7.5.4.2 Effects on the Large-Scale Circulation

Several studies have considered the response of a GCM with a mixed-layer ocean to indirect aerosol effects (Rotstayn et al., 2000; K. Williams et al., 2001; Rotstayn and Lohmann, 2002) or to a combination of direct and indirect aerosol effects (Feichter et al., 2004; Kristjansson et al., 2005; Takemura et al., 2005). All of these, and recent transient simulations (Held et al., 2005; Paeth and Feichter, 2006), found a substantial cooling that was strongest in the NH, with a consequent southward shift of the Inter-Tropical Convergence Zone (ITCZ) and the associated tropical rainfall belt. Rotstayn and Lohmann (2002) even suggest that aerosol effects might have contributed to the Sahelian droughts of the 1970s and 1980s (see Sections 9.5 and 11.2). If in turn the NH is warmed, for instance due to the direct forcing by black carbon aerosols, the ITCZ is found to shift northward (Chung and Seinfeld, 2005).

Menon et al. (2002b) and Wang (2004) found that circulation changes could be caused by aerosols in southeast China. In India and China, where absorbing aerosols have been added, increased rising motions are seen as well as increased subsidence to the south and north (Menon et al., 2002b). However, Ramanathan et al. (2005) found that convection was suppressed due to increased stability resulting from black carbon heating. Drier conditions resulting from suppressed rainfall can induce more dust and smoke due to the burning of drier vegetation (Ramanathan et al., 2001), thus affecting both regional and global hydrological cycles (Wang, 2004). Heating of a lofted dust layer could increase the occurrence of deep convection (Stephens et al., 2004). It can also strengthen the Asian summer monsoon circulation and cause a local increase in precipitation, despite the global reduction of evaporation that compensates aerosol radiative heating at the surface (Miller et al., 2004b). The dust-induced thermal contrast changes between the Eurasian continent and the surrounding oceans are found to trigger or modulate a rapidly varying or unstable Asian winter monsoon circulation, with a feedback to reduce the dust emission from its sources (Zhang et al., 2002).

In summary, an increase in atmospheric aerosol load decreases air quality and reduces the amount of solar radiation reaching the surface. This negative radiative forcing competes with the greenhouse gas warming for determining the change in evaporation and precipitation. At present, no transient climate simulation accounts for all aerosol-cloud interactions, so that the net aerosol effect on clouds deduced from models is not conclusive.