Radiative Forcing and Climatic Impact of the Mount Pinatubo Volcanic Eruption Fanglin Yang Work Done at Climate Research Group Department of Atmospheric Sciences University of Illinois at Urbana-Champaign Current Affiliation Climate Prediction Center Environmental Modeling Center National Center for Environmental Prediction

Objectives To reconstruct optical properties & calculate radiative forcing. To assess the contributions of the Pinatubo volcanic aerosols and El Niño events to the observed surface-air temperature anomalies. To simulate and understand the atmospheric temperature changes.

Optical Properties & Radiative Forcing Points: The Pinatubo aerosol imposed a negative net forcing to the atmosphere. The maximum TOA forcing was about –4.8 W/m2 in DJF 91/92. The Pinatubo aerosol radiatively heated the middle and lower stratosphere and radiatively cooled the troposphere.

Major volcanic eruptions in the past two centuries

Formation and Deposition of Volcanic Aerosol Particles and Their Impact on the Atmosphere SO2 + OH + M  HSO3 HSO3 + O2  HO2 + SO3 SO3 + H2O  H2SO4 Gas Phase Reaction Nucleation Coagulation Condensation Evaporation Sedimentation Rainfall Washout Aerosol Particles H2SO4 /H2O Reflection of solar radiation & absorption of terrestrial and solar near-IR radiation Heterogeneous reactions on aerosol particles N2O5 + H2O  2HNO3 ClONO2 + H2O  HNO3 + HOCl Formation & Growth Deposition Radiative Effect Chemical Effect Changes in Photolysis Rate atmospheric temperature Mt. Pinatubo 15 June 1991 SO2 Ash Redistribution of aerosol particles circulation Ozone losses

Observed Temperature Changes Points: In two years following the Pinatubo eruption, large stratospheric warming and minor tropospheric cooling were observed. Over North America and Eurasia, surface warming occurred in DJF 91/92 and DJF 92/93, and surface cooling occurred in JJA 1992 and JJA 1993.

Signal Separation of El Niño and Volcanic Eruption Points: The signals of El Niños and volcanic eruptions in surface-air temperature (SAT) anomalies are distinguishable. The influence of El Niño events on SAT is more significant over North America than over Eurasia. In JJA 1992, global mean SAT anomaly over land: ENSO-related –0.21°C, Pinatubo-related: –0.42°C.

Ensemble Numerical Simulations Points: The ST-GCM simulates better JJA cooling than DJF warming near the surface. The signal of SSTA is the strongest in the troposphere and near the surface, and the signal of volcanic aerosol forcing is the strongest in the stratosphere. The simulated SAT forced by Pinatubo aerosol is sensitive to initial conditions and prescribed SST, while the simulated SAT in the stratosphere is not. The ST-GCM overestimated the stratospheric warming by 1~2°C in 1992.

Influences of the QBO and Ozone Depletion Points: The QBO-related temperature variation reduced the observed Pinatubo-induced lower stratospheric warming by 1°C before Aug 1992, and increased by 1°C after Aug 1992. The observed ozone depletion after the Pinatubo eruption cooled the lower stratospheric by 0.5°C ~ 1.0°C.