Sunlight-absorbing aerosol amplifies the seasonal cycle in low cloud
fraction over the southeast Atlantic
Abstract
Many studies examining shortwave-absorbing aerosol-cloud interactions
over the southeast Atlantic apply a seasonal averaging. This disregards
a meteorology that raises the mean altitude of the smoke layer from July
to October. This study details the month-by-month changes in cloud
properties and the large-scale environment as a function of the
biomass-burning aerosol loading from July to October, based on
measurements from Ascension Island (8°S, 14.5°W), satellite retrievals
and reanalysis. In July and August, variability in the smoke loading
predominantly occurs in the boundary layer. During both months, the
low-cloud fraction is less and is increasingly cumuliform when more
smoke is present, with the exception of a late morning boundary layer
deepening that encourages a short-lived cloud development. September
marks a transition month during which mid-latitude disturbances can
intrude into the Atlantic subtropics, constraining the land-based
anticyclonic circulation transporting free-tropospheric aerosol to
closer to the coast, and resulting deeper, drier, and cooler boundary
layers with strongly reduced cloud cover near the main stratocumulus
deck. The October meteorology is more singularly dependent on the
strength of the free-tropospheric winds advecting aerosol offshore.
Low-level clouds increase and are more stratiform, when the smoke
loadings are higher. The increased cloud-top moisture and cloud droplet
number concentrations can help sustain a thinner stratiform cloud layer
through microphysical interactions. Overall the monthly changes in the
large-scale circulation and aerosol/moisture vertical structure act to
amplify the seasonal cycle in low-cloud amount and morphology, raising a
climate importance as cloudiness changes dominate the top-of-atmosphere
radiation budget.