A climatology of stratospheric aerosol

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Abstract
A global climatology of stratospheric aerosol is created by combining nearly a decade (1979–1981 and 1984–1990) of contemporaneous observations from the Stratospheric Aerosol and Gas Experiment (SAGE I and II) and Stratospheric Aerosol Measurement (SAM II) instruments. One goal of this work is to provide a representative distribution of the aerosol layer for use in radiative and chemical modeling. A table of decadal average 1μm extinction values is included, extending from the tropopause to 35 km and 80°S to 85°N, which allows estimation of surface area density. We find that the aerosol layer is distinctly volcanic in nature and suggest that the decadal average is a more useful estimate of future aerosol loading than a “background” loading, which is never clearly achieved during the data record. This climatology lends insight into the general circulation of the stratosphere. Latitude ‐ altitude sections of extinction ratio at 1 μm are shown, averaged by decade, season, and phase of the quasi‐biennial oscillation (QBO). A tropical reservoir region is diagnosed, with an “upper” and a “lower” transport regime. In the tropics above 22 km (upper regime), enhanced lofting occurs in the summer, with suppressed lofting or eddy dilution in the winter. In the extratropics within two scale heights of the tropopause (lower regime), poleward and downward transport is most robust during winter, especially in the northern hemisphere. The transport patterns persist into the subsequent equinoctial season. Ascent associated with QBO easterly shear favors detrainment in the upper regime, while relative descent and poleward spreading during QBO westerly shear favors detrainment in the lower regime. Extinction ratio differences between the winter‐spring and summer‐fall hemispheres, and differences between the two phases of the QBO, are typically 20–50%. Dynamical implications of the aerosol distributions are explored, with focus on interhemispheric differences, strong subtropical gradients, and the pronounced annual cycle.