Alan Robock and Georgiy Stenchikov

Department of Enviromental Sciences, Rutgers University, New Brunswick, New Jersey

Large volcanic eruptions inject sulfur gases into the stratosphere, which convert to sulfate aerosols with an e-folding residence time of about 1 year. Large ash particles fall out much quicker. The radiative and chemical effects of this aerosol cloud produce responses in the climate system. By scattering some solar radiation back to space, the aerosols cool the surface, but by absorbing both solar and terrestrial radiation. the aerosol layer heats the stratosphere. For a tropical eruption, this heating is larger in the tropics than in the high latitudes, producing an enhanced pole-to-equator temperature gradient, especially in winter. During the winter in the Northern Hemisphere following every large tropical eruption of the past century, surface air temperatures over North America, Europe, and East Asia were warmer than normal, while they were colder over Greenland and the Middle East. This pattern and the coincident atmospheric circulation correspond to the positive phase of the Artic Oscillation. In spite of the decrease in surface solar heating, surface air temperature increases in high and midlatitudes of the Northern Hemisphere in the winter because of changes in tropospheric circulation caused by stratosphere-troposphere dynamical coupling.To test these observations, we have conducted experiments with climate models. To force climate models, we have developed an aerosol data set using satellite and lidar data. The lidar data are used to fill in gaps in satellite coverage. Using the Max Planck Institute ECHAM4 and the Geophysical Fluid Dynamics Laboratory SKYHI GCMs, we have successfully simulated the observed climate response following the 1991 Mount Pinatubo eruption. This result will allow us to produce better seasonal forecasts for the Northern Hemisphere winter following the next large tropical eruption. It also shows that stratospheric forcing of the climate system must be considered along with sea surface temperature anomalies when making seasonal forecasts, especially in mid and high latitudes in the winter.