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P. Braconnot, Laboratoire des Sciences du climat et de l’envrionnement, Gif-sur-Yvette, France

Past ENSO : observations and simulations with the IPSL model for the Holocene and the last Millennium. P. Braconnot, Laboratoire des Sciences du climat et de l’envrionnement, Gif-sur-Yvette, France. With input from different collegues. High resolution proxy records to infer ENSO variability.

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P. Braconnot, Laboratoire des Sciences du climat et de l’envrionnement, Gif-sur-Yvette, France

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  1. Past ENSO : observations and simulations with the IPSL model for the Holocene and the last Millennium P. Braconnot, Laboratoire des Sciences du climat et de l’envrionnement, Gif-sur-Yvette, France With input from different collegues

  2. High resolution proxy records to infer ENSO variability From T. Correge, G. Cabioche, C. E. Lazareth, M. Carré S. Tudhope, M. Helliot

  3. Massive corals Chemical analyses of massive corals: A window on tropical climate T. Correge S. Thudope ~8000 yr old porites coral

  4. Tracers in corals: Sr/Ca Calibration of Sr/Ca vs SST in a live coral from New Caledonia Sr/Ca appears to be a robust SST tracer ("proxy") from Correge

  5. Jarvis coral d18O and NINO3.4 SST r2 = 0.62 for monthly data (Tudhope et al., unpublished)

  6. Marine bivalves

  7. ENSO strength from corals and bivalves From Living and fossil corals (Tudhope et al, 2001) Comparison with bivalves Cabioche and Correge

  8. Paleo-ENSO from mollusk shells • Ex East Pacific R=0.74 DTCallao 50-70% reduction of ENSO activity Annual Niño1+2 index M. Carré

  9. Multi proxy reconstruction for the early to mid Holocene • Need to combine different proxy to reconstruct mean-state, seasonality and variability • Variability reduced in Early to mid-Holocene • Link with the insolation forcing through Bjerknes feedback (Clement et al. 1999, 2000, etc…) or monsoon activity (Liu et al. 2003) • La Niña type mean state ? From literature : Braconnot et al. subm

  10. ENSO in PMIP 6ka simulations • Almost all simulations show reduced ENSO • The increased monsoon counteracts the development of ENSO • Case where the change in large scale dynamics dominate the signal different from future or LGM where thermodynamics seems more important. Niño 3 ENSO reduction underestimated? Pb model? Pb proxy interpretation? Zheng et al 2008

  11. Mid and Early HoloceneResponse to insolation forcing and fresh water fluxes Results from Yihu Luan’s PhD Collaboration S. Brewer, W. Zheng Braconnot et al. Subm.

  12. Insolation 6ka-0ka Insolation 9.5ka-0ka > 20 W/m2 > 25 W/m2 Latitude Latitude Early vs mid-Holocene? • Differences in the response depending on precession? Summer solstice Summer solstice From Berger 1978

  13. Simulations with IPSLCM4 OAGCM • Ocean-atmosphere-sea-ice-land surface • Same version as in AR4 and PMIP2 • Resolution atm : 96x72xL19 • Ocean : 2° (refined at eq) xL31 Test to insolation forcing only Marzin and Braconnot 2009

  14. Role of fresh water in the North Atlantic? Ice-sheet Fresh water interactive with climate, uniformly distributed over the ocean > 40°N in Atlantic and Arctic • Fresh water flux on present day mean state enhances ENSO in a multimodel ensemble simulations (Timmermann et al. 2007) • Test for 9.5ka and 6ka using IPSL-CM4 and a crude representation of interactive ice-sheet melting with climate Following Swingedouw et al. 2006 Test to insolation forcing and fresh water forcing

  15. Change in DJF interannual variability SST : standard deviation 0k Insolation 6k Insolation +wF • Variability larger in model than in observations • Variability increases from 9.5k to 0k • Fresh water flux in NA enhances variability 9.5k

  16. Change in seasonality Niño 3 box : annual mean cycle Wind and precipitation changes in July-August mm/d Insolation Fresh water Braconnot et al. Subm. Luan et al. In prep

  17. Months Yr previous Yr after ENSO composites Typical evolution in Niño3 box Ex: role of EH insolation mm/d Niño Niña Rms computed over this period SST isolines Pr : color Normal= seasonal cycle

  18. Relative changes induced by changes in seasonality, El-Niño and La Niña on SST Braconnot et al. Subm.

  19. The last millenium: ENSO response to volcanic forcing From M. Khodri

  20. Influence of external (volcanic and solar) forcing on SST? Gao et al. 2008 Khodri et al. In prep IPSL-CM4 Model Calendar time Volcanic Forcing • Warm Medieval Period • (≈ 950-1250 Year A.D.) • simulated cooling in Niño 3 in respone to volcanic forcing • Persistant cooling in data (multi-year La Niña or La Niña like state? • Transition to LIA • (≈1400-1700?) • Higher • Low Frequency (decadal) Variability • Modulation of El Niño events? Mann et al 2009

  21. SST (ENSO) response to Volcanic Forcing Selected Eruption (max >2.5 W/m2) Niño 4 Index Niño 3 Index Control Run Control Run SST Anomaly (°C) SST Anomaly (°C) Millennium Run Millennium Run SST Anomaly (°C) SST Anomaly (°C) Volcanic Forcing (W/m2) Volcanic Forcing (W/m2) Random SST with same variance (s) as in CTRL Simulated SST SST > s for at least 9 months

  22. Conclusion • Insolation and fresh water flux damp the SST seasonal cycle in the east Pacific (link with large scale dynamics) • Increase ENSO magnitude from 9.5 ka to 0k in response to insolation • Early and mid-Holocene ENSO development is damped in Autumn • Fresh water flux enhances ENSO magnitude • Volcanic eruptions favors increased decadal variability at the MWP/LIA transition in last millennium simulation with insolation and volcanic forcing • Colder SST (more La Niña? ) after volcanic eruption? What about the hypothesis of El-Niño 1 year after? • Need to better understand relative impact of seasonality and variability on relationship between proxy records and ENSO variability to produce reliable model-data comparisons.

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