Observations of the spectral clear‐sky aerosol forcing over the tropical Indian Ocean

During the first field phase (FFP) of the Indian Ocean Experiment (INDOEX) in February and March, 1998, the spectral global and direct beam irradiance have been measured between 350 and 1050 nm wavelengths using a 512‐channel, fixed grating, photodiode array spectroradiometer. A detailed analysis of the instrument's reliability, the absolute calibration, and the corrections for deviation from the ideal cosine response are presented. For most of the spectral region the total uncertainty is shown to be <2%. The spectral optical depth, the spectral aerosol forcing, and the aerosol forcing for the photosynthetically active radiation have been derived from direct beam measurements and global irradiance measurements. The optical depth at 500 nm wavelength decreases from ∼0.5 in the northern Arabian Sea to as low as 0.05 south of the Intertropical Convergence Zone (ITCZ) near ∼15°S latitude. The surface aerosol forcing efficiency is defined as the rate of change of net irradiance at the surface due to an increase by 1 in optical depth at 500 nm. The normalization procedure we adopt to determine the aerosol forcing efficiency with respect to a reference pristine day in the Southern Hemisphere eliminates most of the radiometric calibration uncertainties. The continental aerosol south of the ITCZ shifts the peak in the direct solar radiation from 470 nm (for pristine conditions) to ∼580 nm for the polluted region. The spectral aerosol forcing efficiency peaks around 460 nm, with −1.2, −0.6, and +0.6 W m −2 nm −1 for the direct, global, and diffuse irradiance, dropping for the lower and higher wavelengths to about −0.3, −0.25, and 0.05 W m −2 nm −1 at 350 nm and −0.3, −0.1, and +0.2 at 1050 nm. Integrated over 400–700 nm, the aerosols decrease the noontime solar flux by as much as −38 W m −2 in the Arabian Sea to as little as −2 W m −2 south of the ITCZ. This introduces a strong north to south gradient in the climate forcing of the ocean. In addition, the strong aerosol modulation of the photosynthetically active radiation (400–700 nm) and its north to south gradient have important implications for biomass production of the ocean.