the altitude of the ozone change; increases below 29 km produce surface warming, and increases above 29 km produce surface cooling. Model simulations suggest that such radiative coupling effects can alter the strength of the Hadley cell circulation, with attendant effects on, for example, Atlantic hurricane flows (Haigh, 2003).

Solar-induced indirect effects on climate may also involve altered modes of variability. Model simulations and analyses of patterns of variability suggest that the Arctic Oscillation (AO), or Northern Annular Mode (NAM), and its subset the North Atlantic Oscillation (NAO) propagate from the stratosphere to the troposphere (Baldwin and Dunkerton, 1999). Radiative forcings that impact the stratosphere could alter this coupling. Contemporary observations suggest that the NAM manifests itself primarily in the North Atlantic sector, as the NAO, during solar cycle minima, and extends more uniformly over all longitudes, as the AO, during solar maxima (Kodera, 2002). The effect of the Sun on the NAM may further depend on the phase of the quasi-biennial oscillation (QBO) in stratospheric equatorial winds (Ruzmaikin and Feynman, 2002). Reduced solar activity in the Maunder Minimum may have produced a negative NAO phase (compared with the current positive phase), based on empirical analysis of historical surface temperature fields and model simulations (Shindell et al., 2001b). Additional evidence that the phase of the QBO changes with the solar cycle (Salby and Callaghan, 2000) underscores the complicated, multifaceted nature of indirect solar effects on climate.

VOLCANIC ERUPTIONS

Emissions from volcanic eruptions have multiple effects on climate as listed in Table 2-3 (Robock, 2002). A number of studies have evaluated the role of volcanic forcing in climate change during the twentieth and earlier centuries (Free and Robock, 1999; Crowley, 2000; Bertrand et al., 2002; Bauer et al., 2003). These studies suggest that volcanic forcing is the dominant source of natural global radiative forcing over the past millennium. The greater prevalence of explosive volcanic activity during both the early and the late twentieth century and the dearth of eruptions over the interval from 1915 to 1960 represents a significant natural radiative forcing of twentieth century climate (e.g., Crowley, 2000). Similarly, the longer-term volcanic radiative forcing has been associated with a significant long-term forced cooling from A.D. 1000 to A.D. 1900 resulting from a general increase in explosive volcanic activity in later centuries (Crowley, 2000; Bertrand et al., 2002; Bauer et al., 2003; Crowley et al., 2003; Hegerl et al., 2003). Some spatially resolved simulations of volcanic forcing indicate a large continental summer cooling but a tendency for a dynamically induced, offsetting winter warming (Stenchikov et al., 2002; Shindell et al.,



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