Verifying the water vapor feedback using ENSO

A. E. Dessler and S. WongDepartment of Atmospheric Sciences

Texas A&M University

2

forcing

e.g., CO2

increase

Ts

increase

Atmospheric

humidity

increases

feedback

How can we measure this?

3

Volcano

ENSO+interannual

seasonal cycle

decade-scale warming

Ts

increase

Constant Relative

Humidity

Partial Radiative

Perturbation

Atmospheric

humidity

increases

4

VolcanoSoden et al., 2002; Forster and Collins,

2004

ENSO+interannualSoden, 1997; Dessler et al., 2008;

Minschwaner and Dessler, 2004;

Gettelman and Fu, 2008;

seasonal cycleInamdar and Ramanathan, 1998;

Wu et al., 2008

decade-scale warmingHall and Manabe, 1999; Soden et al.,

2005

Observational tests of the

water vapor feedback

Partial Radiative Perturbation

5

Ts-a

qa

Tatm-a

icea

Ts-b

qb

Tatm-b

iceb

Wetherald and Manabe, 1988

Colman, 2003; Forster and Collins, 2004;

Soden and Held, 2006

6

Ts-a

Tatm-a

icea

Ts-b

Tatm-b

iceb

TOA net flux Rb

qa qb

new

Wetherald and Manabe, 1988?

Colman, 2003; Forster and Collins, 2004;

Soden and Held, 2006

TOA net flux Ra

7

Ts-a

Tatm-a

icea

Ts-b

Tatm-b

iceb

TOA net flux Rb

qa qb

new

Wetherald and Manabe, 1988?

Colman, 2003; Forster and Collins, 2004;

Soden and Held, 2006

!R = change in global average TOA

flux due to !q

!Ts = change in global avg. surface

temperature change (Ts-b-Ts-a)

associated with !q

!R = change in global average TOA flux

due to !q between two climate states

!Ts = change in global avg. surface

temperature change (Ts-b-Ts-a) associated

with !q

8

Wetherald and Manabe, 1988?

Colman, 2003; Forster and Collins, 2004;

Soden and Held, 2006

!

" =#R

#TS

the water vapor feedback

!

Ts=

"G

#o

+ #1+ #

2+!

Method

• AMIP models from the PCMDI archive

• Reanalysis: ERA40 & MERRA

• For each strong DJF ENSO month (e.g., Jan. 1983),

calculate !R and !Ts between that month and the long-

term average for all Januaries in the model run

• Obtain one estimate of !R and !Ts for every strong ENSO

month

9

10

Each point is a strong ENSO DJF

!

" =#R

#TS

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

!R (

W/m

^2

)

0.80.40.0-0.4

!Ts (K)

NCAR CCSM

11

12

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

q f

eedback (

W/m

^2

/K)

A B C D E F G H I J K L

Model

(a)

ER

A40

ME

RR

A

AMIP models

Des

sler

2008

!

" =#R

#TS

Soden and Held estimate of long-term feedback

Conclusions 1

• Models, reanalysis, and pure obs. agree that

the feedback is positive and strong

• Reanalysis is larger than the models

– if reanalysis is correct, then models are

underestimating the feedback

– uncertainties generally overlap

• ENSO WV feedback larger than in response

to long-term warming (e.g., Colman, Soden and Held)

13

Questions for the rest of the talk

• What is the source of disagreements among

the models and between the models and

reanalyses?

• Speculation: Why is the ENSO feedback

larger than the feedback in response to long-

term warming?

14

15Fig. 2 of Soden et al., 2008

Water vapor feedback is primarily a “tropical” phenomenon

Change in R per unit change in q(x,y,z): !R/!q(x,y,z)

* !R determined by tropical UT !q

* tropical q controlled by tropical

surface temperaturese.g., Minschwaner and Dessler, 2004

* !R (and the WV feedback) is

controlled by tropical surface T

!R

(!q)

!(global avg. T)

Regress !R vs. global surface

temperature

!R

(!q)

!(tropical avg. T)

Regress !R vs. tropical surface

temperature

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

q f

eedback (

W/m

^2

/K)

A B C D E F G H I J K L

Model

(a)

!

"R

"TS

ER

A40

ME

RR

A

AMIP models

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

!R

q v

s. !

TT (

W/m

^2

/K)

A B C D E F G H I J K L

Model

(b)

!

"R

"TT

What does this mean?

• A consistent relationship exists between

tropical surface !TT and the radiative

response to water vapor !R

– GCMs

– reanalyses

– pure obs.

19

+1

+1

+1

!

" =#R

#TS

+1

-1

-1

!R for these two worlds is the same

!Ts is different

smaller feedback larger feedback

21

1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

!T

G/!

TT (

K/K)

4.54.03.53.02.52.0

Feedback strength (W/m^2/K)

A

B

C D

EF

G

H

I

J K

L

Extratropics

warms more

Extratropics

warms less

!

" =#R #q #TT( )( )

#TG

Summary• In response to ENSO climate change, models, reanalysis,

and pure obs. show a strong and positive water vapor feedback

• Models, reanalysis, and pure obs. show that the radiative response to WV between two climate states is determined by change in the tropical surface T

• WV feedback determined by tropical UT water

• Tropical UT water is controlled by surface T described by Minschwaner and Dessler [2004] (see me for reprints)

• Differences among models and between models and reanalyses are due to differing estimates of extratropical surface T changes --- they are NOT due to the radiative response of WV

• This work is in press at J. Climate. See me for preprints.