Investigating the relative responses of regional monsoon dynamics to snow darkening and direct radiative effects of dust and carbonaceous aerosols over the Indian subcontinent

Abstract

A large uncertainty exists today in quantifying the absorbing aerosol snow darkening effect (SnoDE) and its relative role compared to the direct radiative effect (DRE), especially at the regional scale. Using a regional climate model over an enlarged South Asia Coordinated Regional Climate Downscaling Experiment (CORDEX) domain, here we isolate these effects and examine their impacts on the dynamics of the Indian pre-monsoon and summer monsoon (ISM) circulation. Our results clearly indicate that the regional dynamics associated with the pre-monsoon and ISM are highly sensitive to both aerosol SnoDE and DRE, with the SnoDE dominating during the pre-monsoon, and the DRE during the ISM. During the pre-monsoon, the deposition of aerosols on the Himalayas and Tibetan Plateau (HTP) reduces the surface albedo, leading to an increase in absorption of solar radiation and accelerated snow melting. The associated increase in surface and tropospheric temperature draws dry air from central Asia towards northern India and along the Himalayan foothills, creating a low-level anti-cyclonic anomaly over eastern India and the Bay of Bengal. This limits the northward propagation of southwest winds and leads to a decrease in precipitation over the central and eastern parts of India during the pre-monsoon and across northern India in the following ISM. Conversely, an increase in precipitation during the ISM is due to the strengthening of southwest winds caused by large-scale convergence over the Arabian Peninsula. The shift of winds from central Asia towards northern India in the SnoDE and towards the Arabian Peninsula in DRE leads to contrasting effects and can possibly modulate the monsoon onset dates. The SnoDE, with a prevailing contribution from dust, leads to an increase in precipitation over the HTP, which is amplified during the ISM when DRE is also considered. Our study provides evidence that both these aerosol effects have substantial impacts on the pre-monsoon and ISM dynamics and hence need to be considered in climate model simulations over the region.