Abstract
Abstract. The response of the Indian summer monsoon (ISM) circulation and precipitation to Middle East dust aerosols on sub-seasonal timescales is studied using observations and the Weather Research and Forecasting model coupled with online chemistry (WRF-Chem). Satellite data show that the ISM rainfall in coastal southwest India, central and northern India, and Pakistan is closely associated with the Middle East dust aerosols. The physical mechanism behind this dust–ISM rainfall connection is examined through ensemble simulations with and without dust emissions. Each ensemble includes 16 members with various physical and chemical schemes to consider the model uncertainties in parameterizing short-wave radiation, the planetary boundary layer, and aerosol chemical mixing rules. Experiments show that dust aerosols increase rainfall by about 0.44 mm day−1 (~10 % of the climatology) in coastal southwest India, central and northern India, and north Pakistan, a pattern consistent with the observed relationship. The ensemble mean rainfall response over India shows a much stronger spatial correlation with the observed rainfall response than any other ensemble members. The largest modeling uncertainties are from the boundary layer schemes, followed by short-wave radiation schemes. In WRF-Chem, the dust aerosol optical depth (AOD) over the Middle East shows the strongest correlation with the ISM rainfall response when dust AOD leads rainfall response by about 11 days. Further analyses show that increased ISM rainfall is related to enhanced southwesterly monsoon flow and moisture transport from the Arabian Sea to the Indian subcontinent, which are associated with the development of an anomalous low-pressure system over the Arabian Sea, the southern Arabian Peninsula, and the Iranian Plateau due to dust-induced heating in the troposphere. The dust-induced heating in the mid-upper troposphere is mainly located in the Iranian Plateau rather than the Tibetan Plateau. This study demonstrates a thermodynamic mechanism that links remote desert dust emissions in the Middle East to ISM circulation and precipitation variability on sub-seasonal timescales, which may have implications for ISM rainfall forecasts.
Highlights
Aerosols, from both natural sources and anthropogenic emissions, can influence climate by scattering and absorbing solar and terrestrial radiation and by serving as cloud condensation nuclei and ice nuclei and altering cloud microphysical properties
Intensive irrigation activities occurring during JJA over the IndoGangetic Plain (IGP) can increase local evapotranspiration, and increase rainfall (Douglas et al, 2009; Guimberteau et al, 2012)
The positive response of rainfall to aerosol optical depth (AOD) is primarily located in the IGP, central India, and coastal southwest India (CSWI), while a weak negative response is seen in southeast India
Summary
From both natural sources (e.g., mineral dust, sea salt, and volcanic eruptions) and anthropogenic emissions (e.g., black carbon and sulfate), can influence climate by scattering and absorbing solar and terrestrial radiation (the direct effect) and by serving as cloud condensation nuclei and ice nuclei and altering cloud microphysical properties (the indirect effect). Vinoj et al (2014; hereafter V2014) found a positive relationship between the ISM rainfall in southern India and dust aerosols over the AS, west Asia and the Arabian Peninsula (AP) using a GCM They proposed that dust-induced convergence over eastern North Africa and the AP by heating the atmosphere increases moisture transfer over India, which in turn modulates monsoon rainfall over south India within a week. Most recently, Solmon et al (2015; hereafter S2015) studied the interaction between Middle East dust and ISM rainfall on interannual to decadal timescales using a regional climate model (RCM) They found that the dust aerosols could increase rainfall in southern India, while it decreased rainfall in central and northern India (CNI) and Pakistan during the period of 2000 to 2009. Is the 13-day maximum cross-correlation found in observations in J2014 captured by WRF-Chem? This question is critical because if the AOD– ISM rainfall hypothesis is true, AOD must lead the ISM rainfall response in the model
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