Weak, low-level dry convection over Angola determines biomass-burning aerosol entry into the marine boundary layer

Variability in cloud droplet number concentrations (N_d) within the large subtropical stratocumulus decks can strongly impact outgoing shortwave radiation. The southeast Atlantic subtropical stratocumulus deck is particularly prone to elevated N_d, attributed to continental African fire emissions.  The highest stratocumulus N_d occur when Angolan agricultural fires coincide with weak surface warming during the austral winter months (June-early August). Dry convection fills a shallow continental boundary layer with smoke and a nighttime land breeze advects the aerosol into or slightly above the marine boundary layer. The offshore transport is strengthened by low-level easterlies from a continental high to the southeast of Angola that is stronger when the Angolan land is cooler. Simultaneously, the south Atlantic subtropical high (SASH) is weaker when Angolan land warming is more muted, allowing the biomass-burning aerosol to also disperse further south. The shortwave-absorbing aerosol can either reach the remote boundary layer by direct low-lying easterly transport, or through entrainment over longer time scales after being transported south. While the weak Angolan land heating in June-July correlates with higher offshore N_d, these coincide with lower cloud fractions and thinner clouds, primarily because the SASH is also weaker. This meteorological co-variation fully compensates for any aerosol brightening of the cloud deck. Marine cloud brightening by emissions from a southeast Atlantic shipping lane is more evident when Angolan land heating is stronger, coinciding with a stronger SASH, as the background N_d is less and the background cloud fraction is higher. Most of the year-to-year variability from 2003 to 2023 in the June-July marine shortwave cloud radiative effect can be constrained using the surface-level temperature over Angola (r2 = 0.4). While Angolan land has warmed slightly in June-July since 1980 in reanalysis, no trend is evident in synoptic variations of warmer versus cooler heating. Fire emissions have slightly increased since 2003. A continuing warming trend would deepen the continental boundary layer, and could place more of the transported smoke above the marine boundary layer, stabilizing the lower atmosphere through shortwave absorption.