Unraveling the Mystery of Extreme trans-Atlantic African Dust Events: Insights from Case Studies

North Africa is the world’s largest source of dust. Each summer, the Saharan Air Layer transports massive amounts of mineral dust across the Atlantic Ocean, affecting weather, climate, ecosystem, and public health across large areas. While considerable attention has been devoted to extreme events like the "Godzilla" dust storm of 2020, gaps remain in our understanding of the genesis and consequences of other extreme trans-Atlantic African dust events. This research aims to reduce these gaps through a detailed examination of several key events between 2003 and 2022, specifically focusing on the June 2015 event—the second strongest over the tropical North Atlantic during the studied period—as well as contrasting cases from 2008, 2017, and 2022.We found that the June 2015 event was characterized by moderately high dust emissions over western North Africa in a relatively short period and an extremely high AOD over the tropical North Atlantic for at least three days. The high dust loading over the tropical North Atlantic is associated with atmospheric circulation extremes that are similar to the June 2020 “Godzilla” dust event. Both the African easterly jet (AEJ) and Caribbean low-level jet (CLLJ) are greatly intensified, along with a westward extension of the North Atlantic subtropical high (NASH). Over western North Africa, the enhanced dust emissions are related to anomalously strong surface winds, reduced vegetation density, and decreased soil moisture over the northern Sahel.The dust plume reduced surface net shortwave radiation over the eastern tropical North Atlantic by about 25 W m-2 but increased net longwave flux by about 3 W m-2, where regional mean aerosol optical depth (AOD) maximized around 1.4. The magnitude of surface radiative effects is largely proportional to AOD magnitude and reduces towards the Caribbean Sea. In contrast to the 2020 event, the 2015 event does not appear to have had a strong adverse impact on air quality in the U.S., with only a few sites over the southern U.S. slightly exceeding the daily PM2.5 guideline of 25 µg m-3 by the World Health Organization. This lower impact on air quality is associated with an extremely intensified CLLJ (the strongest in June over 1979-2022) that dispersed the dust plume further over Mexico and the southeastern Pacific.The case studies of 2008, 2015, 2017, and 2022 reveal that high dust loadings over the tropical North Atlantic can occur in the absence of exceptionally high emissions over source regions. This challenges the notion that intense emissions are a necessary precondition for extreme trans-Atlantic dust events. 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.

Grantor dc:publisher

University of Kansas

Year dc:date.issued

2023

Author dc:creator

• Harr, Brian

Advisor dc:contributor.advisor

• Pu, Bing

• Atmospheric sciences
• Dust Aerosol
• Extreme
• Saharan Dust event
• trans-Atlantic dust

Statement dc:rights

• Copyright held by the author.

Language dc:language.iso

en

Dc Identifier Other

http://dissertations.umi.com/ku:19250

OAI identifier oai:identifier

oai:kuscholarworks.ku.edu:1808/38729