Abstract
Abstract. 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 the Community Earth System Model (CESM) to quantify two radiative effects of absorbing BC and dust on the SAM. Specifically, 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, ARIs). 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 ARIs 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. Precipitation increases of 2 mm d−1 or more (a 60 % increase in June) result across much of northern India from May through August, with larger anomalies (+5 to +10 mm d−1) in the western Indian mountains and southern Tibetan Plateau (TP) mountain ranges due to orographic and anabatic enhancement. Across the Tibetan Plateau foothills, SDE by BC aerosols drives large precipitation anomalies of > 6 mm d−1 (a 21 %–26 % increase in May and June), comparable to ARI of absorbing aerosols from April through August. Runoff changes accompany BC SDE-induced snow changes across Tibet, while runoff changes across India result predominantly from dust ARI. Finally, there are large differences in the simulated SDE between the VR and traditional 1∘ simulations, the latter of which simulates a much stronger SDE and more effectively modifies the regional circulation.
Highlights
The South Asian monsoon (SAM) and Tibetan Plateau (TP) snow cover are critical to the security of water resources across India, Pakistan, and the Bay of Bengal region
Positive dust-induced direct radiative effect (DRE) values of +4 to +9 W m−2 are simulated across the Tarim Basin and the Gobi desert
The reasons for the differences in runoff– precipitation phase between India–Indo-Gangetic Plain (IGP) and the TP foothills (TPF) may be the result of larger runoff effects associated with BCD snow darkening effect (SDE) across the TPF compared to the IGP and India, which contributes to runoff increases of more than 3.5 mm d−1 (26 %) in June (Fig. S6c, e)
Summary
The South Asian monsoon (SAM) and Tibetan Plateau (TP) snow cover are critical to the security of water resources across India, Pakistan, and the Bay of Bengal region. Chemistry parameterizations across the entire model domain; (ii) decrease the model grid spacing over the most complicated terrain of southern and central Asia, which has been identified as being critical to SAM dynamics and evolution; and (iii) estimate the relative importance of the SDE compared to ARI in affecting the premonsoonal and monsoonal environment for BC and dust separately. We venture through this exploration in the following manner.
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