Polar ampliﬁcation dominated by local forcing and feedbacks...POLAR-f CO2 forcing and effective forcing 90S 60S 30S 0 30N 60N 90N 200 400 600 800 1000 1200 CO 2 concentration [ppmv]

Polar amplification dominated by local forcing and feedbacks

Malte F. Stuecker

[email protected]

Co-authors:

C. Bitz, K. Armour, C. Proistosescu, S. Kang, S.-P. Xie, D. Kim, S.

McGregor, W. Zhang, S. Zhao, W. Cai, Y. Dong, F.-F. Jin

mailto:[email protected]

What is polar amplification (PA)?

• Our definition: ratio of surface warming in the polar regions (60N-90N, 60S-90S) compared to the tropics (30S-30N)

IPCC AR5 WG 1, chapter 12

Pithan & Mauritsen (2014)

Figure shows the “warming contributions” (i.e., how

much each feedback contributed to warming in 4xCO2 CMIP5 simulations) for both the Arctic and the

Tropics

Planck = Planck curvature CO2 = radiative forcing F

Recent diagnostic study: What is causing Arctic amplification?


Dominant role of the lapse rate feedback in this study



However, previous studies highlighted instead

dominant roles of either the albedo feedback and

atmospheric heat transport!



In their analysis, radiative forcing, ocean heat uptake

(OHU), cloud- and water vapor feedbacks reduce Arctic amplification (AA)!


Research Questions

Stuecker et al. 2018

Research Questions


How important is remote tropical forcing for PA/AA in a fully coupled experimental setting (previous studies utilized aqua planet simulations,

AGCMs, and reanalysis diagnostics)?

Research Questions


Does the relative importance of different feedback processes (diagnostic framework) depend on the spatial pattern of the forcing?

How important is remote tropical forcing for PA/AA in a fully coupled experimental setting (previous studies utilized aqua planet simulations,

AGCMs, and reanalysis diagnostics)?

Our Approach

fully coupled CAM4-CESM (allowing all feedbacks to operate)

Abrupt 4xCO2 with three different forcing structures: TROP, MLAT, POLAR

(plus a GLOBAL forcing experiment)

year 11-60 average after the perturbation (transient response), ensemble mean

SOM experiments for the equilibrium response (not discussed today)

90S 60S 30S 0 30N 60N 90N

0

2

4

6

8

10

Rad

iativ

e fo

rcin

g [W

m-2]GLOBAL-fTROP-f

MLAT-fPOLAR-f

CO2 forcing and effective forcing

90S 60S 30S 0 30N 60N 90N200400600800

10001200

CO

2 con

cent

ratio

n [p

pmv]

TROPMLATPOLAR

use SST/SIC climatology from coupled control and conduct AGCM experiments with different CO2 forcing structures;

the resulting time-mean net TOA imbalance is the effective forcing F


CO2 forcing and effective forcing

90S 60S 30S 0 30N 60N 90N200400600800

10001200

CO

2 con

cent

ratio

n [p

pmv]

TROPMLATPOLAR

90S 60S 30S 0 30N 60N 90N

0

2

4

6

8

10

Rad

iativ

e fo

rcin

g [W

m-2]GLOBAL-f

SUM-fTROP-fMLAT-fPOLAR-f

the SUM of the effective forcings from the individual experiments (TROP,MLAT,POLAR) is approximately equal to the effective forcing in

GLOBAL


Climate response in the coupled experiments is linear

Only POLAR experiment shows very strong polar amplification!

02468

101214

[°C

]Reference height temperature response

TROPMLATPOLARGLOBALSUM

90S 60S 30S 0 30N 60N 90N



02468

101214

[°C



90S 60S 30S 0 30N 60N 90N

Due to their equivalent spatial coverage, the relative contributions of TROP and POLAR to surface warming can be directly compared:



02468

101214

[°C



90S 60S 30S 0 30N 60N 90N

both induce similar warming in the tropics, but POLAR induces ~4x more global-mean warming and ~10x more POLAR warming



02468

101214

[°C



90S 60S 30S 0 30N 60N 90N

largest global-mean warming comes from MLAT, but this is due to its forcing being applied over a greater area, and the warming is not

polar amplified



linearity in effective forcing, surface warming, SW and LW feedbacks (not shown), and meridional heat transport (not shown) suggests that

the polar warming in GLOBAL can be decomposed in terms of the responses to forcing in each region individually:

TROP, MLAT, and POLAR


Contributions of processes to polar warming

From the zonal mean energy balance:

we can calculate the zonal mean temperature contributions due to the bulk local feedback ( ):

HjO = Fj + λjΔTjS − H

jA,

ΔTjS =1

−λ0[λ′ �jΔTjS − H

jO + F

j − HjA]

λ′�j = λj − λ j0



90S 60S 30S 90N60N30N0

d) GLOBAL-CPL

-3

0

3

6

9

[°C]

