Revisiting the observed correlation between weekly averaged Indian monsoon precipitation and Arabian Sea aerosol optical depth

Dust influences the Indian summer monsoon on seasonal time scales by perturbing atmospheric radiation. On weekly time scales, aerosol optical depth retrieved by satellite over the Arabian Sea is correlated with Indian monsoon precipitation. This has been interpreted to show the effect of dust radiative heating on Indian rainfall on synoptic (few-day) time scales. However, this correlation is reproduced by Earth System Model simulations, where dust is present but its radiative effect is omitted. Analysis of daily variability suggests that the correlation results from the effect of precipitation on dust through the associated cyclonic circulation. Boundary layer winds that deliver moisture to India are responsible for dust outbreaks in source regions far upwind, including the Arabian Peninsula. This suggests that synoptic variations in monsoon precipitation over India enhance dust emission and transport to the Arabian Sea. The effect of dust radiative heating upon synoptic monsoon variations remains to be determined. Plain Language Summary Small particles of dirt are commonly found suspended in the air near deserts, where they are lifted by the wind. Scientists refer to these particles as soil dust or else as just dust aerosols. These particles dim the surface by intercepting sunlight. Scientists have shown that over the course of a summer, dust aerosols change the amount of rainfall. Recently, scientists showed that weekly rainfall during the Indian summer monsoon is larger when there are more soil particles in the air over the Arabian Sea. They asked whether weekly variations in suspended soil change Indian monsoon rainfall from week to week. In the present study, we used a computer simulation of climate to show that in fact it is the rainfall that increases the amount of suspended soil downwind of the Arabian Desert. The monsoon winds that accompany rainfall over India are part of a larger wind pattern that lifts soil particles into the air and transports them to the Arabian Sea. Whether suspended soil particles are important for forecasting variations in monsoon rainfall from week to week remains unknown.