Direct Impacts of Aerosols on Two Successive Contrasting Indian Summer Monsoon Seasons Using a Regional Climate Model.

Abstract:

The impacts of aerosol direct radiative forcing on the atmospheric temperature and wind circulation during the two contrasting monsoon (Jun-Sep) seasons of 2009 (deficit year) and 2010 (normal year) are examined using a regional climate model, RegCM over India. RegCM is coupled with an online aerosol module, which considers both natural dust and anthropogenic aerosols (hydrophilic and hydrophobic black carbon and organic carbon and sulfate). Comparison with satellite and ground-based observations reveal that the model captures the spatial variation of aerosol optical depth (AOD) over the Great Indian Desert and Indo-Gangetic Basin (IGB) appreciably well with a bias in AOD within ±15% of the observations. The spatial patterns in surface shortwave radiative forcing (SWRF) are similar in both the years with a larger dimming observed in the 2010 relative to the previous 2009 season, which is attributed to larger dust transport in 2010 by stronger westerlies. The SWRF at TOA is observed to be < -21 W m^-2 over the IGB during 2010 compared to -15 to -20 W m^-2 during 2009. Changes in near surface air temperature at 2m show a spatial dipole pattern with the aerosol effect dampening out above mid-troposphere with a larger change observed for natural aerosols relative to anthropogenic aerosols. Aerosols tend to strengthen the summer monsoon zonal mean wind at 850 hPa over the regional hotspots, whereas there is negligible impact on the corresponding meridional wind component. This has resulted in a southward shift of the monsoon circulation during the 2010 monsoon season, leading to an increase in upward motion over the core monsoon region and thereby increasing the cloud fraction. In 2009, the upward motion is enhanced to the south of the core monsoon region. These results suggest a positive feedback of the aerosol direct SWRF on the monsoon circulation over India and imply that dynamic impacts are as important as aerosol microphysical impacts.