TASEffective forcingλHAHO

90S 60S 30S 90N60N30N0

c) POLAR-CPL

-3

0

3

6

9 TASEffective forcingλHAHO

90S 60S 30S 90N60N30N0

b) MLAT-CPL

-3

0

3

6

9

[°C]


90S 60S 30S 90N60N30N0

a) TROP-CPL

-3

0

3

6


The very small amount of Arctic warming in response to TROP

forcing is due to atmospheric heat transport



90S 60S 30S 90N60N30N0

d) GLOBAL-CPL

-3

0

3

6

9

[°C]


90S 60S 30S 90N60N30N0

c) POLAR-CPL

-3

0

3

6


90S 60S 30S 90N60N30N0

b) MLAT-CPL

-3

0

3

6

9

[°C]


90S 60S 30S 90N60N30N0

a) TROP-CPL

-3

0

3

6


Arctic warming in MLAT is due to a combination of both atmospheric heat transport and local feedback!



90S 60S 30S 90N60N30N0

d) GLOBAL-CPL

-3

0

3

6

9

[°C]


90S 60S 30S 90N60N30N0

c) POLAR-CPL

-3

0

3

6


90S 60S 30S 90N60N30N0

b) MLAT-CPL

-3

0

3

6

9

[°C]


90S 60S 30S 90N60N30N0

a) TROP-CPL

-3

0

3

6


Arctic warming in POLAR is due to a

combination of both effective

forcing and local feedback!

Heat transport is damping!



90S 60S 30S 90N60N30N0

d) GLOBAL-CPL

-3

0

3

6

9

[°C]


90S 60S 30S 90N60N30N0

c) POLAR-CPL

-3

0

3

6


90S 60S 30S 90N60N30N0

b) MLAT-CPL

-3

0

3

6

9

[°C]


90S 60S 30S 90N60N30N0

a) TROP-CPL

-3

0

3

6


Difference between Arctic and

Antarctic warming in POLAR is due to

difference in feedback strength



90S 60S 30S 90N60N30N0

d) GLOBAL-CPL

-3

0

3

6

9

[°C]


90S 60S 30S 90N60N30N0

c) POLAR-CPL

-3

0

3

6


90S 60S 30S 90N60N30N0

b) MLAT-CPL

-3

0

3

6

9

[°C]


90S 60S 30S 90N60N30N0

a) TROP-CPL

-3

0

3

6


Difference between Arctic and

Antarctic warming in MLAT is due to

difference in ocean heat uptake



Next we calculate warming contributions by each feedback using the method employed by Pithan & Mauritsen (2014) for the time-mean

ensemble-mean response of each experiment




Arctic

amplif

ication

Tropic

al amp

lificatio

n

-2 -1 0 1 2 3-2

-1

0

1

2

3

Arctic

amplif

ication

Tropic

al amp

lificatio

n

Tropical warming [°C]

Arct

ic w

arm

ing

[°C]

-2 -1 0 1 2 3-2

-1

0

1

2

3


Lapse rate

AlbedoCloud

ResidualPlanck

Effectiveforcing

Atmospherictransport (HA)

Ocean (HO)

Water vapor

Lapse rate

Residual


Ocean (HO)

Water vaporPlanck

Albedo

Effectiveforcing

Cloud

GLOBAL-CPL POLAR-CPL

-2 -1 0 1 2 3-2

-1

0

1

2

3


MLAT-CPL

Arctic

amplif

ication

Tropic

al amp

lificatio

n

Lapse rateResidual


Ocean (HO)

Water vaporPlanck Albedo

Effectiveforcing

Cloud

largest contribution to AA is the lapse rate feedback in GLOBAL and POLAR




Arctic

amplif

ication

Tropic

al amp

lificatio

n

-2 -1 0 1 2 3-2

-1

0

1

2

3

Arctic

amplif

ication

Tropic

al amp

lificatio

n


Arct

ic w

arm

ing

[°C]

-2 -1 0 1 2 3-2

-1

0

1

2

3


Lapse rate

AlbedoCloud

ResidualPlanck

Effectiveforcing


Ocean (HO)

Water vapor

Lapse rate

Residual


Ocean (HO)

Water vaporPlanck

Albedo

Effectiveforcing

Cloud


-2 -1 0 1 2 3-2

-1

0

1

2

3


MLAT-CPL

Arctic

amplif

ication

Tropic

al amp

lificatio

n

Lapse rateResidual


Ocean (HO)


Effectiveforcing

Cloud

Secondary are Planck curvature and albedo feedbacks




Arctic

amplif

ication

Tropic

al amp

lificatio

n

-2 -1 0 1 2 3-2

-1

0

1

2

3

Arctic

amplif

ication

Tropic

al amp

lificatio

n


Arct

ic w

arm

ing

[°C]

-2 -1 0 1 2 3-2

-1

0

1

2

3


Lapse rate

AlbedoCloud

ResidualPlanck

Effectiveforcing


Ocean (HO)

Water vapor

Lapse rate

Residual


Ocean (HO)

Water vaporPlanck

Albedo

Effectiveforcing

Cloud


-2 -1 0 1 2 3-2

-1

0

1

2

3


MLAT-CPL

Arctic

amplif

ication

Tropic

al amp

lificatio

n

Lapse rateResidual


Ocean (HO)


Effectiveforcing

Cloud

However, the exact contribution of each feedback depends strongly on the forcing location!



