Multi-year observations of the spatial and vertical distribution of aerosols and the genesis of abnormal variations in aerosol loading over the Arabian Sea during Asian summer monsoon season

Intense aerosol plumes engulf a vast region of the Arabian Sea during the Asian summer monsoon season (ASM: June-September). The largest value of aerosol optical depth (AOD) at 550 nm in this region generally occurs in July when the mean AOD attains its annual peak value of >0.8. However, the AOD over this region is abnormally large during the ASM in some years, especially during the June-July period. Long term satellite observations using MODIS reveal that the largest AOD during the 11-year period of March 2000-February 2011 occurred in June 2008 with a regional mean AOD of 1.1, which was ~97% larger than that of the corresponding long term mean value in June and ~49% larger than that in July. The availability of CALIPSO data since June 2006 provides a unique opportunity to quantify the role of continental aerosols transported from the West Asian desert regions in the genesis of the above abnormality over the Arabian Sea. We examine the spatial and vertical distributions of aerosols over the Arabian Sea and adjoining continents using multi-year data from MODIS and CALIPSO and explore the genesis of the above abnormal enhancement in AOD. The observed anomalies in AOD are substantially larger than that can be attributed to changes in wind-generated sea salt aerosols. The CALIPSO observations show that the anomalous enhancement in aerosol loading over the Arabian Sea during June 2008 was primarily caused by an enhancement in aerosol abundance in the altitude range of ~1-4 km with a distinctly large volume depolarization ratio of >0.25, clearly indicating the dominance of highly non-spherical mineral dust. Although the aerosol loading over the Arabian Sea during the ASM is observed to be mainly caused by the mineral dust transported from the West Asian Deserts at northwest of the Arabian Sea, the abnormal enhancement in the observed AOD during June 2008 was primarily caused by a distinct increase in dust storms over the northern continents and subsequent transport into the Arabian Sea.