Simulation of the direct and semidirect aerosol effects on the southern Africa regional climate during the biomass burning season

International audienceThe regional climate model RegCM3 was used to simulate the direct and semidirect radiative effects of biomass burning and dust aerosol over southern Africa during the austral winter season. Simulated aerosols were found to induce changes in the regional surface fluxes and atmospheric dynamics. Clear-sky surface radiative forcing decreased by up to −60 W/m2 in the main biomass burning region, resulting in decreased surface turbulent fluxes and PBL height as well as reduced surface temperatures. The positive temperature bias over the western half of the subcontinent was thus reduced. Radiative absorption by biomass burning aerosols resulted in diabatic warming of the atmosphere, peaking near 700 hPa at a rate of up to 1°C/d. Simulated surface cooling and heating at altitude stabilized the lower troposphere below 700 hPa. Above 700 hPa, stability was reduced in the equatorial region between 5°N and 5°S through an elevated heat pump mechanism, enhancing deep convection and precipitation. The southern branch of the African Easterly Jet was enhanced and shifted southward, likely as a result of the changes in the surface temperature gradient induced by both the reduction in solar radiation reaching the surface and through precipitation-induced surface cooling in the equatorial region. Daily-scale aerosol outflow events to the southwest Indian Ocean were also investigated, these events occurring with the passage of a westerly wave. It was found that the aerosol loading enhanced baroclinicity along the leading edge of the frontal system, thus intensifying and narrowing the band of precipitation in this zone