A new approach to reproduce the 20 th century ENSO variability in an OAGCM

1. A new approach to reproducethe 20th century ENSO variability in an OAGCM Mathieu Joly ( Météo-France / CNRM )PhD-Thesis Director:S. Janicot( IPSL / LOCEAN )Research Guidance:J.-F. Royer & A. Voldoire( Météo-France / CNRM )

2. Framework of this study (1) • Observations(1901– 2000) CRU & HadISST Leading mode of a Maximum Covariance Analysis of JJAS west African precipitation & tropical SSTs • IPCC Coupled Simulations The Pacific teleconnection looks quite different from one model to another. (Joly et al. 2007) Météo-France CNRM-CM3 coupled model (IPCC-AR4 version) SST - African monsoon interannual teleconnections

3. Framework of this study (2) • IPCC Coupled Simulations The Pacific teleconnection looks quite different from one model to another. (Joly et al. 2007) • This could be because of: • The timing of the simulated ENSO • and / or the position of the SST anomalies in the Pacific. • The atmospheric teleconnection that links the Pacific SST anomalies • with the monsoon. In this study, we try to improve the ENSO variability in the coupled model, in order to assess the consequences for the teleconnection. The pacemaker experiment would be a good solution, but we have developped an other method: • A reanalyzed Wind-Stress is used here to drive the ocean, while keeping all other fluxes fully coupled.

4. Wind-Stress forcing: How? ERA40 Wind-Stress ARPEGE-Climat Forcing OASIS ERA40 daily Wind-Stress Buffer Zone CNRM-CM3 Version Coupling OPA Ocean Model Spin-up: 10 yrs Simulation: 1960-2001 • Over the tropical Pacific (PACIF) • Over all the oceans (GLOBE)

5. SST biases JAS (Monsoon Season) SST Mean SST St-Dev HadISST Coupled REF PACIF GLOBE

6. SST annual cycles • ENSO area Anomalies • Gulf of Guinea Gulf of Guinea Niño-4 Niño-3

7. Does it work? Where? GLOBE experiment JFM JAS Grid-point correlations between Observed and Simulated SST anomalies

8. ENSO variability Band-Pass filter: 17.5 months – 7.3 years YEARS Filtered Niño 3.4 SST index Filtered Niño 3.4 SST Spectrum

9. El Niño SST composite JAS OND JFM AMJ HadISST JAS OND JFM AMJ Coupled REF JAS OND JFM AMJ PACIF JAS OND JFM AMJ GLOBE

10. La Niña SST composite JAS OND JFM AMJ HadISST JAS OND JFM AMJ Coupled REF JAS OND JFM AMJ PACIF JAS OND JFM AMJ GLOBE

11. Wind-Stress forcing: Synthesis • Using a reanalyzed Wind-Stress to drive the ocean in the Coupled Model allows us: • To reproduce quite accurately the observed 1960-2001 ENSO timing • To improve the SST biases and annual cycles in the equatorial Pacific, • but also in the Gulf of Guinea (GLOBE experiment) • To yield rather realistic ENSO composites  What consequences for the African monsoon?

12. African monsoon interannual variability 45% Coupled REF CRU 26% 32% PACIF GPCC 30% 35% GLOBE JAS Precipitation EOF leading mode (with prior filtering of the decadal variability)

13. Links with SST? Coupled REF PACIF CRU GPCC GLOBE Correlation of the leading EOF timeseries with the JAS SST anomalies (with prior filtering of the decadal variability)

14. ENSO-monsoon teleconnection Correlation of the leading precipitation EOF timeseries with the lagged Niño-3.4 SST indices

15. ENSO-monsoon teleconnection: Synthesis (1) JAS JAS JAS JAS OND OND JFM JFM AMJ AMJ HadISST JAS JAS OND JFM AMJ Coupled REF PACIF

16. ENSO-monsoon teleconnection: Synthesis (2) JAS JAS JAS JAS JAS JAS JAS JAS OND OND OND OND JFM JFM JFM JFM AMJ AMJ AMJ AMJ HadISST JAS JAS JAS JAS OND OND JFM JFM AMJ AMJ Coupled REF Coupled REF PACIF PACIF GLOBE ?

17. Conclusion A new approach to reproducethe 20th century ENSO variability in an OAGCM • ENSO – African monsoon teleconnections are poorly reproduced in IPCC simulations (Joly et al. 2007) • Using a reanalyzed Wind-Stress to drive the ocean in our coupled model allows us to reproduce the historical ENSO timing • The idea of such experiments is similar to the "pacemaker" protocol: • Thanks to the realistically simulated ENSO timing it is now possible to make year-to-year comparisons between simulations and observations • Furthermore, forcing the wind-stress preserves the heat budget at the ocean surface, and partially preserves the ocean-atmosphere interactions • But this protocole has a strong impact on the ENSO – monsoon teleconnection, which remains to be understood in terms of atmospheric mechanisms…

18. End of this talk… … Thank you for your attention.