Variability of aerosol optical thickness in the tropical Indian Ocean and South China Sea during spring intermonsoon season

The variability of aerosol optical thickness (AOT) in the tropical Indian Ocean (IO) and South China Sea (SCS) during the intermonsoon (February-May) is investigated using shipboard and satellite data from 2011 to 2014, in order to understand the mechanism controlling AOT production and transport. Overall AOT in tropical IO is significantly smaller than those in SCS, and a strong intraseasonal variability of AOT, along with significant dependence on wind speed (especially when <8m s(-1)), is observed in both basins. Our analysis showed that in tropical IO, aerosols are mostly of natural marine production in spring, and the AOT shows greater dependence on the higher wind speeds in May, while in SCS, AOT dependence on wind speed increases when the prevailing wind turns from offshore (February, March) to onshore (April, May), due to less transport of terrestrial dust and anthropogenic aerosols from land. A better agreement between shipboard and once-daily satellite AOT is observed in the Equatorial belt (5 degrees S-5 degrees N, i.e. remote areas of IO) than in the Non-Equatorial area (5 degrees N-25 degrees N), and can be explained by the overall lower AOT values and the smaller diurnal variation in the Equatorial belt. Changes of wind regime during the monsoon evolution and the consequent transport of land-based aerosols reduce AOT dependence on wind speed and are the major drivers for the AOT variability