Variability of Anthropogenic aerosols and its impact on Indian Summer Monsoon

Abstract

Understanding the variability of aerosols in troposphere over the South Asian region is highly essential since aerosol in this region plays a vital role in the modulating Indian summer monsoon. The rapid increase in the anthropogenic emissions of sulphur aerosols is of serious concern. All around the globe emissions of SO2 have become two-fold over the century (Baron et al., 2017). Developing countries in Asia, mainly India produces SO2 emissions at high growth rates. Past study shows increase of SO2 aerosols of the total Aerosol Optical Depth (AOD) by 4.8% per annum over India ). Large amounts of anthropogenic emissions of sulfur dioxide (SO2) over India play an important role in radiative forcing, circulation changes and hydrology. Here, using a state of art chemistry-climate model, ECHAM6-HAMMOZ, we investigate impacts of South Asian anthropogenic Sulfate aerosols on the Indian summer monsoon rainfall during three typical years (1) 2010 a La Niña, (2) 2015 an El Niño, and (3) a normal year 2016. Our sensitivity simulations for 48% enhancement in South Asian SO2 emissions is based on a trend of 4.8% yr-1, estimated from Ozone Monitoring Instrument (OMI) satellite observations during 2006-2017. The model simulations show Sulfate aerosols reduces summer monsoon precipitation (by 7.85 mm day-1; 7.1 % in 2010 La Niña; 6.88 mm day-1, 10 % in 2015 El Niño and 5.35 mm -1, 9.3% in 2016 normal year. The larger reduction in precipitation seen in 2010 La Niña is due to strong radiative cooling at the surface due thicker Sulfate aerosol column over the Indian region than in 2015 El Nino and 2016 a normal year. The Sulfate aerosols loading over the Indian region caused weakening of the Hadley circulation, increases upper tropospheric stability, and reduced moisture transport over the Indian region leading to precipitation reduction. The South Asian Sulfate aerosols are transported(towards the high latitudes) by the monsoon convection. This transport occurred in the UTLS during 2010 (La Niña), mid troposphere during 2015 (El Nino) and 2016 (a normal year). The strong ascend over the Indian region in 2010 La Niña lifts the aerosols into the UTLS which are then transported to Arctic in the UTLS. While, this transport occurs through mid-troposphere during 2015 El Nino and 2016 a normal year. This study will mainly focus on the impacts of sulfate aerosols on the Indian Monsoon.