Impact of atmospheric aerosols on the radiation budget of the Earth-atmosphere system and climate is well recognized.
The spatial distribution and the properties of atmospheric aerosols over the oceanic areas around the Indian subcontinent
during the Asian dry season (November-April period) are significantly governed by the transport of continental air from
the adjoining landmass. In the present study, we report the estimated spatial distribution of monthly mean aerosol direct
radiative forcing (ADRF) at the top-of-atmosphere (TOA), within the atmosphere, and at the earth's surface over the
Arabian Sea, Bay of Bengal and Equatorial Indian Ocean during the Asian dry season, based on the regional distribution
of aerosol optical depth (AOD) derived from NOAA14/16-AVHRR data during 1996-2003, and the model estimates of
the diurnal mean normalized aerosol radiative forcing (NADRF) which also compares fairly well with the earlier
observations reported over this region. The diurnal mean NADRF varies with latitude and Julian day. Its value at TOA
(for AOD at 550nm) is in the range -26.5 Wm-2AOD -1 to -29.5 Wm-2 AOD-1 while the corresponding value at surface is
in the range -77 to -95 Wm-2 AOD-1. In the Northern Hemisphere, ADRF is in the range -4 to -14 Wm-2 at TOA, -2 to -
42 Wm-2 at the surface and 8 to 28Wm-2 in the atmosphere. During the Asian dry season highest DMADRF is observed
in the northwest Bay of Bengal followed by the southeast Arabian Sea during the March-April period.
The regional distribution of atmospheric aerosols over the Indian Ocean, Bay of Bengal and Arabian Sea during the Asian winter monsoon period of November to April, and its inter annual variability during 1996 to 1999 are studjed. During the winter monsoon period, the predominantly northeasterly winds from the Asian continent transport large amount of polluted air mass deep into the otherwise pristine ocean areas. The aerosol optical depth (AOD) is obtained by inverting the observed radiance in channel 1 of NOAA- 14 AVHRR after accounting for multiple scattering and absorption due to aerosols and molecules and the wind dependence of sea surface reflectance. The Scattering phase function and single scattering albedo used for the estimation of AOD are based on the in situ measurements of physical, chemical and optical properties of aerosols in this region during the Indian Ocean experiment (1NDOEX). Transport of aerosols from continental areas into the oceanic environments is studied using a combination of regional atmospheric circulation and the aerosol distribution over the oceanic regions. It is observed that the Bay of Bengal region is significantly affected by large-scale transport of aerosols from the Southeast Asia and the Indian subcontinent. Aerosol distribution over the Arabian Sea is mainly controlled by the transport from the Indian subcontinent and the Arabia. Continental aerosols are transported deep into the Indian Ocean up to the Inter Tropical Convergence Zone (ITCZ). The aerosol distribution shows significant monthly and year-to-year variability at the north of ITCZ.
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