Boundary Layer Clouds and Absorbing Aerosols in the Southeast Atlantic

Ascension Island, a remote island located in the middle of the Atlantic Ocean within the trade-wind region oat 8S, 14.5W, experiences the outflow of biomass-burning aerosols from continental Africa, over 2000 km away, from July through November, peaking in August and September. The shortwave-absorbing free-tropospheric aerosols, located in a region of high solar irradiance, provide a strong warming influence on the regional climate that is poorly represented in global aerosol climate models. The low clouds can respond to the smoke layer in myriad possible ways that are not yet well-documented. The warming through shortwave absorption can stabilize the free-troposphere, enhancing the low cloud fraction. Our analysis also highlights that the transport of the smoke over the southeast Atlantic is accompanied with moisture that alters the radiation field the low clouds respond to. The deepening boundary layer and subsiding smoke layer also increase the likelihood of aerosol-cloud microphysical interactions as compared to further south over the main stratocumulus deck. Interest in this climate regime is supporting an observational strategy of a year-long DOE ARM Mobile Facility deployment to Ascension (Layered Atlantic Smoke Interactions with Clouds, or LASIC), and an NSF aircraft campaign (ObservatioNs of Fireʼs Impact on the southeast atlantic REgion, or ONFIRE) based upwind on Sao Tome Island. These campaigns will be integrated with other NASA, UK and African activities with similar goals based further south in Namibia.

The forward trajectories of emissions from over 24,000 fire sources on continental Africa show that a free-tropospheric jet can advect aerosols to above Ascension island in just one-two days. The fast transport time appears to allow the retention of signatures of the fire sources, in particular the radiatively-crucial single-scattering albedo value, suggested by AERONET sun photometer data. Thereafter, a deep land-based anticyclonic high encourages the recirculation of over one-third of these trajectories back to the African continent, explaining the widespread extent of the aerosol layer over the Atlantic. The free-tropospheric jet also affects the vertical distribution of both the aerosol and underlying cloud layer by affecting the larger-scale subsidence field: a stronger jet reduces the mean atmospheric subsidence independently of shortwave absorption by the aerosols. Modeling studies are needed to better distinguish the low cloud response to the aerosol effects from the dynamical effects.