Black Carbon and West African Monsoon precipitation: observations and simulations

Abstract

Abstract.We have recently investigated large-scale covariability between aerosol and precipitation and other meteorological variables in the West African Monsoon (WAM) region using long term satellite observations and reanalysis data. In this study we compared the observational results to a global model simulation including only direct radiative forcing of black carbon (BC). From both observations and model simulations we found that in boreal cold seasons anomalously high African aerosols are associated with significant reductions in cloud amount, cloud top height, and surface precipitation. These results suggest that the observed precipitation reduction in the WAM region is caused by radiative effect of BC. The result also suggests that the BC effect on precipitation is nonlinear.

Interannual Tropospheric Aerosol Variability in the Late Twentieth Century and Its Impact on Tropical Atlantic and West African Climate by Direct and Semidirect Effects

“…Biomass burning and forest fires over the Sahel region and central Africa result in carbonaceous aerosols over central Africa and the equatorial and south tropical Atlantic Ocean from July to September (e.g., Haywood et al 2003Haywood et al , 2004Matichuk et al 2007). Observational and modeling studies reveal that increased aerosols over tropical Africa and tropical Atlantic tend to reduce the West African rainfall as well as cloud amount (e.g., Huang et al 2009a). Increased aerosols are also associated with increased shallow cloud cover over the north and south tropical Atlantic off the west coast of Africa (e.g., Kaufman et al 2005).…”

“…Aerosols have been found to play a particularly important role in the climate of the tropical Atlantic and West Africa (e.g., Prospero and Lamb 2003;Yoshioka et al 2007;Huang et al 2009a;Sakaeda et al 2011), the Amazon basin (e.g., Koren et al 2004;Pö schl et al 2010), and Southeast Asia (e.g., Ramanathan et al 2001Ramanathan et al , 2005Chung and Ramanathan 2006;Lau et al 2006;Collier and Zhang 2009). In this study, we focus on the impact of variability in aerosols on the climate of the tropical Atlantic and West Africa.…”

“…Sulfate aerosols are advected to the north tropical Atlantic from emissions in North America and Europe. Dust emitted from the Sahara desert and the Sahel region travels westward across the tropical North Atlantic between 58 and 308N, peaking in the boreal summer (e.g., Prospero and Carlson 1972;Huang et al 2009b). Biomass burning and forest fires over the Sahel region and central Africa result in carbonaceous aerosols over central Africa and the equatorial and south tropical Atlantic Ocean from July to September (e.g., Haywood et al 2003Haywood et al , 2004Matichuk et al 2007).…”

“…However, only a few studies (e.g., Huang et al 2009a;Mahowald et al 2010) focus on the impact of the interannual variability of aerosols on the climate system. Studies of aerosol induced trends in the past century also focus on the long-term changes (e.g., Ramanathan et al 2005) with little analysis of aerosol-induced interannual variations.…”

“…In this study, we focus on the impact of variability in aerosols on the climate of the tropical Atlantic and West Africa. The tropical Atlantic and the African continent bear an influx of different aerosol species over different latitudinal bands throughout the year (e.g., Kaufman et al 2005;Huang et al 2009a). Sulfate aerosols are advected to the north tropical Atlantic from emissions in North America and Europe.…”