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 = 0.65 R = -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

UniformEx-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

GlobalLow 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.