Impact of dust aerosols on the Indian Summer Monsoon Rainfall on intra-seasonal time-scale

The Indian subcontinent is a major hotspot of aerosol loading and several studies have demonstrated the importance of interaction between Asian dust aerosols and the Indian Summer Monsoon Rainfall (ISMR). However, a few studies have suggested a declining trend in the pre-monsoon dust loading over the Indian region. The present study investigates how the decrease in dust emissions influences the intensity of the monsoon rainfall over the Indian region using a fully coupled Weather Research and Forecasting model coupled with chemistry (WRF-Chem). We have conducted two sets of simulation experiments for the June-July-August-September (JJAS) season of 2017: (1) a control experiment with the default dust aerosol emissions, and (2) a sensitivity experiment by reducing dust emissions by 60%. Our analyses show that the reduction in dust emissions significantly alters the spatial distribution of seasonal mean monsoon rainfall with enhanced rainfall over the Bay of Bengal (BoB) and eastern coastal regions, and decreased rainfall over the lower Indo-Gangetic Plain (IGP), and parts of Central India (CI). It is found that the low aerosol loading conditions alter the large-scale circulation patterns along the tropical/sub-tropical region centred between 15 degrees N and 35 degrees N and 40 degrees E -130 degrees E, and through dynamical feed-back, strengthens the convective rainfall over the BoB region. As a result, the 10-20 days sub-seasonal mode of the ISMR strengthens, facilitating enhanced westward propagation of rain-bearing systems formed over northern BoB which provides substantial rainfall over vast regions of the Indian subcontinent.