Solar geoengineering has been suggested as a potential approach to counteract the anthropogenic global warming. Major volcanic eruptions have been used as natural analogues to large-scale deployments of stratospheric aerosol geoengineering, yet difference in climate responses to these forcings remains unclear. Among many factors characterizing the difference between the two, durations of the additional aerosol layer in the stratosphere differ substantially between volcanic eruptions and the SAI geoengineering. Sulfate aerosols from volcanic eruptions typically stay in the stratosphere for one to two years. Stratospheric aerosol geoengineering, however, if used to counteract anthropogenic warming, would need to be deployed quasi-continuously and thus the additional aerosols would stay in the stratosphere persistently. Using the NCAR CESM model, we compare the climate response to two highly idealized stratospheric aerosol forcings that have different durations: a short-term pulse representative of volcanic eruptions and a long-term sustained forcing representative of geoengineering. For the same amount of global mean cooling, the pulse case causes much larger reductions in surface temperature over land relative to the sustained case. This greater cooling over land leads to a larger decrease in the vertical motion of air over land in lower atmosphere, and reduces water vapor transport from the ocean to land. For similar amounts of global cooling, the decrease in land runoff caused by a short-term pulse aerosol forcing is about twice as large as that caused by a sustained aerosol forcing. Our results clearly demonstrate difference in the climate response to volcanic-like and geoengineering-like stratospheric aerosol forcings, and suggest that caution should be exercised when extrapolating results from volcanic eruptions to the SAI geoengineering deployments. However, observations and simulations of climate impacts from volcanic eruptions test many of the same physical mechanisms that would come into play in a stratospheric aerosol geoengineering scenario, and thus major volcanic eruptions remain as valuable analogues for solar geoengineering deployment.

Monsoon low pressure systems (LPS) are synoptic scale tropical disturbances that form over the Indian subcontinent along the quasi-stationary trough axis during the monsoon (June to September) period. Around 14 LPS form every year, accounting for around 44% of monsoon precipitation and 78% of extreme precipitation events over the country. Many past studies have investigated the influence of various topographical features on the Indian monsoon. This study investigates the influence of the Himalayan and Tibetan orography (HTO) on various LPS-related characteristics/features (genesis location, number, tracks, and intensity). The NCAR Community Earth System Model (CESM1.2.2) is used to study the influence of HTO on monsoon and LPS activity over India. Simulations from CESM1.2.2 are obtained at 0.9°×1.25° horizontal resolution by considering the present-day height (h) of HTO, and altered heights (zero, 0.5h, and 1.5h). A 9.3% increase in the average monsoon precipitation is simulated over India when the height of HTO is increased to 1.5h, while a decrease in the same by 11.5% (44%) is simulated when the height of HTO is reduced to 0.5h (zero). These results are consistent with previous modeling studies. The changes in monsoon precipitation are attributed to a strong (weak) mean meridional temperature gradient (MTG) associated with an increase (a decrease) in the height of HTO and the prevention of cold dry mid-latitude air mixing with the warm humid air over India. Furthermore, we find that the simulated number of LPS per year increases when the height of HTO is reduced. The number of LPS is 17.2, 16.1, 13.6, and 12.4, respectively, in the simulations where the height of HTO is zero, 0.5h, h, and 1.5h. The mean meridional width of the LPS active region also increases when the height of HTO is reduced (Fig. 1). Contrary to the expectation of a southward shift in the LPS median track with a decrease in MTG (when the height of HTO is reduced), a slight northward shift is simulated in the track’s location. We attribute this to an increase (a decrease) in barotropic instability on reducing (increasing) the height of HTO, which results in a larger latitudinal spread in the location of genesis and tracks. The increased (decreased) barotropic instability also causes an increase (a decrease) in the frequency of LPS over the Indian subcontinent.