A satellite-observation based study on responses of clouds to aerosols over South Asia during IOD events of south-west monsoon season

The present study investigated the possible effects of aerosols on cloud properties during positive and negative Indian Ocean Dipole (IOD) months over South Asia using 20 years (2002-2021) of long-term satellite-based observations and reanalyses datasets. Positive aerosol optical depth (AOD) differences between the two IOD phases are found mainly over the land areas and some parts of the Indian Ocean, with the maximum value observed over Indonesia, Malaysia, Northern China, Western India, and the Northern Arabian Sea. The negative AOD differences are seen over the equatorial Indian Ocean, some regions of Indian land mass, the North Indian Ocean, and some parts of the South China Sea, with the highest dip over the central Indo-Gangetic Plain region. The highest increase (decrease) in rainfall is observed over the western (eastern) pole. The reduced rainfall is responsible for the increased AOD over Indonesia, Malaysia, and neighboring regions. The strong negative relationships of cloud fraction (CF) with cloud top pressure and cloud optical depth (COD) suggest persisting optically thick middle and high-level clouds during the IOD months. Twomey effect is noticed over the study region during both the IOD phases irrespective of the meteorological conditions. CF and COD both showed an upsurge with increased AOD in the presence of high relative humidity and stronger updrafts, suggesting the role of prevailing meteorology in forming thicker clouds. However, the height of the cloud tops is getting lowered in polluted conditions. The aerosol-cloud relationships observed during IOD scenarios could be attributed to the prevailing meteorology.