Intraseasonal modulation of Indian summer monsoon by Middle East dust : an observational and numerical modeling study
As one of the world's strongest monsoon systems, the Indian summer monsoon affects one-third of the global population. The temporal and spatial variability of the ISM rainfall has great socio-economic impacts, particularly on agriculture and food supply in South Asia. Additionally, South Asia is an area with heavy atmospheric loading of aerosols from both wind-blown mineral dust and manmade pollutants. These aerosols can significantly influence the ISM rainfall through their radiative and microphysical effects. In our study, we focus on three questions. 1) How does the ISM rainfall respond to the Middle East dust in observations on intraseasonal timescales? 2) Can the regional climate model reproduce the observed relationship between Middle East dust and the ISM rainfall, and what are the model uncertainties and how do they influence our results? 3) How does the ISM system respond to different types of aerosols in different source regions on the intraseasonal timescales? I use multiple satellite retrievals, reanalysis datasets, and ensemble modeling experiments to study the dust-monsoon connection. Multivariate empirical orthogonal function is performed on aerosol optical depth and rainfall from satellite observations, and winds and geopotential height from reanalysis to identify the coupled spatial patterns among these variables. Cross-correlation analyses between satellite-retrieved AOD in the Middle East and the ISM rainfall are calculated to characterize the timescales of dust-monsoon connections. Furthermore, ensemble numerical experiments are conducted to examine the causal relationship and physical mechanisms between Middle East dust aerosols and the ISM monsoon. The ensemble experiments are created by perturbing physical and chemical model schemes to examine the uncertainties in parameterizing the shortwave radiation, dust diffusion in the boundary layer, and aerosol chemical mixing rules. The primary scientific findings are summarized here. (1) Middle East dust aerosols are positively correlated with the ISM rainfall in Pakistan, central and northern India, and Coastal South West India. (2) The timescale of the dust–monsoon connection is about 11 to 13 days. (3) Middle East dust aerosols can enhance the southwesterly monsoon flow over the Arabian Sea due to their direct radiative heating effect in the lower troposphere, which can increase the south–north ocean–land thermal contrast. The enhanced monsoon flow can transport more water vapor from the Arabian Sea to the Indian subcontinent, thereby resulting in more monsoon rainfall. (4) Middle East dust aerosols play a dominant role in modulating the ISM rainfall compared to dust aerosols from other regions; local anthropogenic aerosols in India, although with much lower concentrations than dust, can play a similar role to Middle East dust aerosols. Our findings demonstrate that a better representation of Middle East dust aerosols and their interactions with meteorological fields is important for understanding and modeling the variability of the ISM rainfall.