Arctic

amplif

ication

Tropic

al amp

lificatio

n

-2 -1 0 1 2 3-2

-1

0

1

2

3

Arctic

amplif

ication

Tropic

al amp

lificatio

n


Arct

ic w

arm

ing

[°C]

-2 -1 0 1 2 3-2

-1

0

1

2

3


Lapse rate

AlbedoCloud

ResidualPlanck

Effectiveforcing


Ocean (HO)

Water vapor

Lapse rate

Residual


Ocean (HO)

Water vaporPlanck

Albedo

Effectiveforcing

Cloud


-2 -1 0 1 2 3-2

-1

0

1

2

3


MLAT-CPL

Arctic

amplif

ication

Tropic

al amp

lificatio

n

Lapse rateResidual


Ocean (HO)


Effectiveforcing

Cloud

The relative importance of meridional heat transport also strongly depends on where the forcing is applied:

Atmospheric heat transport strongly contributes to AA in MLAT, but acts to decrease AA for POLAR forcing



Arctic

amplif

ication

Tropic

al amp

lificatio

n

-2 -1 0 1 2 3-2

-1

0

1

2

3

Arctic

amplif

ication

Tropic

al amp

lificatio

n


Arct

ic w

arm

ing

[°C]

-2 -1 0 1 2 3-2

-1

0

1

2

3


Lapse rate

AlbedoCloud

ResidualPlanck

Effectiveforcing


Ocean (HO)

Water vapor

Lapse rate

Residual


Ocean (HO)

Water vaporPlanck

Albedo

Effectiveforcing

Cloud


-2 -1 0 1 2 3-2

-1

0

1

2

3


MLAT-CPL

Arctic

amplif

ication

Tropic

al amp

lificatio

n

Lapse rateResidual


Ocean (HO)


Effectiveforcing

Cloud

Limitation of interpreting causal mechanisms of AA from GLOBAL forcing:

e.g., in GLOBAL effective forcing is seen to amplify warming in the tropics rather than in the Arctic, yet when forcing is applied region by region, effective forcing in polar regions is seen to be a primary driver of AA



Arctic

amplif

ication

Tropic

al amp

lificatio

n

-2 -1 0 1 2 3-2

-1

0

1

2

3

Arctic

amplif

ication

Tropic

al amp

lificatio

n


Arct

ic w

arm

ing

[°C]

-2 -1 0 1 2 3-2

-1

0

1

2

3


Lapse rate

AlbedoCloud

ResidualPlanck

Effectiveforcing


Ocean (HO)

Water vapor

Lapse rate

Residual


Ocean (HO)

Water vaporPlanck

Albedo

Effectiveforcing

Cloud


-2 -1 0 1 2 3-2

-1

0

1

2

3


MLAT-CPL

Arctic

amplif

ication

Tropic

al amp

lificatio

n

Lapse rateResidual


Ocean (HO)


Effectiveforcing

Cloud

Thus, reducing local effective forcing at the poles may be a more effective policy to minimize AA than previously thought!


Conclusions

Stuecker et al. (in review)

Conclusions


Climate response in the coupled system is remarkably linear both in transient coupled CGCM simulations and equilibrium SOM simulations

Conclusions



Forcing in the midlatitudes and subtropics (MLAT) accounts for a substantial fraction of polar warming.

However, MSE diffusion leads to flat temperature profile with little PA

Conclusions





Forcing in the high latitudes leads to large polar warming, due to strong radiative feedbacks. Lapse rate feedback dominates (with

additional contributions from Planck curvature and albedo)

MSE diffusion not as efficient at taking heat out of high latitudes

Conclusions





Forcing in the high latitudes leads to large polar warming, due to strong radiative feedbacks. Lapse rate feedback dominates (with

additional contributions from Planck curvature and albedo)

MSE diffusion not as efficient at taking heat out of high latitudes

Relative importance of each feedback strongly depends on the spatial structure of the forcing

Additional slides

Same results hold other for CAM5-CESM:

Atmospheric and oceanic heat transport

Strong heat uptake in Southern Ocean and moderate heat uptake in subpolar Northern Hemisphere for both MLAT and POLAR


Oceanic heat transport is generally equatorward (especially MLAT and POLAR)


Only very little anomalous OHT into the Arctic, and only for MLAT experiment!


Poleward atmospheric heat transport increases strongly nearly everywhere under TROP and MLAT forcing (MSE diffusion) —> relative uniform TS warming for both TROP and MLAT

Documents

Polar ampliﬁcation dominated by local forcing and feedbacks...POLAR-f CO2 forcing and effective forcing 90S 60S 30S 0 30N 60N 90N 200 400 600 800 1000 1200 CO 2 concentration [ppmv]