Climate Change and the Middle Atmosphere. Part II: The Impact of Volcanic Aerosols

The effects of volcanic aerosols on the middle atmosphere are investigated with the Goddard Institute for Space Studies (GISS) Global Climate/Middle Atmosphere model. Volcanic aerosols with a visible optical depth of 0.15 are put into the lower stratosphere, and their influence is explored for different time scales: instantaneous effect (sea surface temperatures not allowed to adjust); influence for the first few years, with small tropospheric cooling; and long-term effect (50 years) with significant tropospheric cooling. The aerosols induce a direct stratospheric response, with warming in the tropical lower stratosphere, and cooling at higher latitudes. On the shorter time scales, this radiative effect increases tropospheric static stability at low- to midlatitudes, which reduces the intensity of the Hadley cell and Ferrel cell. There is an associated increase in tropospheric standing wave energy and a decrease in midlatitude west winds, which result in additional wave energy propagation into the stratosphere at lower midlatitudes in both hemispheres. Convergence of this flux in the middle atmosphere increases the residual circulation, producing low-latitude cooling and high-latitude warming near the stratopause. The dynamical changes are on the order of 10%, and are generally similar to occurrences following major volcanic eruptions in the last 30 years. On the longer time scale, a strong hemispheric asymmetry arises. In the Northern Hemisphere eddy energy decreases, as does the middle-atmosphere residual circulation, and widespread stratospheric cooling results. In the Southern Hemisphere, the late increase in sea ice increases the tropospheric latitudinal temperature gradient, leading to increased eddy energy, an increased middle-atmosphere residual circulation, and some high-latitude stratospheric warming. The different experiments emphasize that the middle-atmosphere response to climate change depends on both the direct and indirect (i.e., tropospheric) effects. Similarly, the tropospheric changes are not simply the products of the direct climate perturbation; they depend as well on what happens to the stratosphere. Such examples of the coupled systems underline the need to include both the troposphere and middle atmosphere in studying the effects of climate change.