Association of higher (lower) rainfall with lower (higher) Aerosol Optical Depth (AOD) is consistent with the understanding that increased washout (build-up) and shorter (longer) life-time of aerosols occur in wetter (drier) conditions. Given the life-time of aerosols, it is imperative to examine how aerosols impact active/break (wetter/drier than normal) spells, prominent intraseasonal variability (ISV) of Indian summer monsoon (ISM), through their composite analysis using recent satellite observations of aerosols and cloud properties, circulation and rainfall. Dust aerosols can act as CCN and participate efficiently in cloud processes during active phase. During breaks, build-up of desert dust transported by prevalent circulation, is associated with lower cloud effective radius implying aerosols’ indirect effect where they can inhibit cloud growth in the presence of reduced moisture and decrease precipitation efficiency/rainfall. Correspondingly, correlation albeit small, between intraseasonal anomalies of AOD and rainfall is negative, when AOD leads rainfall by 3–5 days implying that indirect aerosols impact is effective during breaks, though it is not the dominant responsible factor. During breaks, lower shortwave flux at top of atmosphere hints at dust-induced semi-direct effect. As breaks are permanent features of ISM, incorporation of dust-induced feedbacks in models, is essential for improved ISV simulation and ISM prediction.
$\bullet$ Active (break) spell of summer monsoon is found to be associated with lower (higher) aerosol optical depth over India.
$\bullet$ The build-up of desert dust transported to India by prevalent circulation during summer monsoon breaks, is associated with lower cloud effective radius which indicates the indirect effect of aerosols.
$\bullet$ Predominant indirect effect induced by dust aerosols along with secondary semi-direct effect can lead to further rainfall reduction during intense and persistent breaks.
$\bullet$ Proper incorporation of dust aerosol induced heating during breaks in models, is essential for simulation of intraseasonal variation inherent to Indian summer monsoon and thereby improving its prediction.
Volume 130, 2021
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