A meteorological overview of the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) campaign over the southeastern Atlantic during 2016–2018: Part 1 – Climatology

Abstract

Abstract. In 2016–2018, the ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) project undertook 3-month-long deployments to the southeastern (SE) Atlantic Ocean using research aircraft to better understand the impact of biomass burning (BB) aerosol transport to the SE Atlantic Ocean on climate. In this (part 1 of the meteorological overview) paper, the climatological features at monthly timescales are investigated. The southern African easterly jet (AEJ-S), defined as the zonal easterlies over 600–700 hPa exceeding 6 m s−1 around 5–15∘ S, is a characteristic feature of the mid-level circulation over southern Africa that was also during the deployment months of August 2017, September 2016, and October 2018. Climatologically, the AEJ-S develops at lower altitudes (∼ 3 km; 700 hPa) between 5–10∘ S in August, while it develops at around 4 km (∼ 600 hPa) and further south (5–15∘ S) in September and October, largely driven by the strong sensible heating over the African plateau. Notable meteorological anomalous characteristics during the 3 deployment months, compared to climatology (2000–2018), include the following: (1) during August 2017, the AEJ-S was weaker than the climatological mean, with an additional anomalous upper-level jet aloft (∼ 6 km) around 10∘ S. August 2017 was also drier over the SE Atlantic at 600–700 hPa than climatology, with a stronger Benguela low-level jet (LLJ) at 925–950 hPa along the Namibian coast of the SE Atlantic. Consistent with this, the southern Atlantic anticyclone was also stronger and closer to the coast than the August climatological mean. (2) During September 2016, the AEJ-S intensity was similar to the climatological mean, although the heat low and vertical motion over the land was slightly stronger compared to the September climatology. The LLJ and the large-scale southern Atlantic anticyclone were stronger than the climatological mean. (3) During October 2018, the AEJ-S was slightly weaker compared to the climatological mean, as was the LLJ and the southern Atlantic anticyclone. October 2018 was wetter over the Benguela coastal region at 600 hPa than the climatological mean. During all the deployment months, the sea surface temperatures (SST) over the SE Atlantic were warmer than the climatological means, but the monthly mean low cloud fraction was only noticeably reduced in August 2017. A weak August 2017 AEJ-S can explain low offshore black carbon (BC) mixing ratios within the European Centre for Medium-Range Weather Forecasts (ECMWF) Copernicus Atmosphere Monitoring Service (CAMS) reanalysis, although the BC peak altitude, at 2–3 km, is below that of the AEJ-S. The upper-level wave disturbance and the associated anomalous circulation also explain the weakening of AEJ-S through the reduction of the strength of the heat low over the land during August 2017.

Non-reversible aging can increase solar absorption in African biomass burning aerosol plumes of intermediate age

“…5. Some recirculation back to land around the south Atlantic anticyclone also apparent for these three days (see also Ryoo et al (2021) for more synoptic detail). Days in which the aerosol is constrained closer to the coast (Fig.…”

“…The interannual variation in aerosol loading over the SEA is primarily driven by when the climatological AEJ-S sets in, as this is responsible for its geographic distribution of aerosol at higher altitudes. The AEJ-S, though weaker than climatology in August 2017 (Zhang and Zuidema, 2021;Ryoo et al, 2021), was present on 31 August, 2017.…”

“…The 8/31/2016 profile, occurring further south (16.4 • S, 6.5 • E), sampled aerosol that was constrained near the coast (Fig. 2, top row) by an impinging mid-latitude disturbance (Ryoo et al, 2021), a meteorological condition that is common in September when the AEJ-S is less strong (Zhang and Zuidema, 2021). The associated synoptically-driven ascent mixes moisture upward, generating a linear increase in water vapor mixing ratio (Fig.…”

“…km at approximately 10 • S, known as the African Easterly Jet-South(Adebiyi and Zuidema, 2016;Pistone et al, 2021;Ryoo et al, 2021), advected an aerosol layer with the highest OA:BC mass ratio measured during the 2016 campaign (approximately…”

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

“…After showing that the overall realism of the ensemble with radiatively active BBA for aerosol and cloud distribution is significantly improved, we discuss the BBA radiative effects in accelerating the AEJ-S and in modifying the horizontal and vertical transport of BBA in comparison with the ensemble of radiatively inactive BBA simulations. Note that September 2017 has a slightly weaker AEJ-S than the climatological mean (Ryoo et al, 2021) and that the microphysical BBA-cloud interaction (the indirect effect of BBA) is not considered here.…”