Climatic Effects of the 1991 Mt. Pinatubo Volcanic Eruption

Following the June 1991 Mt. Pinatubo eruption in the Philippines, the winter of 1991-1992 was very warm over most of North America, Europe, and Siberia, and cold over Greenland and the eastern Mediterranean. It snowed in Jerusalem that winter and corals at the bottom of the Red Sea died because of the cold surface waters. During the next summer of 1992 continental regions of North America and Eurasia were unusually cold. Global warming halted for several years as the planet cooled temporarily. All these climatic effects were due to the response to sulfate aerosols in the stratosphere from the Pinatubo eruption. In this talk we review the climatic response to volcanic eruptions using Pinatubo as a prime example. Both observational studies and general circulation model experiments support these conclusions. By scattering some solar radiation back to space, the stratospheric 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. Together with the forcing from ozone depletion and surface cooling in the subtropics, the dynamical response of the climate system produces temperature changes due to anomalous advection which dominate over the radiative effects in the winter. This atmospheric circulation pattern corresponds 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 caused by stratosphere-troposphere dynamical coupling. This new understanding 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.