The role of the southern African easterly jet in modifying the southeast Atlantic aerosol and cloud environments

The westward transport of biomass‐burning (BB) aerosols by mid‐tropospheric winds over the southeast Atlantic stratocumulus deck has long been recognized, but the coupling to the large‐scale circulation has yet to be investigated fully. This goal is furthered here using satellite observations and reanalysis datasets spanning 2001–2012, as well as 10 day forward trajectory calculations of satellite‐detected smoke emissions. The results highlight the important role of a mid‐tropospheric zonal wind maximum, the Southern African Easterly Jet (AEJ‐S), in transporting BB aerosol west off the African continent. The AEJ‐S, defined through daily‐mean 600 hPa easterly wind speeds exceeding 6 m s −1 between 5°S and 15°S and centred zonally on the coastline, is most pronounced during September–October. The AEJ‐S is part of a meridional circulation that is diabatically forced by the temperature–moisture gradient between the southern hot–dry and northern cool–moist convective structures over land. 45% of 24 264 total identified smoke trajectories exit the continent to its west between 5°S and 15°S. These thereafter follow three major pathways: northwestward (8%), directly westward (55%) and anticyclonically recirculated (37%). The AEJ‐S induces an upward motion directly below the jet that enhances prevailing updraughts over land, lifting emissions and transporting aerosols more efficiently over the southeast Atlantic. Offshore, the prevailing large‐scale mean subsidence is reduced, with an associated increase in the nearby cloud‐top heights and reduction in the nearby marine low‐level cloud fraction. Further from the jet, increased warm continental temperature advection at 800 hPa associated with the strengthened land‐based anticyclone decreases mean low‐level cloud heights.