Respective forcing of the Indian OceanAnd Western Pacific warming on theNorthern hemisphere atmosphericcirculation

Bologna, Feb. 2008Cassou, Sanchez, Hodson, Keenlyside, Okumura, Zhou, Sutton

SST [1950-1975] – SST[1960-1990]

SST [1976-2001] – SST[1960-1990]

1. Quick summary of the experimental design

SST [1976-2001] – SST[1960-1990]

SST averaged over the entire basin

Goal: quantify the influence of the INDO-PACwarming on the Northern hemisphere circulation

1. ARPEGE4.42. HADAM3. ECHAM54. IAP5. CAM3

40-year WIP40-year CIP

CIP

WIP

NCEP Z500 JJA[1976-2001] – [1950-1975]

NCEP Z500 DJF[1976-2001] – [1950-1975]

BL

NAO+

2. Models responses for DJF Precip

ARPEGE HADAM

ECHAM IAP

[40-yr WIP]-[40-yr CIP]

CAM Multi

2 groups of models:

ARP / HAD / ECH : Increased precipAlong a Madagascar/Sumatra line

IAP / CAM : Weak local response

3. Models responses for DJF Z500

ARPEGEHADAM

ECHAMIAP

CAMMulti

[40-yr WIP]-[40-yr CIP]

4. Models responses for DJF Z500

ARPEGE HADAM

ECHAM IAP

CAM Multi

NCEP Z500 DJF[1976-2001] – [1950-1975]

[40-yr WIP]-[40-yr CIP]

5. Models responses for JJA Precip

ARPEGE HADAM

ECHAM IAP

Northward shift of the ITCZIn all the models

[40-yr WIP]-[40-yr CIP]

CAM Multi

Regional discrepancies are associated with different mean states

6. Relationship between response and climatology

ARPEGE HADAM

ECHAM IAP

CAM Multi

Impossible to have a clear/deterministic response

over monsoon regions

[40-yr WIP]-[40-yr CIP]

ARPEGE HADAM

ECHAM IAP

CAM Multi

7. Models responses for JJA Precip over Africa

[40-yr WIP]-[40-yr CIP]

Similar conclusions to e.g. Giannini et al (2003)

Large decrease of rainfallOver Africa

ARPEGE HADAM

ECHAM IAP

CAM Multi

8. Models responses for JJA VP200

[40-yr WIP]-[40-yr CIP]

Large change in Walker cells

10. Models responses for JJA Z500

ARPEGE HADAM

ECHAM IAP

CAMMulti

[40-yr WIP]-[40-yr CIP]

9. Models responses for JJA Z500

ARPEGE HADAM

ECHAM IAP

CAMMulti

NCEP Z500 JJA[1976-2001] – [1950-1975]

[40-yr WIP]-[40-yr CIP]

1. West Pac : OUT

WIP-CIP

WIO-CIO

Western Pacific removed and set to [1961-1990]

Indian SSTs are strictly identical

2. Model response for JJA precip

ARPEGE HADAM

ECHAM IAP

CAM Multi

INDO-PAC INDO-ONLYNorthward shift of the ITCZ

In all the models

Southward shift of the ITCZIn all the models

3. Model response for JJA Precip

ARPEGE HADAM

ECHAM IAP

CAM Multi

INDO-PAC INDO-ONLY

NO SIGNIFICANTCHANGES

4. Model response for JJA VP200

ARPEGE HADAM

ECHAM IAP

CAM Multi

INDO-PAC

INDO-ONLYLOCALCHANGES

5. JJA Z500

ARPEGE HADAM

ECHAM

Multi

IAP

CAM

INDO-PAC INDO-ONLY

WEAK ANDNO CONSISTENT

CHANGES

6. DJF Z500

ARPEGE HADAM

ECHAM

Multi

IAP

CAM

INDO-PAC INDO-ONLY

VERY STRONGAND UNREALISTIC

CHANGES

7. DJF U200(clim)+V200

ARPEGE HADAM

ECHAM IAP

CAM

INDO-PAC

Zonally chains of anomalies associated withDisturbances in the vicinity of the mean jets (particulartly the jet that stretches across South Asia

Branstator (2002)

The Pacific Route along the wave guide

8. DJF U200(clim)+V200

ARPEGE HADAM

ECHAM IAP

CAM

INDO-PAC INDO-ONLY

The Pacific RouteAlong the wave guide

9. Conclusions

1. We need to be very careful in what we call “indian ocean”

2. Differences in responses to Indian ocean warming in literature are due to modelclimatologies and also to the selected forcing domain

3. Indian-West-Pacific warming leads to changes in the North Atlantic that projectsOnto the positive phase of the NAO.

4. True NAO in two models (HAD/ARP) and two wavy patterns (CAM/ECHAM)… two mechanisms (direct route via the Pacific –jet wave guide) and indirect via the walker Cell (still under investigation)

5. Removing the west Pacific warming leads to unrealistic results (very strong in winter,Very weak in summer)… The WP warming tends to counteract the direct Indian warming…Separating the two of them might be not physical.