Aerosol modulation of atmospheric and surface solar heating over the tropical Indian Ocean

The major finding of this study is that aerosols over the tropical Indian Ocean enhance clear sky atmospheric solar heating significantly and decrease the surface solar heating by even a larger amount. The results presented here are based on aerosol chemical, microphysical, and optical and radiometric data collected at the island of Kaashidhoo (4.97°N, 73.47°E) during February and March of 1998, as part of the first field phase of the Indian Ocean experiment (INDOEX). The aerosol optical properties were integrated with a multiple scattering Monte Carlo radiative transfer model which was validated at the surface with broadband flux measurements and at the top of the atmosphere (TOA) with the clouds and earth's radiant energy system (CERES) radiation budget measurements. We consider both externally and internally mixed aerosol models with very little difference between the two models in the estimated forcing. For the February-March period, the aerosols increase the monthly mean clear sky atmospheric solar heating by about 12 W/m² (about 15% of the total atmospheric solar heating) and decrease the sea surface clear sky solar heating by about 16 W/m² with a daily range from 5 to 23 W/m². The net aerosol forcing at the top of the atmosphere is about -4 W/m² with a daily range from -2 to -6 W/m². Although the soot contributes only about 10% to the aerosol optical thickness, it contributes more than 50% to the aerosol induced atmospheric solar heating. The fundamental conclusion of this study is that anthropogenic aerosols over the tropical Indian Ocean are altering the clear sky radiation budget of the atmosphere and surface in a major manner.