processes responsible for polar amplification of climate change Peter L. LangenCentre for Ice and Climate Niels Bohr InstituteUniversity of Copenhagen

Polar amplification of model-projected climate change Peter L. Langen, COGCI School, Jan Holland and Bitz (2003) 15 model intercomparison:

Spatial distribution of warming Peter L. Langen, COGCI School, Jan Holland and Bitz (2003) Models with “high” PA

Seasonal distribution of warming Peter L. Langen, COGCI School, Jan Holland and Bitz (2003) … but when is the albedo feedback active?

Latitude of maximum warming vs. sea ice extent Peter L. Langen, COGCI School, Jan Holland and Bitz (2003) R = -0.80

Degree of PA vs. amount of sea ice Peter L. Langen, COGCI School, Jan Holland and Bitz (2003) R = 0.65 R = PA seems weakly related to amount of sea ice in weak-to-medium-strength-PA models

Snow on land? Peter L. Langen, COGCI School, Jan Holland and Bitz (2003) R = 0.67 … but snow extent correlates also with sea ice extent and not with change in snow

Clear-sky and cloud shielding of SAF Peter L. Langen, COGCI School, Jan Qu and Hall (2006) The surface albedo feedback is given by, where

In Fourth Assessment Report simulations Peter L. Langen, COGCI School, Jan Qu and Hall (2006)

The surface albedo sensitivity Peter L. Langen, COGCI School, Jan Qu and Hall (2006) Inter-model differences in SAF strength derive from parameterizations of surface processes rather than from clouds!

Fixed-albedo experiment Peter L. Langen, COGCI School, Jan Hall (2004)

Fixed cloud experiment (I) Peter L. Langen, COGCI School, Jan Vavrus (2004)

Fixed cloud experiment (II) Peter L. Langen, COGCI School, Jan Vavrus (2004)

Effect of fixing low or high latitude clouds only Peter L. Langen, COGCI School, Jan Vavrus (2004) The warming due to low-latitude cloud feedback contributes to high-latitude warming!

Peter L. Langen, COGCI School, Jan Ghost forcing experiments Exp 1 Exp 2 Exp 3 Exp 2 Exp 2 + Exp 3 Exp 1 Forcing Response Alexeev, Langen and Bates (2005)

Peter L. Langen, COGCI School, Jan 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 Fixed temperature perturbation (II) Temperature change (K) Abs. humidity change (g/kg) Ex-trop Uniform Tropical Ex-trop Uniform Ex-trop Tropical Increased heat transport warms and moistens high- latitude troposphere Vertical Profiles at 80N Alexeev, Langen and Bates (2005)

Vertical structure of recent Arctic warming Peter L. Langen, COGCI School, Jan Graversen et al. (2007)

Peter L. Langen, COGCI School, Jan Polar amplification as an excitation of a 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 Surface budget and mixed-layer model Vector containing all surface temperatures Heat capacity of water column Vector containing surface fluxes. Collection of “external” parameters: CO 2, solar constant, etc. Langen and Alexeev (2007)

Peter L. Langen, COGCI School, Jan Linearization Langen and Alexeev (2007)

Peter L. Langen, COGCI School, Jan External forcing 0 in new equilibrium “forcing” Climate change Langen and Alexeev (2007)

Peter L. Langen, COGCI School, Jan Forcing excites least stable mode Linear estimate Actual run Langen and Alexeev (2005) Forcing expanded in basis of eigenvectors If k’th term dominates Least stable mode

Peter L. Langen, COGCI School, Jan Perturbation decay in linear system Time (years) Langen and Alexeev (2007)

Peter L. Langen, COGCI School, Jan Two-box energy balance model Langen and Alexeev (2007)

Peter L. Langen, COGCI School, Jan EBM (II) Alexeev et al. (2005): Tropical perturbation Global Extra-tropical Langen and Alexeev (2007)

Peter L. Langen, COGCI School, Jan Eigenanalysis Fast mode Slow mode Langen and Alexeev (2007)

Peter L. Langen, COGCI School, Jan Response to steady forcing Langen and Alexeev (2007)

Peter L. Langen, COGCI School, Jan Decay in GCM ensemble High lats Global Low lats High lats Global Low lats SW Total Tot-clear LW Langen and Alexeev (2007)

Peter L. Langen, COGCI School, Jan Extraction of cloud fields Cloud radiative forcing: SW Total LW positive is warming Langen and Alexeev (2007)

Peter L. Langen, COGCI School, Jan 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.