents have a tendency to have tendency to have maximum warming at maximum warming at lower latitudes, why is lower latitudes, why is this?this?

Sea Ice ExtentSea Ice Extent

Larger ice extents generally cause Larger ice extents generally cause higher global sensitivity because the higher global sensitivity because the earth has more available ice to melt earth has more available ice to melt (only for models with low-moderate (only for models with low-moderate polar amplification) why not high polar amplification) why not high amplification models?amplification models?

No significant correlation between ice No significant correlation between ice extent and normalized warming, extent and normalized warming, considering all models togetherconsidering all models together

Sea Ice Sea Ice ThicknessThickness

More highly More highly correlated to correlated to normalized normalized warming than sea warming than sea ice extent, R=-.40ice extent, R=-.40

Sea Ice ThicknessSea Ice Thickness

Suggests that because thin ice is Suggests that because thin ice is more easily melted, warming causes more easily melted, warming causes greater loss of ice extentgreater loss of ice extent

Greater albedo feedback and loss of Greater albedo feedback and loss of insulation between ocean and insulation between ocean and atmosphere amplify the warming atmosphere amplify the warming

Still, weak correlations suggest some Still, weak correlations suggest some other factors may be at work...other factors may be at work...

Snow Coverage on LandSnow Coverage on Land

No significant No significant correlation between correlation between snow coverage and snow coverage and normalized warming, normalized warming, considering all models considering all models togethertogether

However, as with ice However, as with ice extent, a significant extent, a significant correlation exists correlation exists among the models with among the models with low-moderate polar low-moderate polar amplification, R=.67amplification, R=.67

Snow CoverageSnow Coverage

Correlation (among low-moderate Correlation (among low-moderate amplification models) is probably a amplification models) is probably a result of the snow coverage on land result of the snow coverage on land being highly correlated to the extent being highly correlated to the extent of ice, and not indicative of the snow of ice, and not indicative of the snow extent influencing the polar extent influencing the polar amplification of warmingamplification of warming

Cloud coverageCloud coverage Does increased cloud coverage have a Does increased cloud coverage have a

positive or negative effect on polar positive or negative effect on polar amplification of warming?amplification of warming?

Cloud Coverage – Temperature Cloud Coverage – Temperature FeedbacksFeedbacks

Cloud coverage

Absorbed solarradiationat surface

Temperature Cloud coverage

Absorbed solarradiationat surface

Temperature

High CloudsHigh Clouds Low CloudsLow Clouds

Summer CloudsSummer Clouds

Changes in ice-Changes in ice-albedo, and thus albedo, and thus absorbed solar absorbed solar radiation, did not radiation, did not result in changes result in changes in cloud cover in cloud cover during summerduring summer

However, the high However, the high amplification amplification models do have a models do have a tendency to have tendency to have increased summer increased summer cloud covercloud cover

Winter CloudsWinter Clouds

Also, decreased winter Also, decreased winter cloud cover in control cloud cover in control climate is correlated to climate is correlated to higher polar amplification, higher polar amplification, R=-.50R=-.50

Problem: model bias? Problem: model bias? Control climate cloud Control climate cloud conditions related to conditions related to change in clouds, R=-.76, change in clouds, R=-.76, Lower levels of original Lower levels of original cloud cover allow for cloud cover allow for greater changegreater change

Larger winter cloud cover Larger winter cloud cover changes correlated to changes correlated to higher amplification, higher amplification, R=.74, R=.74,

Cloud CoverageCloud Coverage

Problems:Problems: No distinction between high and low No distinction between high and low

cloudsclouds Don’t know enough about cloud Don’t know enough about cloud

relationships and feedbacksrelationships and feedbacks Don’t have data from all of the modelsDon’t have data from all of the models

Ocean Heat TransportOcean Heat Transport

ECH-ECH-33

CCSM

CERCERFF

MRI BMRC

CCSR

CCC GFDL-GFDL-AA

GISS

ECH-ECH-33

CCSM

CERCERFF

MRI

BMRC

CCSR

CCC

GFDL-GFDL-AA

GISS

Ocean Heat TransportOcean Heat Transport

Control climate Control climate ocean heat ocean heat transport and transport and change in transport change in transport are both are both significantly significantly correlated to polar correlated to polar warmingwarming

Ocean Heat TransportOcean Heat Transport

Because increased heat transport by Because increased heat transport by the ocean is correlated with both sea the ocean is correlated with both sea ice extent and thickness, it seems ice extent and thickness, it seems possible that this then decreases possible that this then decreases surface albedo by melting sea ice surface albedo by melting sea ice and amplifies warmingand amplifies warming

ConclusionConclusion

Possible factors affecting increased Possible factors affecting increased polar amplification based on these polar amplification based on these models:models: Thin sea-ice coverThin sea-ice cover Increased ocean heat transport to polesIncreased ocean heat transport to poles Increase in winter cloud coverIncrease in winter cloud cover

ConclusionConclusion

Further work is needed within single models Further work is needed within single models to clarify and better understand the to clarify and better understand the relationships suggested by this group of relationships suggested by this group of modelsmodels

From this study it is clear that very accurate From this study it is clear that very accurate depictions of current sea ice characteristics, depictions of current sea ice characteristics, such as extent and thickness, are necessary such as extent and thickness, are necessary to receive valid data concerning the arctic to receive valid data concerning the arctic responses to increased warming by responses to increased warming by greenhouse gasesgreenhouse gases

Arctic picturesArctic pictures