Impact of Biomass Burning Aerosols (BBA) on the tropical African climate in an ocean-atmosphere-aerosols coupled climate model

Abstract. The impact of biomass burning aerosols (BBA) emitted in Central Africa on the tropical African climate is studied using the ocean-atmosphere global climate model CNRM-CM, including prognostic aerosols. The direct BBA forcing, cloud feedbacks (semi-direct effects), effects on surface solar radiation, atmospheric dynamics and precipitation are analysed for the 1990–2014 period. During the June-July-August (JJA) season, the CNRM-CM simulations reveal a BBA semi-direct effect exerted on low-level clouds with an increase in cloud fraction of ∼5–10 % over a large part of the tropical ocean. The positive feedback of BBA radiative effects on low-level clouds is found to be mainly due to the sea surface temperature response (decrease of ~-0.5 K) associated with solar heating at 700 hPa, which increases the lower tropospheric stability. Over land, results also indicates a positive effect of BBA on the low cloud fraction especially for the coastal regions of Gabon and Angola with a potentially enhanced impact in these coupled simulations that integrate the response (cooling) of the SST. In addition to the BBA radiative effect on sea surface temperature, the ocean-atmosphere coupled simulations highlight that the oceanic temperature response is noticeable (about -0.2 to -0.4 K) down to ~80 m depth in the JJA between the African coast and 10°W. In parallel to low-level clouds, reductions of ~5–10 % are obtained for mid-level clouds over central Africa, mainly due to BBA-induced surface cooling and lower tropospheric heating inhibiting convection. In terms of cloud optical properties, the BBA radiative effects induced an increase of the optical depth by about ~2–3 south of the equator over the ocean. The result of the BBA direct effect and feedback on tropical clouds modulates the surface solar radiation over the whole Tropical Africa. The strongest surface dimming is over central Africa (~-30 W m⁻²), leading to a large reduction of the continental surface temperature (by ~-1 to -2 K), but the solar radiation at the oceanic surface is also affected up to the Brazilian coast. With respect to the hydrological cycle, the CNRM-CM simulations show a negative feedback on precipitation over the West African coast with a decrease of ~-1 to -2 mm per day. This study highlights also a persistent impact of BBA radiative effects on low-level clouds (increase in cloud fraction, liquid water content and optical depth) during the September-October-November (SON) period, mainly explained by a residual cooling of sea surface temperature over most of the tropical ocean. In SON, the feedback on precipitation is mainly simulated over the Gulf of Guinea with a reduction by ~-1 mm per day. As for JJA, the analysis clearly highlights the important role of the slow response of the ocean in SON and confirms the need to use coupled modelling platforms to study the impact of BBA on tropical African climate.