Dust Source Activation Frequency in the Horn of Africa

Mineral dust aerosols play an important role in Earth's climate through interactions with incoming solar radiation, clouds, and the atmosphere. However, dust sources in the Horn of Africa (HoA) and controls on their activation are poorly documented. Here, we use fifteen-minute Meteosat Second Generation Spinning Enhanced Visible and Infrared Imager dust index images to identify HoA dust source areas and to quantify their activation frequency in 1 degrees x 1 degrees resolution from 2006 to 2010. Around half of all recorded dust events occur in boreal summer, mostly between 8:00 and 16:00 local time. They are driven by meso- to regional scale meteorological mechanisms including the breakdown of the nocturnal low-level jets, land-sea breezes, and haboobs. By far the most dust-active region in the HoA is the Afar Triangle (>77% of all recorded dust events) which features the Afar and Danakil depressions and is fed by the Awash River. Despite experiencing strong and persistent southwest summer monsoon winds, dust activation on the Somali Peninsula is less significant. A composite of our map with data for North Africa and westernmost Asia shows that the HoA is a striking latitudinal anomaly with dust activation extending deep into the equatorial belt. Our data also reveal that dust activation is unusually seasonal with similar to 40% of events occurring in June and July. Our findings show that aridity and mean wind strength alone are poor predictors of dust activation and underscore the strong control exerted by the availability of readily deflated unconsolidated riverine and lacustrine sediments. Plain Language Summary Mineral dust aerosols play an important role in Earth's climate through interactions with incoming solar radiation, clouds, and processes in the atmosphere. North Africa is the biggest contributor of mineral dust to global atmospheric loading and dust activation is relatively well understood. However, the dust activation in the Horn of Africa (HoA) is currently poorly documented. We use high-resolution remote sensing imagery to identify main dust hot spots in this region over 4 years. Dust activation primarily occurs throughout the day during boreal summer and is driven by medium to regional scale meteorological mechanisms. By far the most dust-active region is the Afar Triangle, which features the lower Awash River system and the Afar and Danakil depressions. Dust activation within the Somali Peninsula is less significant. A composite of our data together with data for North Africa and westernmost Asia reveals a striking latitudinal anomaly. Modern day dust activation in the northern hemisphere is typically confined to latitudes north of similar to 13 degrees N but in the HoA, it extends deep into the equatorial belt. Our findings show that aridity and wind strength are poor predictors of dust activation, highlighting the strong control exerted by the availability of dried surface sediments.