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processes responsible for polar amplification of climate change
Peter L. Langen Centre for Ice and ClimateNiels Bohr Institute University of Copenhagen
Peter L. Langen, COGCI School, Jan 27 2009 2
Polar amplification of model-projected climate change
Holland and Bitz (2003)
15 model intercomparison:
Peter L. Langen, COGCI School, Jan 27 2009 3
Spatial distribution of warming
Holland and Bitz (2003)
Models with “high” PA
Peter L. Langen, COGCI School, Jan 27 2009 4
Seasonal distribution of warming
Holland and Bitz (2003)
… but when is the albedo feedback active?
Peter L. Langen, COGCI School, Jan 27 2009 5
Latitude of maximum warming vs. sea ice extent
Holland and Bitz (2003)
R = -0.80
Peter L. Langen, COGCI School, Jan 27 2009 6
Degree of PA vs. amount of sea ice
Holland and Bitz (2003)
R = 0.65R = -0.66
PA seems weakly related to amount of sea ice in weak-to-medium-strength-PA models
Peter L. Langen, COGCI School, Jan 27 2009 7
Snow on land?
Holland and Bitz (2003)
R = 0.67
… but snow extent correlates also with sea ice extentand not with change in snow
Peter L. Langen, COGCI School, Jan 27 2009 8
Clear-sky and cloud shielding of SAF
Qu and Hall (2006)
The surface albedo feedback is given by,
where
Peter L. Langen, COGCI School, Jan 27 2009 9
In Fourth Assessment Report simulations
Qu and Hall (2006)
Peter L. Langen, COGCI School, Jan 27 2009 10
The surface albedo sensitivity
Qu and Hall (2006)
Inter-model differences in SAF strength derive from parameterizations of surface processes rather than from clouds!
Peter L. Langen, COGCI School, Jan 27 2009 11
Fixed-albedo experiment
Hall (2004)
Peter L. Langen, COGCI School, Jan 27 2009 12
Fixed cloud experiment (I)
Vavrus (2004)
Peter L. Langen, COGCI School, Jan 27 2009 13
Fixed cloud experiment (II)
Vavrus (2004)
Peter L. Langen, COGCI School, Jan 27 2009 14
Effect of fixing low or highlatitude clouds only
Vavrus (2004)
The warming due to low-latitude cloud feedback contributes to high-latitude warming!
Peter L. Langen, COGCI School, Jan 27 2009 15
Ghost forcing experiments
Exp 1
Exp 2
Exp 3
Exp 3
Exp 2
Exp 2 + Exp 3
Exp 1
Forcing Response
Alexeev, Langen and Bates (2005)
Peter L. Langen, COGCI School, Jan 27 2009 16
Fixed temperature perturbation (I)
Tropical-only SST change gives positive high-latitude tendency:
Turbulent
Radiative
Wm
-2
Alexeev, Langen and Bates (2005)
Peter L. Langen, COGCI School, Jan 27 2009 17
Fixed temperature perturbation (II)
Temperaturechange (K)
Abs. humiditychange (g/kg)
Ex-trop
Uniform
Tropical
Tropical
Ex-tropUniform
Uniform
Ex-trop
Tropical
Increased heat transport warms and moistens high-latitude troposphere
VerticalProfiles at
80N
Alexeev, Langen and Bates (2005)
Peter L. Langen, COGCI School, Jan 27 2009 18
Vertical structure of recent Arctic warming
Graversen et al. (2007)
1979-2001
Peter L. Langen, COGCI School, Jan 27 2009 19
Polar amplification as an excitation ofa preferred mode of response
Two different forcings
… and very similar responses
Manabe and Wetherald (1980): Early GCM experiments
Manabe and Wetherald (1980)
Peter L. Langen, COGCI School, Jan 27 2009 20
Surface budget and mixed-layer model
Vector containing all surface
temperatures
Heat capacity of water column
Vector containing surface fluxes.
Collection of “external” parameters: CO2, solar
constant, etc.
Langen and Alexeev (2007)
Peter L. Langen, COGCI School, Jan 27 2009 21
Linearization
Langen and Alexeev (2007)
Peter L. Langen, COGCI School, Jan 27 2009 22
External forcing
0 in new equilibrium
“forcing”
Climate change
Langen and Alexeev (2007)
Peter L. Langen, COGCI School, Jan 27 2009 23
Forcing excites least stable mode
Linear estimate
Actual run
Langen and Alexeev (2005)
Forcing expanded in basis of
eigenvectorsIf k’th term dominates
Least stable mode
Peter L. Langen, COGCI School, Jan 27 2009 24
Perturbation decay in linear system
Time (years)
Langen and Alexeev (2007)
Peter L. Langen, COGCI School, Jan 27 2009 25
Two-box energy balance model
Langen and Alexeev (2007)
Peter L. Langen, COGCI School, Jan 27 2009 26
EBM (II)
Alexeev et al. (2005):
Tropical perturbation
Global
Extra-tropical
Langen and Alexeev (2007)
Peter L. Langen, COGCI School, Jan 27 2009 27
Eigenanalysis
Fast mode
Slow mode
Langen and Alexeev (2007)
Peter L. Langen, COGCI School, Jan 27 2009 28
Response to steady forcing
Langen and Alexeev (2007)
Peter L. Langen, COGCI School, Jan 27 2009 29
Decay in GCM ensemble
High lats
Global
Low lats
High lats
Global
Low lats SW
Total
Tot-clearLW
Langen and Alexeev (2007)
Peter L. Langen, COGCI School, Jan 27 2009 30
Extraction of cloud fields
Cloud radiative forcing:
SW
Total
LW
positive is warming
Langen and Alexeev (2007)
Peter L. Langen, COGCI School, Jan 27 2009 31
Regimes of heat transport sensitivity
Caballero and Langen (2005)
Climate change experiments are often found to give nearly unchanged heat transports.
These experiments follow approximately iso-lines of transport.
So when iso-lines are vertical ( ) there is no polar amplification.