Direct observations of clear-sky aerosol radiative forcing from space during the Indian Ocean Experiment

This study presents the regional estimates of the seasonal and diurnal mean broadband (0.3-5.0 mum) clear-sky aerosol radiative forcing at the top of atmosphere (TOA) due to both the natural and the anthropogenic aerosols over the tropical Indian Ocean from 25 degreesN to 25 degreesS. We propose two new methods, the slope method and the differencing method, to obtain clear sky aerosol forcing from solely satellite measurements. The focus of the study is January to March 1997, 1998, and 1999. The TOA clear-sky aerosol forcing was obtained by integrating satellite data for aerosol optical depth (AOD) and the broadband radiation budget. Over 30,000 pixels were collocated to estimate that the diurnal and seasonal mean reflected broadband solar radiation at TOA increases by about 24 W m(-2) per unit increase in AOD at the wavelength of 500 nm. The observed TOA clear-sky aerosol forcing varied between -4 and -14 W m(-2) in the Northern Hemisphere (NH) and between 0 and -6 W m(-2) in the Southern Hemisphere. Assuming a ratio of surface to TOA clear-sky aerosol forcing of 3 which was observed over Kaashidhoo Climate Observatory (4.96 degreesN, 73.46 degreesE) during the same period [Satheesh and Ramanathan, 2000], this leads to a clear-sky aerosol forcing of -12 to -42 Wm(-2) at the surface in the NH. The difference between the TOA forcing and the surface forcing is the atmospheric forcing. As a result, the atmosphere is subject to a large net forcing of about 8-28 Wm(-2) in the NH, largely due to the presence of black carbon. Of equal importance is the fact that the Indian Ocean aerosols introduce a large interhemispheric gradient in the solar heating during the wintertime. The implications for climate and monsoonal circulation may be major and need to be explored with coupled model studies.