Acceleration of the southern African easterly jet driven by the radiative effect of biomass burning aerosols and its impact on transport during AEROCLO-sA

Abstract

Abstract. The direct and semi-direct radiative effects of biomass burning aerosols (BBAs) are investigated over southern Africa and the southeastern Atlantic during the Aerosols, Radiation and Clouds in southern Africa (AEROCLO-sA) field campaign in September 2017. A reference convection-permitting simulation has been performed using the Meso-NH model with an online dust emission scheme, a strongly absorbing BBA tracer emitted using the daily Global Fire Emissions Database and online-computed backward Lagrangian trajectories. The simulation captures both the aerosol optical depth and the vertical distribution of aerosols as observed from airborne and spaceborne lidars. The occurrence of stratocumulus over the southeastern Atlantic, deep convective clouds over equatorial Africa and the large-scale circulation are all reproduced by the model. If the radiative effects of BBA are omitted in the model, we show that (i) the smoke plume is too low in altitude, (ii) the low-cloud cover is too weak, (iii) the deep convective activity is too frequent but not intense enough, (iv) the Benguela low-level jet is too strong, and (v) the southern African easterly jet is too weak. The Lagrangian analysis indicates that the radiative effect of BBA leads to the transport of BBA to higher altitudes, farther southwest and with a stronger diurnal oscillation. The higher smoke plume altitude can be explained by a combination of three factors: increased upward motion induced by the stronger southern African easterly jet, self-lofting of BBA and reduced subsidence associated with less frequent deep convective activity over western equatorial Africa.

Assessing Radiative Impacts of African Smoke Aerosols Over the Southeastern Atlantic Ocean

“…The southern branch of the African Easterly Jet (AEJ‐S) strengthens (i.e., increase in mid‐level (600 hPa) wind speed) in July and achieves a maximum in October (Adebiyi & Zuidema, 2016). This is a large contributing factor to the long‐range, westward transport of central African biomass burning smoke plumes over the remote South Atlantic Ocean (Adebiyi & Zuidema, 2016; Chaboureau et al., 2022; Kuete et al., 2020; Ryoo et al., 2021). Note that non‐refractory aerosols (e.g., sea salt) can also contribute to the total aerosol profile, especially during clean episodes (Zuidema et al., 2018).…”

“…The 600 hPa wind product from the National Centers for Environmental Prediction (Kalnay et al., 1996) is used as a proxy for the magnitude and direction of the mid‐troposphere smoke aerosol transport to the AMF‐ASI site (Chaboureau et al., 2022; Hsieh & Cook, 2007; Kuete et al., 2020; Ryoo et al., 2022). The NASA Modern Era Retrospective Analysis for Research and Applications Reanalysis version 2 (MERRA‐2) platform incorporates a sophisticated data assimilation system from the Goddard Earth Observing System version 5 (GEOS‐5) model (Buchard et al., 2017; Gelaro et al., 2017).…”

“…In the Southern Hemisphere, extensive regions in Central Africa are the primary sources of nearly year-round biomass burning due to agricultural and wildfire activity, most notably within the sub-Sahel and Congo jungle terrains (Kacarab et al, 2020;Marquardt Collow et al, 2020;Redemann et al, 2021;Ryoo et al, 2021Ryoo et al, , 2022. Large scale mid-tropospheric (e.g., 600 hPa) wind patterns advect smoke aerosols westward over the southern Atlantic Ocean during the major burn season months of June-October (Adebiyi & Zuidema, 2016;Chaboureau et al, 2022;Zhang & Zuidema, 2019). Several multi-national and multi-agency efforts have investigated the Southern Hemisphere with respect to aerosols, clouds, precipitation, and meteorological patterns during the 2016-2017 period.…”

Impacts of an aerosol layer on a midlatitude continental system of cumulus clouds: how do these impacts depend on the vertical location of the aerosol layer?

“…These layers are lifted and transported to the southeastern Atlantic (SEA) region and S. S. Lee et al: Impacts of an aerosol layer on a midlatitude continental system of cumulus clouds located above or around the top of a large layer or deck of warm cumulus and stratocumulus clouds (Roberts et al, 2005;van der Werf et al, 2010;Che et al, 2022). Note that aerosols in the transported aerosol layers contain organic and black carbon, and these aerosols act as radiation absorbers as well as CCN (Wilcox, 2010;Deaconu et al, 2019;Chaboureau et al, 2022). Reflecting this interest, to better understand roles of aerosol layers above or around cloud tops in cloud development, there have been international field campaigns in the SEA such as the National Aeronautics and Space Administration ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES; https:// espo.nasa.gov/oracles/content/ORACLES, last access: 5 January 2023), the United Kingdom Clouds and Aerosol Radiative Impacts and Forcing (CLARIFY; Redemann et al, 2021), and the French Aerosol, Radiation and Clouds in southern Africa (AEROCLO-sA; Formenti et al, 2019).…”

Source attribution of cloud condensation nuclei and their impact on stratocumulus clouds and radiation in the south-eastern Atlantic

“…Among them, biomass burning (BB) aerosol advected from continental Africa, where one-third of the global BB emissions are produced from July to October (Roberts et al, 2009;van der Werf et al, 2010), plays a unique role in modulating the cloud properties due to its short-wave absorption ability as well as its ability to act as CCN. Previous studies have suggested that, as BB aerosols are mainly located above and near the inversion layer, the main role of their radiative effect in the SEA is to strengthen the capping inversion and reduce dry-air entrainment from cloud tops, thereby increasing the LWP and low-level cloud fraction, resulting in a significant impact on the radiation balance (Wilcox, 2010;Gordon et al, 2018;Deaconu et al, 2019;Mallet et al, 2020;Herbert et al, 2020;Chaboureau et al, 2022). When BB aerosols are located in the marine boundary layer, their radiative effect can enhance the decoupled boundary layer and result in a reduction in cloud cover and the LWP, shifting the stratocumulus-to-cumulus transition to the upwind area (Zhang and Zuidema, 2019;Ajoku et al, 2021).…”