Impacts of aerosols on dynamics of Indian summer monsoon using a regional climate model

A regional climate model, RegCM has been utilized to examine the dynamic impacts of large aerosol radiative forcing on the atmospheric temperature and circulation in India during the monsoon (Jun-Sep) seasons of 2009 and 2010. Surface shortwave radiative forcing at the aerosol hot spots is in the range - 25 to -60 W m(-2) with the larger values observed during the summer monsoon season of 2010 (due to larger dust load) relative to that in 2009. It is important to note that the summer monsoon rainfall in 2010 was declared to be a normal monsoon as against the deficit rain in 2009. Changes in near surface air temperature show a spatial dipole pattern with the aerosol effect dampening out above 500 hPa with a larger change observed for natural aerosols relative to anthropogenic aerosols. The dipole pattern is characteristics of aerosol-induced change. Aerosols tend to strengthen the summer monsoon zonal mean wind at 850 hPa over the hotspots (larger effect in 2009 than in 2010) whereas there is negligible impact on the corresponding mean meridional wind component. This has resulted in a southward shift of the monsoon circulation during 2010 summer, leading to an increase in upward motion over the core monsoon region and thereby increasing the cloud fraction. This may also be facilitated by the aerosol induced heating in the lower troposphere. In 2009, the upward motion is enhanced to the south of the core monsoon region. The dynamic effects imply a positive feedback of the aerosol direct radiative forcing on the summer monsoon circulation over India.