PNNL

Abstract

Meridional distribution of aerosol optical thickness (AOT) over the ocean was analyzed by using the eight-year MISR and MODIS-Terra data sets, from March 2000 to February 2008, as well as the five-year MODIS-Aqua data set, from July 2002 to June 2007. The three independent sensors show that there was a pronounced meridional aerosol asymmetry. It was found that there were strong seasonal variations in the hemispheric aerosol asymmetry: it was pronounced during the half-year period, from March to August (the most pronounced asymmetry was observed from April to July). There was no noticeable asymmetry during the season from September to December. Not only has the Northern hemisphere, where the main sources of aerosols are located, but also the Southern hemisphere contributed to the formation of noticeable aerosol asymmetry. The increase in AOT, averaged over the Northern hemisphere during the season of pronounced hemispheric aerosol asymmetry, was accompanied by a decrease in AOT, averaged over the Southern hemisphere. In both hemispheres, amplitudes of seasonal AOT variations decreases from mid-latitudes (60N - 30N and 30S - 60S) to low latitudes (30N - 0 and 0 - 30S) respectively, indicating that the contribution of AOT averaged over mid-latitudes to the formation of pronounced meridional hemispheric asymmetry and its seasonal variations is more significant than the contribution of AOT averaged over low latitudes. For the season of prominent hemispheric aerosol asymmetry, from April to July, during the eight-year period under consideration, a declining long-term tendency of AOT prevailed at latitudes between 30oN and 60oN, suggesting brightening over the cloud-free ocean.