1. 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

2. 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.

3. 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.

4. Major volcanic eruptions in the past two centuries

5. 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

6. 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.

7. 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.

8. 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.

9. 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.