Fast and Slow Responses of the South Asian Monsoon System to Anthropogenic Aerosols


Summer monsoons deliver about three-quarters of South Asia’s annual rainfall, influencing fresh water supplies, agriculture, and energy production. Small changes in monsoons can have large impacts on local living conditions, affecting crop yields, prolonging droughts, or fostering floods. Recent studies have suggested various mechanisms and effects for how pollution aerosols in South Asia impact the monsoon. Aerosols cool the underlying surface and reduce the north-south temperature gradient leading to slow-response climate effects. High-altitude absorbing aerosols may cause short-term, localized enhancement of convective uplift. Using a global climate model with a fully predictive aerosol life cycle, U. S. Department of Energy researchers from Pacific Northwest National Laboratory investigated the fast and slow responses of the South Asian monsoon system to anthropogenic aerosol forcing. They show that the feedbacks associated with the slower sea surface temperature (SST) change caused by aerosols play a more important role than the aerosol’s direct impact on radiation, clouds, and land surface (rapid adjustments) in shaping the total equilibrium climate response of the monsoon system to aerosol forcing. Inhomogeneous SST cooling caused by anthropogenic aerosols eventually reduces the north-south tropospheric temperature gradient and the easterly shear of zonal winds over the region, slowing the local Hadley cell circulation, decreasing the northward moisture transport, and causing a reduction in precipitation over South Asia. Although total responses in precipitation are closer to the slow responses in general, the fast component dominates over land areas north of 25°N. The results also show an east-west asymmetry in the fast responses to anthropogenic aerosols causing increases in precipitation west of 80ºE but decreases east of it. This study provides insights into the various impacts of aerosols on the South Asian monsoon.


Ganguly, D., P. J. Rasch, H. Wang, and J.-H. Yoon. 2012. “Fast and Slow Responses of the South Asian Monsoon System to Anthropogenic Aerosols,” Geophysical Research Letters 39, L18804. DOI: 10.1029/2012GL053043.