Didier Swingedouw

Laboratoire des Sciences du Climat et de l’Environnement

France

Projections of the thermohaline

circulation in OAGCMs: toward

an understanding of

uncertainties

Thermohaline circulation

THC : a system with complex feedbacks

Coupled with the atmosphere : an AOGCM is necessary

Uncertainty for the future

Greenland melting impact on the THC

WIS2

CTRL

NIS2Scenario of CO2 doubling, stabilised during 430 ans

Temps

(années)

CO2

(ppm)

0 70 500

280

560

CTL

Scénario

Temps

(années)

CTRL

WIS2

NIS2Mean global Temperature

Snow

Land Ocean

Glacier

Two different IPSL-CM4 models: one with land-ice melting, the other without

Impact of the THC on global warming after 500 years

NIS2 - CTRLWIS2 - CTRL

Années de simulation

WIS2

CTRL

NIS2

Feedback quantification

E S0G

1

i

n

+-

THCTHC

t=0t=0Sans-CTL

Avec-CTL

THC

Glaciers1

1

i

i

ii

iwith

Density budget at the convection sites :

STTSTSResiduResiduSurfaceSurfaceTransportTransport

i

-0,4

-0,2

0

0,2

0,4

0,6

0,8

1

1 2

Main THC feedbacks

0.3)(1

1

TS

G

Dynamical gain of the THC system

S T

Climate system

Density flux ofTempérature

Density flux of salinity

THCe THCs

)( S

)( T

+-

CNRS project : understanding THC uncertainties in IPCC projections

Apply feedback model to « water hosing » CMIP : quantification of differences in feedback processes among IPCC models

Comparison of models with ocean « observations » in transient phase :

Last decades Paleoclimate 8.2 event

Role played by atmospheric forcing and ocean resolution in scenarios

ModelsModels

High resolution simulation of last decades (MERCATOR, DRAKKAR)

• Analysis of« water-hosing » = evaluate differences in ocean processes among IPCC models

• Coupling ice-sheet model GREMLINS with IPSL-CM4

• Simulation of paleoclimatic event 8.2.

ObservationsObservations

• OVIDE and RAPID section

• Paleoclimate record of 8.2 event

• Representation of existing processes (feedbacks …)

• Missing process : Greenland melting

• Paléoclimatical constrain on IPSL-CM4

• Dynamical constrain of last 50 years

• High resolution constrain on key sections

Uncertainty

2 years

ModelsModels

ObservationsObservations

Uncertainty

• Atmospheric forcing uncertainty

• Ocean resolution impact

• Paleoclimate constrain for all IPCC models

• Climatic impact of THC

• NEMO with • 4 different atmospheric model• different oceanic resolution

• Analysis of PMIP2 database of 8.2 event

• OVIDE and RAPID section

• Paleoclimate record of 8.2 event

High resolution simulation of last decades

(MERCATOR, DRAKKAR)

4 years

Conclusions

Better evaluation of: IPCC models against different observation

datasets Ice-sheet melting interaction with THC in

IPSL-CM4 Ocean resolution issues in coupled models

with NEMO Uncertainty related to atmospherical

forcing of NEMO

mailto: didier.swingedouw@cea.fr

Thank you