Tropical and Extratropical Circulation Effects on the Extent of Biomass Burning Aerosol-Cloud Interactions in the Southeast Atlantic

Aerosol-cloud interactions (ACI) represent one of the largest sources of uncertainty in estimates of changes in Earth’s radiative balance. Although the mechanisms by which increasing cloud condensation nuclei (CCN) leads to increases in liquid cloud droplet number concentration (N_d) are well-studied, the magnitude of the effective radiative forcing from ACI remains poorly constrained because of regime-dependent rapid cloud responses, irreducible uncertainty in preindustrial CCN concentrations, and limited knowledge of the horizontal and vertical distribution of aerosol over the global oceans. This study aims to improve our understanding of the latter by investigating the linkage between tropical and extratropical circulation patterns and ACI over the southeast Atlantic Ocean (SEA) during the southern African biomass burning season.

The SEA region has recently experienced an intensive period of observation by land and air, including the DOE/ARM Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign on Ascension Island from June 2016 to October 2017 and the NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) aircraft campaign based out of Walvis Bay, Namibia, in September 2016 and São Tomé and Príncipe in August 2017 and October 2018. Early results from the ORACLES-2016 deployment have demonstrated that the observational challenge in assessing ACI may be greater than previously recognized because the accumulation of entrained above-cloud aerosol over the course of days often matters more than instantaneous smoke-cloud contact in determining MBL properties. A corollary of this finding is that the impact of smoke above the MBL in the southernmost reaches of the SEA will be manifested upstream days later due to the prevailing southerly MBL flow.

The passage of midlatitude cyclones was observed to influence the location of the smoke plumes and cloud decks during ORACLES deployments. In addition, work done in anticipation of the LASIC and ORACLES campaigns has shown that the column loading and spatial pattern of aerosol over the SEA is influenced by the strength of the southern African Easterly Jet during the biomass burning season. We use reanalysis and satellite products to evaluate what systematic effects these and related circulation patterns have on warm cloud properties over the SEA, focusing on N_d, and how these effects vary geographically and seasonally.