Abstract
Abstract. Atmospheric absorptive aerosols exert complicated effects on the climate system, two of which are through their direct radiative forcing and snow-darkening forcing. Compared to black carbon, the snow-darkening effect of dust on climate has been scarcely explored till now. When depositing in snow, dust can reduce the albedo of snow by darkening it and increasing the snowmelt. In this study, the snow-darkening effect of dust, as well as the direct radiative effect, on the Indian summer monsoon are evaluated by atmospheric general circulation model experiments. The results show that the snow-darkening and direct radiative forcing of dust both have significant impacts on the onset of the Indian monsoon, but they are distinctly opposite. The snow-darkening effect of dust weakens the Indian monsoon precipitation during May and June, opposite to black carbon. The surface temperature over central Asia and the western Tibetan Plateau becomes warmer due to the dust-induced decrease in snow cover, which leads to a local low-level cyclonic anomaly as well as an anticyclonic anomaly over the Indian subcontinent and Arabian Sea. This circulation pattern allows air currents penetrating into the Indian subcontinent more from central Asia but less from the Indian Ocean. In contrast, the direct radiative forcing of dust warms the low troposphere over the Arabian Peninsula, which intensifies moisture convergence and precipitation over the Indian monsoon region. The upper tropospheric atmospheric circulation over Asia is also sensitive to both effects. Compared to previous studies which emphasized the temperature over the Tibetan Plateau, our results further highlight an important role of surface/low tropospheric temperature changes over dust source areas, which can also significantly modify the response of summer monsoon. Thus, links between the climatic impact of dust and complicated thermal conditions over Asia are of importance and need to be clarified accurately.
Highlights
Mineral dust, a kind of natural aerosol in the atmosphere, mainly originates from the global deserts, including the Sahara, the Arabian Peninsula, central Asia and East Asia
The aerosol optical depth (AOD) reaches above 0.2 over major source areas. This simulated pattern is similar to the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)-retrieved AOD over the deserts (Fig. 1b), which indicates that Community Atmosphere Model 4 (CAM4)-bulk aerosol model (BAM)
The simulated absolute values of dust AOD over the Arabian Peninsula and southwestern slope of the Tibetan Plateau (TP) and Taklimakan desert are biased low because the considered dust particles are restricted to less than 10.0 μm and the dust forcing is underestimated due to less coarse dusts in the current global climate models (Kok et al, 2017)
Summary
A kind of natural aerosol in the atmosphere, mainly originates from the global deserts, including the Sahara, the Arabian Peninsula, central Asia and East Asia. In the fifth Assessment Report (IPCC, 2013), the annual mean DRE of dust is approximately −0.10 W m−2 on the global scale, which varies from −0.30 to +0.10 W m−2 among different global climate models. It is still unclear whether dust aerosol has a net warming or cooling effect on global climate (e.g., Tegen and Lacis, 1996; Miller and Tegen, 1998; Mahowald et al, 2014; Kok et al, 2017; Xie et al, 2018a). Due to the underestimation of coarser dust in climate models than in the atmosphere, the considered DRE may be more cooling in the current model ensemble, and the possibility that dust may cause a net warming is highlighted (Kok et al, 2017)
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