Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 4 2005 10.5194/acpd-5-7283-2005 http://www.atmos-chem-phys-discuss.net/5/7283/2005/ http://www.atmos-chem-phys-discuss.net/5/7283/2005/acpd-5-7283-2005.html http://www.atmos-chem-phys-discuss.net/5/7283/2005/acpd-5-7283-2005.pdf 7283 7308 2005-08-22 The initial dispersal and radiative forcing of a Northern Hemisphere mid latitude super volcano: a Yellowstone case study C. Timmreck H.-F. Graf Max-Planck Institut für Meteorologie, Hamburg, Germany Centre for Atmospheric Science University of Cambridge, Cambridge, UK The chemistry climate model MAECHAM4/CHEM with interactive and prognostic volcanic aerosol and ozone, was used to study the initial dispersal and radiative forcing of a possible Yellowstone super eruption. Tropospheric climate anomalies are not analysed since sea surface temperatures are kept fix. Our experiments show that the global dispersal of a Yellowstone super eruption is strongly dependent on the season of the eruption. In Northern Hemisphere summer the volcanic cloud is transported westward and preferentially southward, while in Northern Hemisphere winter the cloud is transported eastward and more northward compared to the summer case. Aerosol induced heating leads to a more global spreading with a pronounced cross equatorial transport. For a summer eruption aerosol is transported much further to the Southern Hemisphere than for a winter eruption. In contrast to Pinatubo case studies, strong cooling tendencies appear with maximum values of –1.6 K/day three months after the eruption in the upper tropical stratosphere. This strong cooling effect weakens with decreasing aerosol density over time and initially prevents the aerosol laden air from further active rising. All-sky net radiative flux changes of more than 32 W/m² at the surface are about a factor of 6 larger than for the Pinatubo eruption. Large positive flux anomalies of more than 16 W/m² are found in the first months in the tropics and sub tropics. These strong forcings call for a fully coupled ocean/atmosphere/chemistry model to study climate sensitivity.