university of washington department of atmospheric sciences

The Response of the Southern Hemispheric Eddy-Driven Jet to Future Changes in Shortwave Radiation in CMIP5

University of WashingtonDepartment of Atmospheric Sciences

Paulo Ceppi, Mark D. Zelinka and Dennis L. Hartmann Southern Ocean Clouds Workshop, 18 March 2014

Outline• Changes in shortwave radiation in

RCP8.5• Impact of SW changes on SSTs and

midlatitude jet• Determining causal relationships• Conclusions

Absorbed SW radiation (ASR)2050-2099 minus 1950-1999, in W m-2

AS

R c

hang

e (W

m-2)

ASR changes (in W m-2)A

SR

cha

nge

(W m

-2)

ASR changes (in W m-2)albedo

AS

R c

hang

e (W

m-2)

ASR changes (in W m-2)albedo clouds

AS

R c

hang

e (W

m-2)

Absorbed SW radiation (ASR)

• considerable inter-model spread• wide variety of meridional gradients

of ASR response

Effect of ASR changes• Atmospheric circulation is very

sensitive to meridional SST gradient• Idea: can the different ASR

gradient responses explain spread in SST and eddy-driven jet response?

• Define simple ASR and SST gradient indices

Change in ASR gradientASR index = <ASR>20-50 – <ASR>50-90

AS

R c

hang

e (W

m-2)

ASR index vs Tsfc index

change in ASR gradient change in SST gradient

ASR index vs jet shift

increase in ASR/SST gradient poleward jet shift

poleward jet shift

ASR index vs jet shift

But wait! What if the jet shifts are causing the different ASR responses?

poleward jet shift

Causality

• To solve this, use AMIP + AMIPfuture experiments (11 models)

• AMIP: 1979–2005 SSTs• AMIPfuture: AMIP SSTs + patterned anomaly• prescribed SSTs = SW effects are suppressed

jet shift cloud shift SST anomalies1

2

cloud response SST anomalies jet shift

Causality

• If no relationship between ASR and jet shift in AMIPfuture

• If jet shift and ASR changes are related

jet shift cloud shift SST anomaliescloud response SST anomalies jet shift1

2

In AMIPfuture:

1

2

jet shift vs ASR indexRCP8.5

jet shift vs ASR indexRCP8.5

jet shift vs ASR indexRCP8.5 + AMIPfutureRCP8.5

no relationship between jet shift and ASR change in AMIPfuture

cloud response SST anomalies jet shift

ASR changes: AMIP vs RCP8.5

SW changes are the same! (despite different jet shifts)

jet shift cloud shift ASR/SST anomalies12

ASR response to natural jet variability (in W m-2)

AS

R re

spon

se (W

m-2)

ASR response to natural jet variability (in W m-2)

jet shift cloud shift ASR/SST anomalies2

AS

R re

spon

se (W

m-2)

Conclusions• Large spread in ASR changes in

RCP8.5• ASR changes cause spread in SST

gradient change and jet shift• Spread in ASR response due to

model-dependent sensitivity to SST warming

• Poleward jet shift seems to play a minor role in ASR responseThe End Thank you!

Extra plots

ASR changes in AMIPfuture2050-2099 minus 1950-1999, in W m-2 K-1

AS

R c

hang

e (W

m-2)

Effect of clouds2050-2099 minus 1950-1999

AS

R c

hang

e (W

m-2)

Effect of surface albedo2050-2099 minus 1950-1999

sea ice

AS

R c

hang

e (W

m-2)

jet s

hift

(deg

S K

-1)

jet s

hift

(deg

S K

-1)