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. Westward-moving 600 hPa winds at the northern edge of a land-based anticyclone become the southern African easterly jet (AEJ-S, blue contours, 6-10 m/s) in September-October. 10-day smoke trajectories (red to yellow indicating age), for September of 2007, visualize shortwave-absorbing aerosol transport from satellite-detected fire emissions (fire-counts in maroon) far offshore, over the southeast Atlantic stratocumulus deck (greyscale, cloud fractions of 0.5 to 1.0). We further examine the impact of the AEJ-S's secondary circulation on the stratocumulus clouds and aerosol distribution.