1. Interactions between volcanic eruptions and El Niño: Studies with a coupled atmosphere-ocean model C. Timmreck, M. Thomas, M. Giorgetta, M. Esch, H.-F. Graf 1, H. Haak, J. Jungclaus, W. Müller, E. Roeckner, H. Schmidt and G. Stenchikov 2 Max Planck Institute for Meteorology, Hamburg 1 Centre Atmospheric Sciences University, Cambridge 2 Rutgers-The State University of NJ, New Brunswick AGU Spring meeting 2007

2. o The last three biggest eruptions (Agung (1963), El Chichón (1982) and Pinatubo (1991)) occurred at the time as an El Niño event. Dynamic response due to the combined effects of El Niño and volcanic eruptions are still uncertain. Paleo reconstructions (Adams et al, 2003) show that large volcanic eruptions enhance the probability of an El Niño event. AOGCMs are an important tool to improve our understanding of the atmospheric and oceanic response to the combined effects of El Niño and large volcanic eruptions. Coupled El Niño Volcanic runs

3. We have carried out a series of volcanic simulations with the AOGCM, ECHAM5/MPIOM. The volcanic radiative forcing is calculated online in the model. A realistic spatial-temporal distribution of aerosol optical parameters for the Pinatubo episode is used. Optical depth in the visible

4. Niño 3.4 SST anomalies in ECHAM5/MPIOM The frequency analysis of the El Niño events show good agreement with observations. However, the amplitude of 1.7 K is higher than in the IPCC runs (1.5 K) and in the AMIP cases (1.K)

5. Three different cases are selected from a 100 year control run. 5 ensembles for Jan and June, perturbed / unperturbed. Each ensemble run has been performed for two years. Coupled El Niño Volcanic runs Niño 3.4 Case INiño 3.4 Case IIINiño 3.4 Case II

6. Stratospheric temperature response [K] ERA 40 Reanalysis Case I June 30 hPa 100 hPa 50 hPa

7. Temperature response at 50 hPa Case I June Case II June Case III June Case II January Case I January Case III January

8. GPH Anomaly 50 hPa DJF „91/92“ Era 40 Reanalysis Case I JuneCase I January Case II JanuaryCase II June

9. 2m temperature anomaly [k] DJF „91/92“ Era 40 Reanalysis Case I JanuaryCase I June Case II JuneCase II January

10. Case II June 2m temp. anomalies [K] DJF 91/92

11. Atmospheric response The stratospheric tropical temperature response is similar for all cases and in agreement with observations. The dynamical response in the NH high latitudes is highly variable. Only in a few cases the observed pattern is mirrored. However, some ensemble members reflect the observed pattern. Further analysis (EP fluxes, refractive indices) is necessary Note: model top at 10 hPa !

12. January June Niño 3.4 Case III CTRL Niño 3.4 SST anomalies [k] Case III

13. Niño 3.4 SST anomalies [k] Case II January June Niño 3.4 Case II CTRL

14. Niño 3.4 SST anomalies [k] Case I Niño 3.4 Case I CTRL January June

15. U10 [m/s] + 4 - 4 + 2 - 2 0 Ocean T 6 m [K] Hovmoeller diagram of anomalies 5N-5S Case I June Ocean T 100 m [K]

16. Oceanic response Large interannual variability can be detected in the tropical dynamical ocean heat response. Significant ocean response only in two cases: Case I June where the model is triggered into a La Niña state. Ekman pumping in the mid equatorial Pacific leads to significant cooling in the perturbed model run. Case II January where radiative effects damp the El Niño signal

17. Conclusion I Based on our model results we cannot support the hypothesis from Adams et al.(2003) that volcanic eruptions enhance the possibilty of an El Ni ñ o event. The dynamical response in the atmosphere and in the ocean is highly variable and differs between the selected cases, but also quite strongly between the single ensemble members. Further analysis is necessary to learn more about the physical mechanisms behind and why the ensemble members behave differently Ongoing work

18. Conclusion II Thank you very much for your attention! We had some questions before we started. Now we have even more.

20. Coupled El Niño Volcanic runs Oceanic response

21. Temperature differences (K) and respective significances in the equatorial pacific. The grey scale indicates significances of 90, 95, and 99%. Ocean depth (m) 199201, 7 months after eruption Ocean depth (m) Temperature response in the equatorial Pacific Case I June

22. Nino 82/83

23. Nino 91792

24. Coupled El Niño Volcanic runs Atmospheric response

25. Case I June 2m temp. anomalies

26. Case I June GPH anomalies at 50 hPa

27. Case II June 2m temp. anomalies

28. Case II June GPH anomalies at 50 hPa