West African Aerosols and their Impacts on Regional Climate

Abstract

West Africa is one of the most important source of aerosols in the World due to the large extent of Sahara and Sahel regions of Africa which have been identified as the first source of mineral dust. In addition, fires occur in several vegetated ecosystems across the World, especially in tropical and subtropical savannah where fire is widely used by the population, during dry season for mainly social and economic purposes. Therefore West Africa, due to its location (between Sahara and Atlantic Ocean) is subject to complex interaction between dust, combustion (biomass burning and fossil fuel) particles and maritime aerosols which may impact the regional climate. Using a regional climate model (RegCM4) coupled to an interactive dust module, treating dust emission, transport, and deposition processes, investigations of effects of dust and carbonaceous aerosols particles from biomass burning were conducted on the West African climate. The study further investigates the relationship between the Saharan Air Layer located above Atlantic Ocean (OSAL) and West African Monsoon (WAM) features, including Monsoon flow, African Easterly Jet (AEJ), and Tropical Easterly Jet (TEJ) over West Africa. To achieve these set purposes, two sets of experiments from 2000-2010 were performed, one including dust and one without dust effect over the West African domain, encompassing the whole West Africa and a large part of the adjacent Atlantic Ocean. Results from simulations performed in this study show that dust load into the atmosphere has an effect on both the wind and temperature structure at different levels, inducing observed changes in WAM system during June-July-August-September (JJAS) seasons. These changes lead to a westward shift and slight strength of AEJ core over tropical Atlantic which is associated to a weak TEJ. Moreover despite the prescribed Sea Surface Temperature (SST), good correlation was noted to exist between Aerosol Optical Depths in OSAL and regional wind, suggesting that the mechanism between dust and WAM features is well reproduced by RegCM4. Moreover, assessing dust-induced radiative forcing over the study domain revealed that dust induced cooling both at TOA and surface throughout the year. The radiative forcing at the Top of Atmosphere (TOA) is minimum during June-July-August (JJA) both over the Ocean (- 30 to -40 W.m-2) and land (-10 to -20 W.m-2), and maximum during December-January- February (DJF) with transitional value during MAM and SON. Also, the daily satellite products (L3JRC) of burned areas from the SPOT– VEGETATION sensor at a moderate spatial resolution of 1 km × 1 km between 2000 and 2007 were analyzed in this work. Results from seasonal analysis revealed a large increase in burned areas from November to February with consistent peaks in December at regional scale and 30% of the L3JRC pixels were burned in approximately 2 years intervals over the West African Savannah. Dividing West Africa into sub-regions broadly according to climate and vegetation, revealed existence of several fire regimes across the region following climate and vegetation gradient. Fires regime is regular in Guinean and Sudanian savannahs with less impact of climate variability on the fires.