Understanding the Indian Ocean Experiment (INDOEX) aerosol distributions with an aerosol assimilation

We use the aerosol assimilation procedure described by Collins et al. [2000 ] to help explain INDOEX aerosol distributions. The procedure combines mode led aerosol with AVHRR satellite estimates. The result is consistent with s atellite measurements, regular in space and time, and provides information where retrievals are difficult lover (and, coincident with clouds, and at n ight). Extra information on aerosol composition, vertical distribution, and region of origin is also produced. Carbonaceous, sulfate, and sea salt aer osols agree with the in situ measurements to 10-20%. Carbonaceous aerosols were estimated to be the dominant contributor (36%) to the aerosol optical depth (AOD); dust (31%) and sulfate (26%) were also important. The residenc e time for sulfate and carbon is similar to7 and similar to8 days respectiv ely, longer than globally averaged residence times of many modeling studies . Thus aerosols produced here during the winter monsoon may have a larger c limate impact than the same emissions occurring where the residence time is shorter. Three points of entry are found for anthropogenic aerosol to the INDOEX region: a strong near surface southward flow near Bombay; a deeper p lume flowing south and east off Calcutta and a westward flow originating fr om southeast Asia and entering the Bay of Bengal. All three plumes are stro ngly modulated by a low-frequency change of meteorological regime associate d with the Madden Julian Oscillation. The analysis suggests that India is t he dominant source of aerosol in the Arabian Sea and Bay of Bengal near the surface but that Asia, Africa and the rest of world also contribute at hig her altitudes. India and Asia contribute similar to 40% each to the total c olumn mass of air reaching the Maldives, the balance of air comes from othe r source regions. The assimilation procedure produces an analysis that is a synergy in information about aerosols, that is not easily accessible by in dependent estimates from remote sensing, in situ measurements, or global tr ansport models by themselves.