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

Abstract

The effects of volcanic aerosols on the middle atmosphere are investigatedwith the Goddard Institute for Space Studies (GISS) Global Climate/MiddleAtmosphere model. Volcanic aerosols with a visible optical depthof 0.15 are put into the lower stratosphere, and their influenceis explored for different time scales: instantaneous effect (seasurface temperatures not allowed to adjust); influence for the firstfew years, with small tropospheric cooling; and long-term effect(50 years) with significant tropospheric cooling.The aerosols inducea direct stratospheric response, with warming in the tropical lowerstratosphere, and cooling at higher latitudes. On the shorter timescales, this radiative effect increases tropospheric static stabilityat low- to midlatitudes, which reduces the intensity of the Hadleycell and Ferrel cell. There is an associated increase in troposphericstanding wave energy and a decrease in midlatitude west winds, whichresult in additional wave energy propagation into the stratosphereat lower midlatitudes in both hemispheres. Convergence of this fluxin the middle atmosphere increases the residual circulation, producinglow-latitude cooling and high-latitude warming near the stratopause.The dynamical changes are on the order of 10%, and are generallysimilar to occurrences following major volcanic eruptions in thelast 30 years.On the longer time scale, a strong hemispheric asymmetryarises. In the Northern Hemisphere eddy energy decreases, as doesthe middle-atmosphere residual circulation, and widespread stratosphericcooling results. In the Southern Hemisphere, the late increase insea ice increases the tropospheric latitudinal temperature gradient,leading to increased eddy energy, an increased middle-atmosphereresidual circulation, and some high-latitude stratospheric warming.Thedifferent experiments emphasize that the middle-atmosphere responseto climate change depends on both the direct and indirect (i.e.,tropospheric) effects. Similarly, the tropospheric changes are notsimply the products of the direct climate perturbation; they dependas well on what happens to the stratosphere. Such examples of thecoupled systems underline the need to include both the troposphereand middle atmosphere in studying the effects of climate change.