Optical properties of atmospheric aerosols over the Arabian Sea and Indian Ocean: North-South contrast across the ITCZ

Abstract

Extensive estimates of aerosol spectral optical depths are made over the Arabian Sea and south-western Indian Ocean, using a 10-channel multi-wavelength solar radiometer (MWR) and a 4- channel hand held EKO sun-photometer (ESP) on-board the cruise # 133 of ORV Sagar Kanya during the First Field Phase (FFP- 98) of the Indian Ocean experiment (INDOEX) in February and March 1998. High values of optical depths, particularly in the visible wavelengths, are encountered in the coastal areas, with a gradual increase from Goa to Male, The optical depths decrease sharply as the ship moves out to the south western Indian Ocean across the equator; the effect is more significant at shorter wavelengths. Over the pris-tine environment south of the ITCZ, extremely low optical depths appear at the visible wavelengths, while at the NIR wavelengths, the optical depths remain nearly the same as on the northern side. On the return Ieg, again higher optical depths are encountered north of the ITCZ with those at the visible wavelengths sharply increased. Over the north-western Arabian Sea, higher optical depthvalues occur farther away from the coast, suggesting additional input of aerosols over mid ocean, possibly transported by various wind trajectories from the west Asian deserts. Comparing air trajectories both at the surface and 850 h Pa reveals that in addition to those advected from continental India, winds transporting aerosols from various north/west Asian regions contribute significantly to the aerosol optical depths over the Arabian Sea.