The role of reduced aerosol levels during the COVID-19 lockdown on the Asian summer monsoon

Abstract:

Aerosol concentrations over Asia are known to play a key role in modulating the Asian summer monsoon (ASM) rainfall. Restricted anthropogenic activities caused by the COVID-19 pandemic outbreak led to a substantial reduction in observed Asian aerosol loadings. Aerosol Optical Depth from Moderate Resolution Imaging Spectroradiometer indicate a ~40 % reduction in aerosol loading over the Indo-Gangetic plain during the 2020 pre-monsoon season compared to the 2000-2020 average. Here, we use bottom-up estimates of anthropogenic emissions reductions based on Google and Apple mobility data, together with the ECHAM6-HAMMOZ state-of-the-art aerosol chemistry climate model to investigate the role of the reduced aerosol loadings on the 2020 ASM. We show that the associated decrease in the aerosol direct effect leads to an increase in surface solar radiation of up to 4 Wm^-2 over South Asia, resulting in a strengthening of the ASM. Simultaneously, occurrence of a warm core over the Tibetan plateau generated by the accumulated dust aerosols during the pre-monsoon season reinforces the intensification of the Hadley circulation. The associated cross-equatorial moisture influx over the Indian landmass leads to enhanced rainfall amounts of 0.8-3 mm·day^-1 (5–15%) over central India, Western Ghats, and the southern slopes of Himalayas.

Our study indicates that the reduced anthropogenic emissions caused by the COVID-19 restrictions had a positive impact on the hydrological cycle over South Asia, which has been facing increasing water scarcity over the last few decades. This highlights the importance of controlling anthropogenic emissions as it may help reducing water shortages in one of the world's most densely populated regions.