Is the semi-direct effect for smoke overlying stratocumulus overrated? Circulation variability and radiative impacts reconsidered for the southeast Atlantic

Biomass burning aerosols seasonally overlie the major subtropical Southeast Atlantic stratocumulus deck. Previous modeling and observational studies have highlighted a semi-direct effect, whereby shortwave absorption by the aerosol warms and stabilizes the lower troposphere therefore thickening the cloud. We focus on the dynamical and moisture effects that may be convoluted with the semi-direct effect. Almost-daily radiosondes from the remote St. Helena Island (15.9^oS, 5.6^oW) from September-October, 2000 through 2011, are combined with regional averages of the MODIS clear-sky fine-mode aerosol optical depth (τ_af). Increases in τ_af are associated with increases in 750-500 hPa moisture content that are typically capped by sharply-defined temperature and moisture inversions. The additional mid-tropospheric moisture increases the diurnal-mean instantaneous shortwave heating, but this is fully compensated by the additional longwave cooling. The net cooling by moisture counteracts the layer warming from aerosol shortwave absorption, muting the semi-direct effect. The ERA-Interim Reanalysis is found to match radiosonde composites more realistically than either MERRA or NCEP Reanalyses. ERA-Interim spatial composites show that the polluted conditions are associated with a strengthening of a deep anticyclone over southern Africa, facilitating the westward and, offshore, southward transport of both smoke and moisture at 600 hPa, as well as warmer 800 hPa potential temperatures (θ₈₀₀). The shallower surface-based south Atlantic anticyclone exhibits a less pronounced shift to the northeast, strengthening the low-level coastal jet exiting into the stratocumulus deck and cooling 1000 hPa potential temperatures. Both dynamical changes reinforce the lower tropospheric stability (θ₈₀₀-θ₁₀₀₀), strengthening the stratocumulus deck. Thus, the dynamics encouraging smoke transport act in concert with the semi-direct effect to increase cloud fraction and thicken the clouds, while also decreasing cloud-top height.