Radiative Forcing and Climatic Impact of the Mount Pinatubo Volcanic Eruption

Work Done atClimate Research Group

Department of Atmospheric SciencesUniversity of Illinois at Urbana-Champaign

Fanglin Yang

Current AffiliationClimate Prediction Center

Environmental Modeling CenterNational 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 ParticlesH2SO4 /H2O

Reflection of solar radiation & absorption of terrestrial and

solar near-IR radiation

Heterogeneous reactionson aerosol particles N2O5 + H2O 2HNO3

ClONO2 + H2O HNO3 + HOCl

Formation& Growth

DepositionRadiative Effect

Chemical Effect Changes in Photolysis Rate

Changes inatmospheric temperature

Mt. Pinatubo 15 June 1991

SO2

Ash

Redistribution of aerosol particles

Changes in 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.