Meridional distribution of aerosol optical thickness over the tropical Atlantic Ocean

Abstract. Previous studies showed that, over the global ocean, there is hemispheric asymmetry in aerosols and no noticeable asymmetry in cloud fraction (CF). In the current study, we focus on the tropical Atlantic (30° N–30° S) which is characterized by significant amounts of Saharan dust dominating other aerosol species over the North Atlantic. Over a limited area such as the tropical Atlantic, our study showed that strong meridional asymmetry in dust aerosols was accompanied by meridional CF asymmetry, by contrast to the global ocean. During the 10 yr study period (July 2002–June 2012), NASA Aerosol Reanalysis (aka MERRAero) showed that, when the meridional asymmetry in dust aerosol optical thickness (AOT) was the most pronounced (particularly in July), dust AOT averaged separately over the tropical North Atlantic was one order of magnitude higher than dust AOT averaged over the tropical South Atlantic. In the presence of such strong meridional asymmetry in dust AOT in July, CF averaged separately over the tropical North Atlantic exceeded CF averaged over the tropical South Atlantic by 20%. In July, along the Saharan Air Layer, Moderate Resolution Imaging Spectroradiometer (MODIS) CF data showed significant cloud cover (up to 0.8–0.9), which contributed to above-mentioned meridional CF asymmetry. Both Multi-Angle Imaging SpectroRadiometer (MISR) measurements and MERRAero data were in agreement on seasonal variations in meridional aerosol asymmetry. Meridional asymmetry in total AOT over the Atlantic was the most pronounced between March and July, when dust presence over the North Atlantic was maximal. In September and October, there was no noticeable meridional asymmetry in total AOT over the tropical Atlantic.