Climatic Effect of Aerosol on Tropical Rainfall: Evidence from Satellite Observations
Chidong Zhang, University of Miami
Many efforts have been made to investigate whether and how aerosol may affect precipitation. These efforts have yielded inconsistent and therefore controversial results. Especially, in the absence of observed aerosol effects on precipitation on climate scales, the null hypothesis "There is no climatic effect of aerosol on precipitation" has not been rejected. This has led the IPCC 2007 report to state "the sign of the global change in precipitation due to aerosols is not yet known". In this study, we use long-term satellite observations of aerosol and rainfall to document large-scale co-variability of the two variables over the tropical Atlantic and West Africa. When influences due to known climate phenomena (e.g., ENSO, NAO, TAV) and meteorological factors (e.g., water vapor) are ruled out and analysis domains are carefully designed, such co-variability, especially its spatial patterns, suggests possible aerosol effects on rainfall. Large reductions in rainfall are found over the western tropical Atlantic Ocean and over the Gulf of Guinea in months of anomalously high aerosol concentration. Such reductions are statistically different from random and overall interannual variability. We propose that these reductions signify the climatic effect of aerosol on precipitation distribution and variability. Statistical results based on long-term satellite data are confirmed by consistent results based on more recent and higher quality satellite observations.
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