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

Smoke produced by seasonal biomass burning in the southwestern African savannah can be advected westward over the Atlantic Ocean, where it mostly overlies a major planetary stratocumulus deck. Shortwave absorption by the smoke warms the atmosphere, stabilizing it, thereby reducing cloud-top entrainment and encouraging cloud thickening (the semi-direct effect). Associated dynamical and moisture effects that may be convoluted with the semi-direct effect have received less attention, and are examined here. Radiosondes from the remote St. Helena Island (15.9^oS, 5.6^oW) are combined with MODIS fine-mode aerosol optical depth (AOD_f), and composited into pristine and polluted days for the September-October months. Increases in AOD_f are associated with increases in 750-500 hPa moisture content that produce a diurnal-mean shortwave heating rate of ~ 0.2 K day^-1 from the moisture alone. The polluted conditions are associated with a stronger mid-level anticyclone over southern Africa, facilitating the westward and (offshore) southward transport of both smoke and moisture. The shallower surface-based south Atlantic anticyclone shifts to east, strengthening the low-level coastal jet exiting into the stratocumulus deck, and enhancing warm temperature advection above the main stratocumulus deck. This increases the lower tropospheric stability (θ₈₀₀-θ₁₀₀₀) and enhances the surface fluxes, strengthening the stratocumulus deck. Thus, the dynamics encouraging smoke transport and the additional shortwave absorption by moisture act in concert with the semi-direct effect to increase the cloud fraction and thicken the clouds. The uncertainties inherent to the realistic modeling of the smoke, cloud, their interactions and their climatic effects motivate a field deployment, ONFIRE, proposed to the southeast Atlantic in 2016, that will also be discussed.