Spectral aerosol optical characteristics during the northeast monsoon over the Arabian Sea and the tropical Indian Ocean: 1. Aerosol optical depths and their variabilities

Abstract

From the cruise expeditions conducted during 1996-2000 in the northeast winter monsoon seasons over the Arabian Sea (AS), tropical Indian Ocean (TIO) aerosol optical depths (AODs) in the 0.4-0.85 μm wavelength region are measured, and their yearly, monthly mean variabilities and trends are analyzed. The AODs are found to show a steep latitudinal gradient as the ship moves away from the coast towards the pristine oceanic region. AODs are found to exhibit an increasing trend over coastal India (CI) and AS, while the AODs are found to decrease over TIO during the 5-year period. The monthly mean AODs from December to April are found to increase over CI and AS, while they decrease over TIO. AODs obtained over two land locations (Trivandrum (TVM), Vishakapatnam (VSK)) in peninsular India, and two marine locations (Minicoy (MCY), Kaashidhoo (KCO)), situated in the downwind direction of the Indian subcontinent for the 1996-2000 period are analyzed and compared. The 5-year mean AOD spectra over TVM, VSK, and MCY/KCO are found to exhibit a less steeper gradient and the AODs over these locations are comparable. The AODs are found to show an increasing trend in the 4-year period over TVM. Over VSK no definite positive or negative trends are visible in the AODs. The yearly mean AODs over MCY/KCO are found to exhibit a decreasing trend quite consistent with the trends seen over the Indian Ocean. The monthly mean AODs are found to increase over the land and marine locations during the December-April period. Copyright 2004 by the American Geophysical Union.