Results on direct radiative forcing of aerosols obtained over the tropical Indian Ocean

Abstract

Aerosols reduce the surface reaching solar flux (negative radiative forcing) by scattering the incoming solar radiation out to space. Various model studies on climate change suggest that surface cooling induced by aerosol scattering is the largest source of uncertainty in predicting the future climate. Measurements made by ORV Sagar Kanya over the tropical Indian Ocean, as part of the pre-INDOEX campaign, in January to March of 1996, 1997 and a998 consistently present aerosol characteristics with large spatial and temporal variations. The columnar aerosol optical depths measured at different spectral bands using a sunaerosol amuont decreasing from north to south, until the Inter-Tropical Convergence Zone (ITCZ). INdependent surface level aerosol size distribution measurments complement the optical depth data. The submicron size particles of radius less than 0.5 um are responsible for the high aerosol optical depths obtained north of the ITCZ. Surface reaching direct solar and global (solar plus sky) radiation intensities are measured using standard pryheliometers, pyranometers and photodiode radiometers. Data on aerosol optical depth and radiation intensities available simultaneously on radiative forcing by aerosols. Combining the 1996 and 1997 cruise data, the diurnal averaged radiative forcing for the tropical Indian Ocean is obtained as -5.2W m^-2, which is the reduction in the global flux for avery 0.1 increase in aerosol optical depth. THis is the first ever direct experimental evidence on the aerosol radiative forcing which applies to the aerosols found over the tropical Indian Ocean during January to March. This value is higher than the computed global average value, which is about -2W m^-2. The excess forcing could only be explained by an excess absorption by aerosols or by other atmospheric constituents which are hitherto unaccounted for in the model calculations.