Examining the Effects of Black Carbon and Dust on the South-Asian Monsoon: Experiments Using Variable Resolution CESM

Across south Asia, black carbon (BC) and dust (referred to collectively as BCD) aerosols have been suggested to have important impacts for regulating the timing and intensity of the South Asian Monsoon (SAM) through several mechanisms. While this problem has been examined using global climate models, previous studies have not directly separated on-snow versus in-atmosphere effects of BCD across the region, and previous studies have utilized horizontal resolutions (100-200 km) too coarse to resolve mesoscale terrain features. In this study, we make use of a variable-resolution (VR) version of Community Earth System Model (CESM) with a regionally-refined domain over south Asia (with a horizontal resolution of ~12 km) to examine feedbacks that are initiated by the presence of BC and dust across south-Asia, as these aerosols perturb the SAM radiation balance, thermodynamic properties of the atmosphere, and surface albedo through atmospheric aerosol-radiation interactions (ARI) and the snow darkening effect (SDE). Simulations indicate that ARI and SDE contribute similarly to initiating the Tibetan Plateau (TP) warming and Indian moisture/cloud fraction (CF) increases in April. By May, a strong anomalous quasi-horizontal temperature gradient is simulated from central India through northern Tibet, due primarily to BCD ARI. Specifically, ARI-induced warming above snow cover increases the atmospheric stability across the TP, whilst decreasing surface pressures across south Asia. The increased stability across Tibet brings forth lower CF values and surface warming. Meanwhile, the decreases in surface pressures initiate southerly low-level moisture advection from the Indian Ocean, leading to higher CFs and surface cooling across India. The presence of this anomalous temperature gradient alters the simulated mass and wind fields through June, July, and August. ARI due to BCD generally contributes the larger perturbations to the premonsoonal and monsoonal hydrology and meteorology, especially into May and June, compared to the SDE. The simulated temperature gradient, which initiates in May, is largest in June, and propagates west and northward during June, July, and August, coinciding with northwest-ward propagating positive moisture, CF, and precipitation anomalies. These meteorological changes bring forth perturbations in surface runoff that may have consequences for agriculture, hydropower, and overall economic security across south Asia.