Weak, low-level dry convection over Angola determines offshore stratocumulus cloud droplet number concentrations

Boundary-layer cloud interactions involving shortwave-absorbing aerosols remain one of the least understood aerosol influences on climate. Here, we find the highest stratocumulus cloud droplet number concentrations over the southeast Atlantic occur when agricultural fires coincide with synoptically weakened surface warming over Angola, occurring June-early August. Dry convection fills a shallow continental boundary layer with smoke, and a nighttime land breeze transports the aerosol into the marine boundary layer. Offshore aerosol transport is strengthened by low-level easterlies from a continental pressure high southeast of Angola. A stronger continental high ventilates more smoke westward off a cooler Angola. Simultaneously, the south Atlantic subtropical high is weaker, allowing extensive dispersal of aerosol offshore and obscuring marine cloud brightening from shipping. Meteorological co-variation at synoptic scales compensates for any cloud brightening by the smoke, increasing outgoing shortwave radiation by an additional 15%-20% of the monthly mean in June and July.