The Annual Cycle of Fractional Atmospheric Shortwave Absorption in Observations and Models: Spatial Structure, Magnitude, and Timing.

We use the best currently available in situ and satellite-derived surface and top-of-the-atmosphere (TOA) shortwave radiation observations to explore climatological annual cycles of fractional (i.e., normalized by incoming radiation at the TOA) atmospheric shortwave absorption a ˜ on a global scale. The analysis reveals that a ˜ is a rather regional feature where the reported nonexisting a ˜ in Europe is an exception rather than the rule. In several regions, large and distinctively different a ˜ are apparent. The magnitudes of a ˜ reach values up to 10% in some regions, which is substantial given that the long-term global mean atmospheric shortwave absorption is roughly 23%. Water vapor and aerosols are identified as major drivers for a ˜ while clouds seem to play only a minor role for a ˜ . Regions with large annual cycles in aerosol emissions from biomass burning also show the largest a ˜ . As biomass burning is generally related to human activities, a ˜ is likely also anthropogenically intensified or forced in the respective regions. We also test if climate models are able to simulate the observed pattern of a ˜ . In regions where a ˜ is driven by the annual cycle of natural aerosols or water vapor, the models perform well. In regions with large a ˜ induced by biomass-burning aerosols, the models' performance is very limited. [ABSTRACT FROM AUTHOR]