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One component of aerosol–cloud interactions (ACI) involves dust and marinestratocumulus clouds (MSc). Few observational studies have focused ondust–MSc interactions, and thus this effect remains poorly quantified. Weuse observations from multiple sensors in the NASA A-Train satelliteconstellation from 2004 to 2012 to obtain estimates of the aerosol–cloudradiative effect, including its uncertainty, of dust aerosol influencingAtlantic MSc off the coast of northern Africa between 45° W and15° E and between 0 and 35° N. To calculate theaerosol–cloud radiative effect, we use two methods following Quaas et al. (2008)(Method 1) and Chen et al. (2014) (Method 2). These two methods yield similarresults of −1.5 ± 1.4 and −1.5 ± 1.6 W m−2,respectively, for the annual mean aerosol–cloud radiative effect. Thus,Saharan dust modifies MSc in a way that acts to cool the planet. There is astrong seasonal variation, with the aerosol–cloud radiative effect switchingfrom significantly negative during the boreal summer to weakly positiveduring boreal winter. Method 1 (Method 2) yields −3.8 ± 2.5(−4.3 ± 4.1) during summer and 1 ± 2.9(0.6 ± 1) W m−2 during winter. In Method 1, the aerosol–cloudradiative effect can be decomposed into two terms, one representing the firstaerosol indirect effect and the second representing the combination of thesecond aerosol indirect effect and the semidirect effect (i.e., changes inliquid water path and cloud fraction in response to changes in absorbingaerosols and local heating). The first aerosol indirect effect is relativelysmall, varying from −0.7 ± 0.6 in summer to0.1 ± 0.5 W m−2 in winter. The second term, however, dominatesthe overall radiative effect, varying from −3.2 ± 2.5 in summer to0.9 ± 2.9 W m−2 during winter. Studies show that the semidirecteffect can result in a negative (i.e., absorbing aerosol lies above lowclouds like MSc) or positive (i.e., absorbing aerosol lies within low clouds)aerosol–cloud radiative effect. The semipermanent MSc are low and confinedwithin the boundary layer. CALIPSO shows that 61.8 ± 12.6 % ofSaharan dust resides above North Atlantic MSc during summer for our studyarea. This is consistent with a relatively weak first aerosol indirect effectand also suggests the second aerosol indirect effect plus semidirect effect(the second term in Method 1) is dominated by the semidirect effect. Incontrast, the percentage of Saharan dust above North Atlantic MSc in winteris 11.9 ± 10.9 %, which is much lower than in summer. CALIPSO alsoshows that 88.3 ± 8.5 % of dust resides below 2.2 km thewinter average of MSc top height. During summer, however, there are twopeaks, with 35.6 ± 13 % below 1.9 km (summer average of MSc topheight) and 44.4 ± 9.2 % between 2 and 4 km. Because theaerosol–cloud radiative effect is positive during winter, and is alsodominated by the second term, this again supports the importance of thesemidirect effect. We conclude that Saharan dust–MSc interactions off thecoast of northern Africa are likely dominated by the semidirect effect

Impact of Saharan dust on North Atlantic marine stratocumulus clouds: importance of the semidirect effect