KEYNOTE ADDRESS

Aerosol Monsoon Interactions on Climate, Interannual and Subseasonal Scales Across the Indo-Gangetic Plain

D. R. Sikka

40 Mausam Vihar , New Delhi, 110 051

Email: drsikka@yahoo.com

Since the INDEX field phase in Feb-March 1999, there has been considerable scientific interest in the possible impact of anthropogenic as well as natural atmospheric aerosols with regard to the Indian monsoon seasonal climate variability a well as modulation of the monsoon on sub-seasonal scale during the break phase of the monsoon. The paper attempts to review the work done in this area during the last decade and also suggests possible avenues for further extension of the work using modeling and observational methodologies under the ongoing field phases under CAIPEX and CTCZ. There have been two paths followed for the study of interactions of atmospheric aerosol and the Indian monsoon climate system. The line taken by Prof. V. Ramanathan and his collaborators rests on the role of aerosols in disturbing the hydrological cycle of the summer monsoon thereby resulting in the possibility of more frequent seasonal monsoon droughts within the cope of modeling under enhancing CO2 concentrations in the first half of the 21st century. On the contrary modeling and observational studies by Lau and his group in NASA has followed the elevated heat pump hypothesis by which the monsoon rainfall is enhanced in June but depressed in July and August months. Following the hypothesis of Ramanathan and his collaborators, the author has hypothesized in 2008 on the mid-season impact of the aerosols is to enhance the length of the monsoon along the Indo-Gangetic Plain. The plain is the seat of conflict between the marine origin moist monsoon current and Dry and subsiding north westerly flow of desert origin. During active monsoon spell, the moist flow extends up to NW India. In prolonged break monsoon spell dry dust loaded (coated by anthropogenic materials) sweep the Gangetic plain even upto Bihar and direct aerosol effect suppress monsoon rain and may prolong the break spell. The hiatus in the monsoon advance process noticed in June 2009 and the prolonged break spell in August 2009 are cited as example of aerosol-monsoon interaction on sub- seasonal scale. Also excessively severe rain storms in western India could be linked to role of cloud microphysics in enhancing deep convection through the agency of ice nucleation under mineral dust. Indian monsoon research community has laid great emphasis on studying monsoon-aerosol interactions under the two on-going field programs viz. Cloud Aerosol Interactions and Precipitation Enhancement (CAIPEX) and the Continental Trough Convergence Zone (CTCZ) Experiment. The scope of these studies and similar other studies is also discussed.