Quantifying snow-darkening and atmospheric radiative effects of black carbon and dust on the South-Asian Monsoon and hydrological cycle: Experiments using variable resolution CESM

Black carbon (BC) and dust impart significant effects on the south-Asian monsoon (SAM), which is responsible for ~80 % of the region’s annual precipitation. This study implements a variable-resolution (VR) version of Community Earth System Model (CESM) to quantify the impacts of absorbing BC and dust on the SAM. This study focuses on the snow darkening effect (SDE), as well as how these aerosols interact with incoming and outgoing radiation to facilitate an atmospheric response (i.e., aerosol radiation interactions (ARI)). By running sensitivity experiments, the individual effects of SDE and ARI are quantified, and a theoretical framework is applied to assess these aerosols’ impacts on the SAM. It is found that ARI of absorbing aerosols warm the atmospheric column in a belt coincident with the May-June averaged location of the subtropical jet, bringing forth anomalous upper-tropospheric (lower-tropospheric) anticyclogenesis (cyclogenesis) and divergence (convergence). This anomalous arrangement in the mass fields brings forth enhanced rising vertical motion across south Asia and a stronger westerly low-level jet, the latter of which furnishes the Indian subcontinent with enhanced Arabian Gulf moisture. This leads to precipitation increases of +2 mm d−1 or more across much of northern India from May through August, with larger anomalies in the western Indian mountains and southern TP mountain ranges due to orographic and anabatic enhancement. Across the Tibetan Plateau foothills, SDE by BC aerosol drives large precipitation anomalies of >6 mm d−1, comparable to ARI of absorbing aerosols from April through August. Runoff changes accompany precipitation and Tibetan Plateau snow changes, which have consequences for south-Asian water resources.