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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 the cloud fraction of ∼5 %–10 % over a large part of the tropical ocean. The positive effect 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 indicate 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 integrates the response (cooling) of the sea surface temperature (SST). In addition to the BBA radiative effect on SST, 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 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 in the optical depth of about ∼2 –3 over the ocean south of the Equator. The result of the BBA direct effect and feedback on tropical clouds modulates the surface solar radiation over the whole of tropical Africa. The strongest surface dimming is over central Africa (∼-30 W m−2), leading to a large reduction in 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 effect on precipitation over the western African coast, with a decrease of ∼1 to 2 mm d−1. This study also highlights 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 effect on precipitation is mainly simulated over the Gulf of Guinea, with a reduction of ∼1 mm d−1. 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 the tropical African climate. [ABSTRACT FROM AUTHOR]

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 theWest 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. [ABSTRACT FROM AUTHOR]

This study investigates, through regional climate modelling, the surface mass concentration and AOD (Aerosol Optical Depth) evolution of the various (anthropogenic and natural) aerosols over the Euro-Mediterranean region between the end of the 20th century and the mid-21st century. The direct aerosol radiative forcing (DRF) as well as the future Euro-Mediterranean climate sensitivity to aerosols have been also analysed. Different regional climate simulations were carried out with the CNRM-ALADIN63 regional climate model, driven by the global CNRM-ESM2-1 Earth System Model (used in CMIP6) and coupled to the TACTIC (Tropospheric Aerosols for ClimaTe In CNRM) interactive aerosol scheme. These simulations follow several future scenarios called Shared Socioeconomic Pathways (SSP 1-1.9, SSP 3-7.0 and SSP 5-8.5), which have been chosen to analyse a wide range of possible future scenarios in terms of aerosol or particles precursors emissions. Between the historical and the future period, results show a total AOD decrease between 30 and 40 % over Europe for the three scenarios mainly due to the sulfate AOD decrease (between -85 and -93 %), that is partly offset by the nitrate and ammonium particles AOD increase (between +90 and +120 %). According to these three scenarios, nitrate aerosols become the largest contributor to the total AOD during the future period over Europe, with a contribution between 43.5 and 47.5 %. Concerning natural aerosols, their contribution to the total AOD increases slightly between the two periods. The different evolution of aerosols therefore impacts their DRF, with a significant sulfate DRF decrease by 2.6 W m-2 and a moderate nitrate and ammonium DRF increase by 1.4 W m-2, on average according to the three scenarios over Europe. These changes, which are similar under the different scenarios, explain about 65 % of the annual shortwave radiation change but also about 6 % (in annual average) of the warming expected over Europe by the middle of the century. This study shows, with the SSP 5-8.5, that the extra-warming attributable to the anthropogenic aerosols evolution over Central Europe and the Iberian Peninsula during the summer period is due to aerosol-radiation as well as aerosol-cloud interactions processes. The extra-warming of about 0.2 °C over Central Europe is explained by a surface radiation increase of 5.8 W m-2 over this region, due to both a surface aerosol DRF decrease of 4.4 W m-2 and cloud optical depth (COD) decrease of 1.3. In parallel, the simulated extra-warming of 0.2 °C observed over the Iberian Peninsula is due, as for it, to a COD decrease of 1.3 but also to an atmospheric dynamics change leading to a cloud cover decrease of about 2 % and a drier air in the lower layers, signature of the semi-direct forcing. This study thus highlights the necessity of taking into account the evolution of aerosols in future regional climate simulations. [ABSTRACT FROM AUTHOR]

Aerosols play an important role in Europe and the Mediterranean area where different sources of natural and anthropogenic particles are present. Among them ammonium and nitrate (A&N) aerosols may have a growing impact on regional climate. In this study, their representation in coarse and fine modes has been introduced in the prognostic aerosol scheme of the ALADIN-Climate regional model. This new aerosol scheme is evaluated over Europe and the Mediterranean Sea, using two twin simulations over the period 1979-2016 with and without A&N aerosols. This evaluation is performed at local and regional scales, using surface stations and satellite measurements. Despite an overestimate of the surface nitrate concentration, the model is able to reproduce its spatial pattern including local maxima (Benelux, Po valley). Concerning the simulated Aerosol Optical Depth (AOD), the inclusion of A&N aerosols significantly reduces the model bias compared to both AERONET stations and satellite data. Our results indicate that A&N aerosols can contribute up to 40% to the total AOD550 over Europe, with an average of 0.07 (550 nm) over the period 2001-2016. Sensitivity studies suggest that biases still present are related to uncertainties associated with the annual cycle of A&N aerosol precursors (ammonia and nitric acid). The decrease of sulphate aerosol production over Europe since 1980 produces more free ammonia in the atmosphere leading to an increase in A&N concentrations over the studied period. Analyses of the different aerosol trends have shown for the first time to our knowledge, that since 2005 over Europe, A&N AOD550 and A&N Shortwave (SW) Direct Radiative Forcing (DRF) are found to be higher than sulphate and organics, becoming the species with the highest AOD and the highest DRF. On average over the period 1979-2016, the A&N DRF is found to be about -1.7 W m-2 at the surface and -1.4 W m-2 at the Top of the Atmosphere (TOA) in all-sky conditions over Europe, with regional maxima located at the surface over the Po valley (-5 W m-2). Finally, the dimming effect of A&N aerosols is responsible for a cooling of about -0.2°C over Europe (summer), with a maximum of -0.4°C over the Po valley. Concerning precipitations, no significant impact of A&N aerosols has been found. [ABSTRACT FROM AUTHOR]