Regional aerosol distribution and its long‐range transport over the Indian Ocean

Abstract

Aerosol chemical, microphysical, and optical data collected from an island station and a ship during the first field phase of the Indian Ocean Experiment provided a unique opportunity to develop models for retrieving aerosol optical depth from the advanced very high resolution radiometer (AVHRR) onboard NOAA14 during January–March 1998. Columnar aerosol optical depth (AOD) over Arabian Sea, Bay of Bengal, and Indian Ocean was derived for the 630 nm wavelength from the radiance in channel 1 (580–680 nm) of AVHRR. The satellite retrieval model for AOD accounts for several aerosol species (sulfates, nitrates, sea salt, soot, dust, and organics), the in situ measured value of single scattering albedo, and the wind speed dependence of sea surface albedo. Satellite‐retrieved AOD is in good agreement with surface measurements of AOD taken from the Indian Ocean island of Kaashidhoo (4.96°N, 73.46°E) in the Maldives and from the R/V Sagar Kanya cruising between 20°N and 20°S. The success of our model is most likely due to the use of observed single scattering albedo, the use of phase function derived from in situ data, and the limitation of the analysis to the antisolar side of the satellite scan. However, the model relies on atmospheric column data and surface measurements, which need future verification with in situ aircraft data. Regional maps reveal that the entire northern Indian Ocean has large 0.15 AOD with monthly mean values exceeding 0.2 for latitudes north of ∼5°N, for all 3 months. AOD increases northward, reaching values as high as 0.35 toward the coast in the Bay of Bengal and the Arabian Sea. The non‐sea‐salt component of AOD is inferred to be more than 3 times that of the estimated wind‐dependent sea salt component. In the western Indian Ocean and Arabian Sea the high concentration of non‐sea‐salt aerosols are due to transport from the Indian subcontinent and Arabia. The eastern Indian Ocean is influenced by the transport from the Indian subcontinent and southeast Asia, particularly from Indonesia. The 1998 El Niño‐related forest fires from Indonesia resulted in high AOD values (0.25–0.35) in the eastern equatorial Indian Ocean. Minimum AOD is observed between the equator and 10°S, which is the location of the Intertropical Convergence Zone (ITCZ) during the observation period. AOD is generally found to increase to the south of ITCZ and reach a maximum around 20°S.