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Evaluation of volcanic climate effects in the IPCC historic runs

PI: Georgiy Stenchikov
Institution: Rutgers University
Additional Investigators: K. Hamilton, V. Ramaswamy, A. Robock, R. Stouffer
Volcanic aerosols cool the troposphere and the Earth’s surface reflecting solar radiation and warms the lower stratosphere by absorption of thermal IR and solar near-IR radiation. Together with the direct radiative effect, volcanic forcing affects circulation in the stratosphere and troposphere shifting the Arctic Oscillation (AO) toward a positive phase. The duration of the forcing for each particular eruption is about 2 years. We propose to evaluate how the IPCC models simulate global and regional volcanic impacts on climate during historic period since 1860 to 2000.

The global average cooling of the climate system caused by volcanic aerosol is statistically stable however both observed and simulated responses have to be cleaned from ENSO to make fair comparison. The regional responses to individual (even strongest) eruptions , both caused by radiative cooling and change of circulation, are very difficult to evaluate because the observed signal is contaminated by climate variability.

Therefore in this study we propose to conduct a composite analysis of 12 strongest volcanic eruptions since 1860 to 2000 discussed in [Robock and Mao, 1992]. We combine these eruption in the 3 groups because different type of eruptions caused different climate effects:
1. Equatorial eruption that produced volcanic cloud in both hemispheres – 6 eruptions
2. SH eruptions that produced plume in the SH – 3 eruptions
3. High-latitude NH eruptions with plume in the NH – 3 eruptions

For all models we will analyze summer and winter responses for 2 years following the volcanic eruptions and compare results with corresponding composites from observations. The different ENSO phase in simulations and observations could negatively affect comparison but a composite averaging should diminish this effect. The composite analysis will provide more reliable evaluation with observations than analysis of the individual cases. We will analyze surface air temperature, sea level pressure, 50 hPa geopotential height, and temperature at 50 hPa. Only surface air temperature could be evaluated with observations from the Climatic Research Unit for entire period since 1860 to 2000. The other parameters will be evaluated using reanalysis data for 40 years that include strong eruptions of Agung, El Chichon, and Pinatubo.

In addition to the analysis of the seasonal fields we will calculate anomalies of global temperature and radiative fluxes at the top of the atmosphere. We will remove ENSO signal from global means and compare simulated responses with observations. Global surface air temperature will be evaluated for entire period of historic run, but radiative fluxes could be compared with observations for about 20 years when ERBE data are available.
  • Stenchikov, Georgiy, Kevin Hamilton, Ronald J. Stouffer, Alan Robock, V. Ramaswamy, Ben Santer, and Hans-F. Graf, 2006: Arctic Oscillation response to volcanic eruptions in the IPCC AR4 climate model. J. Geophys. Res., 111, D07107, doi:10.1029/2005JD006286. Abstract. Edit.

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