Elucidating Boundary-Layer Aerosol-Cloud Interactions in the Southeast Atlantic.

The southeast Atlantic is home to one of the world’s largest subtropical stratocumulus decks, with the lower free troposphere often polluted by biomass burning aerosol (smoke) emanating from continental Africa. The vertical and horizontal distribution of biomass-burning aerosol impacts the cloud properties both radiatively and microphysically, altering the region's radiation fields and atmospheric circulations. Identification of the transport pathways by which smoke enters the boundary layer has remained elusive. Recent in-situ measurements in the remote Atlantic (8S, 14.5W) indicate smoke is often present in the marine boundary layer from June through August, with strong synoptic variations. These are well represented within the ECMWF CAMS reanalysis. Here, we use CAMS and ERA5 to compute empirical orthogonal functions and HYSPLIT back trajectories during 2003-2020 to illuminate the changes in smoke transport mechanisms from June to November. We find smoke transport can be influenced by the west African monsoon in June, capable of encouraging some significantly increased and expanded southern African easterly jet wind speeds. Midlatitude waves can weaken and displace the southern Atlantic subtropical high to allow for more direct, if slow, westward transport of aerosol at lower altitudes close to and/or within the boundary layer top, most pronounced in June and July when the boundary layer is also deeper. In August, circulation modulations can establish a local, anomalous anticyclonic circulation centered on 15S, 0W that further supports local subsidence, generating a local maximum in biomass-burning boundary layer aerosol in the middle of the southern tropical Atlantic.