Amplified Seasonal Cycle in Low-cloud Fraction over the Remote Southeast Atlantic when Shortwave-absorbing Aerosols are Present

Abstract:

Marine boundary layer clouds over the remote southeast Atlantic interact with biomass burning aerosols, with significant but still uncertain climate impact. Previous work using ARM observations from Ascension Island (8S, 14.5W) documented an altered diurnal cycle during August of the low-cloud when smoke loadings were elevated in the boundary layer (Zhang and Zuidema, 2019, ACP). The current study characterizes the seasonal evolution in the low-cloud response to the absorbing aerosols, as it shifts from predominantly in the boundary layer in July to predominately in the free-troposphere in September and October. An amplified seasonal cycle in the low-cloud cover and morphology is observed when more absorbing aerosol is present. The low-cloud fraction reduces in July-August, with an increase in the occurrence of surface-forced cumuliform clouds and a decrease in the top-driven stratiform cloud occurrence, whereas the opposite is observed in October. During September, the marine boundary layer is shallower and more humid despite a decrease in the free-tropospheric subsidence when more smoke is present. The former is consistent with reduced boundary layer winds and surface fluxes, suggesting a connection to the larger scale atmospheric circulation, confirmed by an anomalous cyclonic circulation within the boundary layer using ERA5 reanalysis data. We also find that as the smoky free-tropospheric air advects off of the continent in September, it becomes more well-mixed, thought to be aided by longwave cooling from the associated water vapor along with the shortwave aerosol heating. Despite this, the cloud top inversion strength does not increase at Ascension under smokier conditions. The more humid boundary layer supports more frequent cumuliform clouds and enhanced afternoon precipitation. Although the near-surface smoke loading is slightly elevated, the satellite-derived cloud droplet number concentrations decrease slightly, consistent with more precipitation. The implications for the stratocumulus-to-cumulus transition are being explored.