Abstract
Abstract The HadGEM2 AGCM is used to determine the most important anthropogenic aerosols in the Indian monsoon using experiments in which observed trends in individual aerosol species are imposed. Sulfur dioxide (SD) emissions are shown to impact rainfall more strongly than black carbon (BC) aerosols, causing reduced rainfall especially over northern India. Significant perturbations due to BC are not noted until its emissions are scaled up in a sensitivity test, resulting in rainfall increases over northern India due to the elevated heat pump mechanism, enhancing convection during the premonsoon, and bringing forward the monsoon onset. Second, the impact of anthropogenic aerosols is compared to that of increasing greenhouse-gas concentrations and observed sea surface temperature (SST) warming. The tropospheric temperature gradient driving the monsoon shows weakening when forced by either SD or imposed SST trends. However, the observed SST trend is dominated by warming in the deep tropics; when the component of SST trend related to aerosol emissions is removed, further warming is found in the extratropical Northern Hemisphere that tends to offset monsoon weakening. This suggests caution is needed when using SST forcing as a proxy for greenhouse warming. Finally, aerosol emissions are decomposed into those from the Indian region and those elsewhere in pairs of experiments with SD and BC. Both local and remote aerosol emissions are found to lead to rainfall changes over India; for SD, remote aerosols contribute around 75% of the rainfall decrease over India, while for BC the remote forcing is even more dominant.
Highlights
More than a billion people in South Asia rely on the monsoon to supply over 80% of annual rainfall between June and September
The observed sea surface temperature (SST) trend is dominated by warming in the deep tropics; when the component of SST trend related to aerosol emissions is removed, further warming is found in the extratropical Northern Hemisphere that tends to offset monsoon weakening
Lengthy discussion of dust aerosol is beyond the scope of this study. This is partly because we focus on anthropogenic aerosols and how the observed increasing trends in their emissions contribute to variations in Indian summer monsoon rainfall (ISMR)
Summary
GUO ET AL. Local and Remote Impacts of Aerosol Species on Indian Summer Monsoon Rainfall in a GCM National Centre for Atmospheric Science, and Department of Meteorology, University of Reading, Reading, United Kingdom Department of Meteorology, University of Reading, Reading, United Kingdom (Manuscript received 15 October 2015, in final form 24 June 2016)
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