Formation of Summertime Extreme Trans-Atlantic Dust Events

North Africa is the world’s greatest dust source, emitting huge quantities of mineral dust throughout the year. Each summer, the hot, dry, and dusty Saharan air layers (SALs) often propagate westward, transporting African dust across the tropical North Atlantic to the Caribbean Basin and the U.S. Through the complex interactions with radiation, cloud, and geochemistry, African dust aerosols affect climate, weather, and ecosystem, along its westward transport pathways. Extreme trans-Atlantic African dust events not only generate high loading of dust over the tropical North Atlantic but also can degrade air quality over the Caribbean Basin and southern U.S. Despite the adverse impacts of African dust, causes of summertime extreme trans-Atlantic dust events are not fully understood. While the recent “Godzilla” trans-Atlantic dust storm in June 2020 was related to enhanced dust emissions in source regions, it is unclear if extremely high dust emissions are a prerequisite for the formation of extreme trans-Atlantic dust events. In this work, we use case studies and composite analysis to explore this question. While moderately high emissions in June 2015 resulted in the second strongest trans-Atlantic dust event in summer during 2003–2022, the example in 2008 shows that extremely high dust loading over the tropical North Atlantic was formed without extremely high emissions over source regions. The other two examples in 2017 and 2022 show that, on the other hand, extreme emissions over land do not necessarily result in high dust loading over the tropical North Atlantic. The composite analysis further reveals that anomalous easterly flows between 925 and 600 hPa from the west coast of Africa to the Caribbean Basin are essential for the formation of extreme trans-Atlantic dust events. While the enhanced near-surface winds over western Algeria and Mauritania indicate increased dust emissions, they are overall weaker than the winds in the composite of extreme dust emission events, suggesting that high emissions are not always necessary for the formation of extreme trans-Atlantic African dust events. Our findings highlight the importance of circulation patterns over the tropical North Atlantic in supporting long-range dust transport and generating dust extremes.