Long-range Aerosol Transport Patterns over Southern Africa as Observed by TOMS

The tropospheric transport of aerosols has important impacts on the radiative forcings, cloud dynamics, and precipitation patterns of regional and global climates, as well as on human health and on biogeochemical processes of ecosystems within and downwind of emission source regions. Wide recognition that some of the largest uncertainties in global climate models arise from uncertainties related to the magnitude of the aerosol effect on regional radiative forcings, particularly over the biomass burning regions of Africa (IPCC 2007), led me to focus my study on southern Africa where a large proportion of the Southern Hemisphere's aerosol emissions occur. Using multidecadal satellite observations of aerosol distribution over large regions, it is now becoming possible to look for long-term patterns and trends and build climatologies for various prevalent aerosol transport pathways over strong emission regions. Visual classification of an initial six years of TOMS Aerosol Index (AI) data indicates that three distinct aerosol distribution patterns (Plume, Gyre, and River of Smoke) resulting from different general aerosol transport pathways previously described in the literature are common over southern Africa during the austral winter dry season (May – October). Based on MANOVA and field significance test results comparing the mean NCEP-NCAR Reanalysis I pressure and wind fields among aerosol distribution patterns, I determined that all three patterns are associated with different underlying general circulation patterns that, when coupled with the generalized spatial and temporal pattern of aerosol emissions from southern Africa, provide an explanation for how these aerosol distribution patterns develop. The spatial characteristics of these aerosol distribution patterns may be sufficiently quantified using GIS such that accurate, consistent, and replicable ii classifications of event days may be made by independent researchers using an approach based on discriminant function analysis. Such a classification of 21 years of TOMS AI data showed that these aerosol distribution patterns are persistent throughout the record and often occur as extended multi-day events that affect broad regions of the surrounding Atlantic and Indian Oceans. Plume events dominate during the early dry season (May – July), followed by a shift to more Gyre and then more River of Smoke events during the later half of the dry season (August – October). The seasonality of the three patterns appears to largely reflect the seasonal progression of fire activity from northwest to southeast across the continent, as well as the progressive strengthening of the Westerlies and deeper intrusion by ridging highs and frontal system lows that takes place during the transition from winter to spring as part of the Southern Hemisphere's Semi-Annual Oscillation (SAO). It is hoped that the aerosol distribution climatologies and descriptions of the seasonal dynamics that resulted from this study will aid other researchers in making better estimates of the regional radiative forcings necessary to determine how changes in aerosol transport patterns may affect long-term changes in regional and global climates.

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