Differences and similarities in Indian monsoon response to aerosols and to land-use/land-cover change

Abstract Aerosols are known to exert a powerful influence on the Indian summer monsoon, reducing precipitation from onset through demise. We analyze model simulations aimed at detection and attribution to compare and contrast the processes that affect the energy budget and water cycle in the presence of aerosols to those associated with land-use/land-cover change (LULCC) over the Indian subcontinent. We find that the response of the water cycle to these external forcings is remarkably similar: precipitation decreases in the presence of either aerosols or LULCC. We also find that this occurs despite opposite responses in the surface energy budget, manifest in surface temperature: aerosols cool the continent, while LULCC warms it. Consequently, the temperature contrast set up by the difference in thermal inertia between land and ocean at peak insolation is weakened in the aerosol case, but it is strengthened in the LULCC case. These results therefore support the perspective that has forcefully emerged in tropical climate dynamics in the past couple decades that the temperature contrast between land and ocean alone is not sufficient to explain variation in monsoon rains. In line with this perspective, we find that viewing humidity as an expression of energy on par with temperature, that is, considering the moist static energy contrast, is critical to explaining outcome. In the simulations analyzed here, the presence of either aerosols or LULCC coherently reduces moist static energy over land, weakening the contrast between land and ocean, and results in reduced precipitation, confirming the monsoon’s sensitivity to external forcing.