1. Volcanic Climate Impacts and ENSO Interaction Georgiy Stenchikov Department of Environmental Sciences, Rutgers University, New Brunswick, NJ Thomas Delworth and Andrew Wittenberg NOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ Motivations: Radiative forcing from volcanic aerosols causes strong “forced” climate variations that interfere with the major modes of climate variability. Volcanic “natural experiments” provide an opportunity to learn more about important aspects of the climate system behavior including the long discussed statistical connection between explosive volcanism and El Nino. However, nonlinearity of this interaction makes it difficult to make sense from observations and model simulations. Therefore we conducted ensembles of specifically designed numerical experiments with comprehensive coupled ocean-atmosphere GCMs to address the following questions: How does ENSO modulate climate response to volcanic forcing? What is volcanic impact on ENSO cycle? How does Volcano/ENSO interaction depend on the strength of ENSO events and magnitude of volcanic forcing? We mostly focused not on the ENSO statistics but on the detailed structure of Volcano/ENSO interaction. We hope this analysis will help to better understand the observed statistical connection between explosive volcanism and ENSO.

2. We have used GFDL Coupled Climate Model CM2.1 with 2ºx2.5ºL24 Atmosphere and 1ºx1ºL50 Ocean and Volcanic Aerosols implemented by Stenchikov et al. (2006) Radiative Impact of Explosive Volcanism in 1980-1995 Dominated other Forcings Producing Significant Transient Cooling Pinatubo: - 0.75 W/m 2 average for 10 years El Chichon: - 0.50 W/m 2 average for 10 years

3. Volcanic Impacts could be seen in the all components of Climate System Time-depth plot of global mean temperature anomaly (K) caused by volcanic and solar forcings calculated as the ensemble mean of the GFDL IPCC AR4 experiments minus the control integration (Delworth, Ramaswamy, and Stenchikov, 2005) 1860 19001940 1980

4. Observed (CRU, Phil Jones) and Simulated Surface Air Temperature Anomalies (K) from the GFDL IPCC AR4 Runs

5. Observed Simulated EL CHICHON PINATUBO in GFDL CM2.1 IPCC AR4 runs Nino3.4 Index from observations and from ensemble of GFDL CM2.1 IPCC AR4 Runs

6. El Nino, La Nina, and Neutral Initial Conditions for 20-year Ensemble Runs

7. ENSO EFFECT ON RESPONSE: Global Surface Air Temperature Response to Pinatubo Forcing in the Runs with the El Nino and La Nina Initial Conditions

8. The top panel shows the three observed ENSO indices used in the iterative method for separating volcano and ENSO signals. Removal of the estimated ENSO signals yields the residual lower tropospheric temperatures shown at the bottom panel [Santer et al., 2001] Observed Surface Air Temperature Response

9. PINATUBO

10. Volcanic Impact on ENSO of Different Amplitude

11. Mechanism of ENSO response to volcanic forcing

12. Effect of Strong Volcanic Forcing: 3xPinatubo and 5xPinatubo

13. Conclusions We used the GFDL coupled ocean-atmosphere CM2.1 modeling system to conduct ensembles of runs with El Nino, La Nina, and Neutral initial conditions varying strength of ENSO events and magnitude of volcanic forcing. Our findings are as follows: El Nino tends to delay maximum cooling caused by volcanic aerosols, while in the La Nina cases cooling develops earlier in time. Global temperature in the coupled system relaxes for about 7 years in agreement with observations. In the runs with El Nino initial conditions volcanic cooling decreases amplitude of El Nino but causes warming of equatorial SST in the year following an El Nino event. In the runs with the neutral initial conditions volcanic impact tends to produce El Nino-like response in the second year after volcanic eruption. However, this effect is fairly weak. The warming effect in the year after El Nino is robust. It gets stronger in the runs with weaker El Nino and with increase of volcanic forcing. The 1xPinatubo and 3xPinatubo cooling impact in the first year after eruption could only decrease El Nino amplitude, while 5xPinatubo forcing reduces El Nino almost completely. Volcanic cooling affects the Bjerknes Feedback. Associated SST warming is caused by reduction of the strength of the upwelling because of weakening of the trade winds according to the Ocean Dynamical Thermostat Mechanism of Clement et al. (1996). Decreasing of El Nino amplitude caused by volcanic cooling also could weaken the relaxing Kelvin wave that makes relaxation process less intensive and could cause a warming effect in the Eastern Equatorial Pacific in the year following El Nino.