Ground-based and satellite observations are used in conjunction with the Rapid Radiative
Transfer Model (RRTM) to assess climatological aerosol loading and the associated cloud-free aerosol direct
radiative effect (DRE) over the Red Sea. Aerosol optical depth (AOD) retrievals from the Moderate Resolution
Imaging Spectroradiometer and Spinning Enhanced Visible and InfraRed Imager (SEVIRI) instruments are first
evaluated via comparison with ship-based observations. Correlations are typically better than 0.9 with very
small root-mean-square and bias differences. Calculations of the DRE along the ship cruises using RRTM also
show good agreement with colocated estimates from the Geostationary Earth Radiation Budget instrument
if the aerosol asymmetry parameter is adjusted to account for the presence of large particles. A monthly
climatology of AOD over the Red Sea is then created from 5 years of SEVIRI retrievals. This shows enhanced
aerosol loading and a distinct north to south gradient across the basin in the summer relative to the winter
months. The climatology is used with RRTM to estimate the DRE at the top and bottom of the atmosphere
and the atmospheric absorption due to dust aerosol. These climatological estimates indicate that although
longwave effects can reach tens of W m 2
, shortwave cooling typically dominates the net radiative
effect over the Sea, being particularly pronounced in the summer, reaching 120 W m 2 at the surface.
The spatial gradient in summertime AOD is reflected in the radiative effect at the surface and in associated
differential heating by aerosol within the atmosphere above the Sea. This asymmetric effect is expected to
exert a significant influence on the regional atmospheric and oceanic circulation.