Objective

1. Inhomogeneous Sea Surface Cooling by Anthropogenic Aerosols Slowing Down South Asian Monsoon Circulation • Objective • To investigate the fast and slow responses of South Asian monsoon system to increased anthropogenic emissions. • Approach • Used century-long coupled-climate simulations with a variant of the Community Earth System Model (CESM1, includes a comprehensive representation of the aerosol lifecycle, and estimates climate response including sea-surface temperatures). • Examined thermal, radiative, dynamical, and hydrological responses to the various effects of anthropogenic aerosols on the South Asian monsoon system. • Decomposed total responses into fast (i.e., response to direct impact on radiation, clouds and land surface) and slow (i.e., feedbacks associated with SST changes) components. Impact:Both fast and slow responses to aerosol forcing are important; however, the resultant changes in precipitation have opposite spatial distribution patterns over South Asia. An east-west asymmetry in the fast component of circulation responses is likely responsible for the simulated and observed asymmetry in trends of monsoon precipitation over the continent (in north). The weakened north-south SST gradient over the Indian Ocean, as part of the slow responses, slows down the local Hadley circulation, decreases the northward moisture transport, and reduces precipitation over South Asia. Summertime (JJAS) mean (a) fast, (b) slow and (c) total responses in precipitation and winds (at 860 hPa) caused by aerosols. Stipples indicate areas where the anomalies exceed the 90% confidence level based on the Student’s t-test. Ganguly D, PJ Rasch, H Wang, and J-H. Yoon. 2012. “Fast and slow responses of the South Asian monsoon system to anthropogenic aerosols,” Geophysical Research Letters39: L18804. DOI:10.1029/2012GL053043