A meteorological overview of the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) campaign over the southeast Atlantic during 2016–2018
Authors: Ryoo Ju-Mee , Pfister Leonhard , Ueyama Rei , Zuidema Paquita , Wood Robert , Chang Ian , Redemann Jens 
Published: April 2021

Abstract. In 2016–2018, the ObsErvation of Aerosols above CLouds and their intEractionS (ORACLES) project undertook three major field campaigns in the Southeast (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 particular, ORACLES was designed to investigate how BB aerosols interact with oceanic stratocumulus clouds, and how that interaction affects the radiation budget. Here, a meteorological analysis has been performed to support the interpretation of the airborne measurements for aerosol transport and its interaction with clouds during the deployments of September 2016, August 2017, and October 2018.The southern African easterly jet (AEJ-S), represented by the zonal wind at 600–700 hPa < −6 m s−1 with entrance over land and exit over the ocean around 5–15° S, is a dominant feature of the mid-level circulation over western Africa during austral winter and spring (August, September, and October). AEJ-S develops at lower altitudes (~3 km, 700 hPa) in the north (5–10° S) in August, while it develops at around 4 km (~600 hPa) in the south (5–15° S) in September and October, largely driven by the strong sensible heating over the African plateau. AEJ-S advects air plumes that are both aerosol-laden and moist, implying a clear potential impact by both on SE Atlantic stratocumulus. Benguela low-level jet (LLJ) also develops off the Namibian coast in the SE Atlantic for all three months. Low-level cloud fraction (low-CF) is positively associated with low-level tropospheric stability (LTS) and negatively associated with boundary layer height (BLH). This relationship is especially strong in September and weaker in August and October. Correlation analysis indicates that simple relationships among low-CF, LTS, and BLH break up when rapidly varying large-scale flow and mid-latitude frontal systems intrude.There are some notable meteorological anomalous characteristics for the three deployment months compared to the climatology: 1) During August 2017, the AEJ-S is slightly weaker than the climatological mean with an anomalous upper-level jet aloft (~6 km) around 10° S. The AEJ-S strength and moisture transport offshore increase at the end of August 2017. August 2017 is also drier over the SE Atlantic than climatology, with a stronger (by ~2 m s−1) LLJ. The large-scale anticyclone associated with St. Helena high in the SE Atlantic is stronger and closer to the coast than the climatological mean. 2) During September 2016, the AEJ-S intensity is slightly weaker than the climatological mean and the mid-level RH is slightly higher than the climatological mean, although differences are small. The LLJ is stronger (~1 m s−1) than the climatological mean. The large-scale anticyclone associated with St. Helena high in the SE Atlantic is stronger than the climatological mean. 3) During October 2018, the AEJ-S is slightly weaker than the climatological mean and slightly wetter compared to the climatological mean. LLJ is also weaker compared to the climatological mean. The large-scale anticyclone associated with St. Helena high in the SE Atlantic is slightly weaker and further southeast than the climatological mean. Precipitation regions migrate southward from August through October due to seasonal change. During all the deployment years, the sea surface temperatures (SST) over the SE Atlantic are warmer than the climatological means, although its impact on low-CF over the deployment region remains unclear at the daily to synoptic time scale.