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 concurrent changes in the burning season and regional climate in southern Africa over the past 18 years (2003-2020) impact the southeast Atlantic. We combine satellite-derived burned area datasets (GFEDv5 primarily) with ECMWF-reanalysis carbon monoxide (CO), 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 (<1 km) contributing significantly to total burned area. In the most recent decade, 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. Free tropospheric winds, also known as the Southern African easterly jet, increase significantly and shift southward, transporting smoke aerosol further southwest over the southeast Atlantic. The advection increases middle-tropospheric CO by 5 % of the total column per decade, and 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 all-sky shortwave top-of-atmosphere direct aerosol radiative effect has become more positive - a climate warming - in recent decades.