More biomass burning aerosol is being advected westward over the southern tropical Atlantic since 2003

Each year, agricultural fires in southern continental Africa emit approximately one third of the world’s biomass burning aerosol. This is advected westward by the prevailing circulation winds over a subtropical stratocumulus cloud deck. The radiative effects from the aerosol and aerosol-cloud interactions impact regional circulations and hydrology. Here we examine how changes in the coupled southern African earth system over the past 20 years impact the southeast Atlantic. We combine satellite-derived burned area datasets with ECMWF-reanalysis carbon monoxide, black carbon, and meteorology from the biomass burning season (May-October) in southern Africa. The burning season begins in May in woody savannas in the northwest and shifts to open savanna and grassland fires in the southeast, with small fires (less than 1 km2) contributing significantly to total burned area. More small fires are occurring in the middle of the biomass burning season and the overall season is shorter, corroborated by reanalysis carbon monoxide fields. Significantly increased free tropospheric winds, shifted southward, transport smoke aerosol further southwest over the southeast Atlantic. The increased aerosol advection is coupled with a southern shift in the south Atlantic subtropical high and an increase in the low cloud fraction on the southern edge of the stratocumulus cloud deck. While smoke emissions sources have not changed significantly, changes in the smoke transport pathway, attributed to increasing surface temperatures in southern Africa and tropical expansion, combined with an altered low cloud distribution, explain how the regional radiation balance has shifted to more top-of-atmosphere cooling in recent decades.