Aerosol optical depth and direct radiative forcing for INDOEX derived from AVHRR: Observations, January–March 1996–2000



[1] Visible and near infrared reflectances from NOAA-14 Advanced Very High Resolution Radiometer (AVHRR) daytime passes are used to derive optical depths at 0.55 μm, an index of aerosol type, continental or marine, and the direct effect of the aerosol on the top of the atmosphere and surface solar radiative fluxes for the oceans in the Indian Ocean Experiment (INDOEX) region (30°S to 30°N and 50°–110°E) during the January–March 1996–2000 winter monsoons. Comparison of aerosol optical depth and radiative forcing in the Northern Hemisphere with those in the Southern Hemisphere suggests that the additional pollution sources augment the 0.55-μm optical depth by, on average, 0.1 in the Northern Hemisphere. As a result of the aerosol, the region of the Indian Ocean in the Northern Hemisphere loses about 1.6 Wm−2 in reflected sunlight and the ocean surface loses about 5 Wm−2 during the months of the winter monsoon. Aerosol burdens and the aerosol direct radiative forcing are a relatively constant feature of the Northern Hemisphere, although the southeastern Arabian Sea experienced considerably larger aerosol burdens during the February–March 1999 INDOEX Intensive Field Phase (IFP) than in other years. Frequency distributions of the optical depth for 1° × 1° latitude-longitude regions are well represented by gamma distribution functions. The day-to-day and year-to-year variability of the optical depth for such regions is correlated with the long-term average optical depth. Interannual variability of the monthly mean optical depths for such regions is found to be as large as the day-to-day variability. Such large variability suggests that long-term records of in situ observations will be required in order to assess the performance of models that generate climatologies of aerosol concentrations.