Volcanoes in EC-EARTH

Volcanoes, although occurring intermittently and highly localized, are nevertheless an important driver for the global climate. Volcanoes are a source for aerosols that reflect – and to a lesser extent also absorb – solar radiation, and thereby alter the radiation balance of the Earth.

Most of the volcanic exhaust stays within the troposphere from where it is removed within a few days and has little impact on the climate. However, the most powerful volcanic eruptions can transport significant amounts of aerosols into the stratosphere where scavenging is much slower and the radiative forcing by volcanic aerosols can have a discernible effect for years.

Volcanoes - an extra source of aerosols

Volcanoes are not explicitly represented in the EC-EARTH model. Like in other climate models, the effects of volcanoes are parameterized as an extra source of aerosols in the stratosphere. For the CMIP5 simulations, the treatment of volcanoes in EC-EARTH has been entirely revised.

The stratospheric aerosol concentrations are taken from the GISS dataset. EC-EARTH transforms the aerosol mixing ratios into aerosol optical depth (AOD) that is the input for the radiation scheme. As an example, Figure 1 shows the increase and decay of stratospheric AOD during and after the Pinatubo eruption.

RC News May 2011 Timeseries of aerosol optical thickness
Figure 1. Timeseries of zonal cross-sections of stratospheric aerosol optical thickness before, during and after the Pinatubo eruption (Sep 1991).
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Impact of volcanoes on climate simulations

Figure 2 illustrates the impact of the volcanoes on a climate simulation with EC-EARTH in one experiment with and one without volcanoes. Both experiments were run from 1960-1999 and cover three major volcanic eruptions: Mt Agung (1962), El Chichon (1982), and Pinatubo (1991).

RC News May 2011 Global mean annual temperature
Figure 2. Global mean annual temperature at 100 hPa (approx. 16 km altitude). The simulations with and without volcanoes cover the 3 major volcanic eruptions Mt Agung (1962), El Chichon (1982), and Pinatubo (1991).
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A clear signal from volcanoes is an increase of the stratospheric temperature due to the absorption by the volcanic aerosols. At the surface we find a temperature drop albeit with smaller magnitude than the stratospheric warming (not shown here). The cooling at the surface results from the increased reflection of solar radiation by volcanic aerosols. The decrease in the radiative forcing at the surface implies that less energy enters the ocean and has thus far-reaching consequences for the climate in long climate simulations.

The new treatment of volcanoes in EC-EARTH has shown promising results in line with the findings by Bender et al. (2010). The on-going simulations for CMIP5 will reveal more about the impact from volcanoes on the present-day climate.

References

Bender F. A.-M., A.M.L. Ekman, and H. Rodhe (2010): Response to the eruption of Mount Pinatubo in relation to climate sensitivity in the CMIP3 models. CLim. Dyn., doi:10.1007/s00382-010-0777-3