The Great Western Ergs dust sources, in the lee of the Atlas Mountains, are most active during the boreal winter and spring, which are the main active seasons of Sharav cyclones and resulting strong surface winds (Alpert & Ziv, 1989) High DAOD does not always occur year-round over the dust source regions ( Figure 3), which is supported by MERRA-2 estimates ( Figure S1).

the central Sahel, the seasonal cycle of dust activation is almost perfectly reconstructed by the seasonal cycle of high wind speed frequency and precipitation, with an adjusted R 2 of 0.93, based on multiple linear regression using high wind speed frequency and precipitation as predictors for the seasonal cycle of dust activation frequency ( Figure S2).

In addition, the occurrence of nocturnal low-level jets, the major driver of dust activation across the central Sahel, is substantially inhibited by the intrusion of the intertropical discontinuity (ITD, the convergence zone of the trade winds) and associated weak geostrophic winds during the boreal summer ( Schepanski et al. These rains produce relatively abundant soil moisture and vegetation cover during the monsoon and post monsoon seasons limiting the mobility of dust on the ground ( Yu et al., 2017). The West African monsoon onset typically occurs in late June in conjunction with an abrupt northward push of the Intertropical Convergence Zone (ITCZ) (Sultan & Janicot, 2003), bringing about 70% and 50% of the annual rainfall to the Sahel and West African monsoon region, respectively, during July to September.

Across West Africa and the central Sahel, including the Bodélé Depression, dust activation occurs most frequently during the climatologically dry boreal winter and spring seasons (December-May), rather than during the monsoon and post-monsoon wet seasons as suggested by Ginoux et al. seasonal distribution of dust activation frequency across North Africa and the Middle East is largely driven by the climatologies of wind and precipitation ( Figure 3 and Figure S2).

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