Georgiy L. Stenchikov and Alan Robock

Department of Environmental 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. 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. These particles also serve as surfaces for heterogeneous chemical reactions, which affect ozone in the lower stratosphere and therefore change absorption of ultraviolet radiation.
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 Arctic 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. These changes in circulation were caused by stratosphere-troposphere dynamical coupling forced by perturbation of radiative heating/cooling in the stratosphere and the troposphere caused by volcanic aerosols.

To test these observations and theoretical hypotheses, we have conducted experiments with climate models. To force climate models, we have developed an aerosol data set using satellite and lidar data used to fill in gaps in satellite coverage. Nevertheless, the existing observing system for stratospheric aerosols has many gaps in time and space. A stratospheric aerosol data assimilation system, capable of blending of different type of observations from numerous sources, is needed to reconstruct past aerosol distributions and to be able to produce a stratospheric aerosol distribution in real time.

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.