Aerosol optical depths over peninsular India and adjoining oceans during the INDOEX campaigns: Spatial, temporal, and spectral characteristics

Abstract

The spatial, temporal, and spectral characteristics of aerosol optical depths τpλ for the Indian Ocean Experiment (INDOEX) study period (January to April) are examined using data collected through a ground‐based network of multiwavelength solar radiometers (MWR) over coastal regions of peninsular India; two island locations, one in the Arabian Sea and another in the southern Indian Ocean at 20°S; in conjunction with estimates made over various locations over the Arabian Sea and Indian Ocean during the INDOEX cruises of 1996, 1998, and 1999. Spatial variations show extremely low values of τp at the shorter (visible) wavelengths (λ<750 nm) to the south of the Intertropical Convergence Zone (ITCZ), but increases substantially at locations due north of the ITCZ due to increased source impact and advection by favorable winds. An enhancement in τp is seen in the central Arabian Sea, which is attributed to air trajectory effects. Ångström parameters, deduced from optical depth spectra, reveal a high value of α (∼0.9) for north of the ITCZ, while for the south α is negative, indicating a change in the aerosol size distribution. Accumulation aerosols dominate in the north, while concentration of coarse aerosols remain nearly about the same, except very close to the coast. A north‐south gradient in aerosol optical depth, with scaling distance of ∼1000 to 2000 km at shorter wavelengths and much higher at longer wavelengths, is observed. The gradient becomes shallower at high wind speeds. The large‐scale dynamics associated with the movement of the ITCZ and its interannual variation appears to significantly influence the aerosol characteristics. As the southwest monsoon sets in over India, considerable wet removal and change in air mass characteristics cause a significant depletion in optical depths, which then became comparable to those prevailing in the southern hemisphere.