A new approach to reproduce the 20 th 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 )

Framework of this study (1) SST - African monsoon interannual teleconnections  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)  Observations (1901– 2000) Leading mode of a Maximum Covariance Analysis of JJAS west African precipitation & tropical SSTs CRU & HadISST

Framework of this study (2)  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.  IPCC Coupled Simulations The Pacific teleconnection looks quite different from one model to another. (Joly et al. 2007) 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.

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

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

SST annual cycles Niño-4Niño-3  ENSO area Gulf of Guinea  Gulf of Guinea Anomalies

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

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

El Niño SST composite AMJ JAS JFMOND HadISST AMJ JAS JFMOND PACIF AMJ JAS JFMOND Coupled REF AMJ JAS JFMOND GLOBE

La Niña SST composite AMJ JAS JFMOND HadISST AMJ JAS JFMOND PACIF AMJ JAS JFMOND GLOBE AMJ JAS JFMOND Coupled REF

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

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

PACIF GLOBE Coupled REF 5 – 7 yrs PACIF Coupled REF 2.9 – 3.7 yrs African monsoon interannual variability (2) Spectrum of the leading EOF time-series CRUGPCC 5 – 6 yrs 2.5 – 4 yrs

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

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

PACIF AMJ JAS JFMOND JAS HadISST AMJ JAS JFMOND JAS ENSO-monsoon teleconnection: Synthesis (1) Coupled REF AMJ JAS JFMOND JAS

PACIF AMJ JAS JFMOND JAS HadISST AMJ JAS JFMOND JAS Coupled REF AMJ JAS JFMOND JAS PACIF AMJ JAS JFMOND JAS Coupled REF AMJ JAS JFMOND JAS ENSO-monsoon teleconnection: Synthesis (2) GLOBE AMJ JAS JFMOND JAS ?

Conclusion  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 A new approach to reproduce the 20 th century ENSO variability in an OAGCM  But this protocole has a strong impact on the ENSO – monsoon teleconnection, which remains to be understood in terms of atmospheric mechanisms…  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

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