The Atmospheric Drivers of the Major Saharan Dust Storm in June 2020

Abstract

This study investigates the atmospheric dynamics of the major dust storm that occurred in June 2020 over the Sahara and during which dust clouds associated with the highest-on-record aerosol optical depths were transported towards the America. An anomalous atmospheric circulation pattern in the mid-latitudes, linked to a circum-global wavetrain, resulted in the development of a subtropical high-pressure system to the west of the Saharan Heat Low. This created a pressure dipole and generated anomalously strong northeasterlies over the Sahara, which caused continuous dust emissions over 4 days. Occurring along the northern fringes of the Intertropical Discontinuity, the dust was transported to higher altitudes (6 km) by the strong updraft in this region. This injected the dust at the African Easterly Jet (AEJ) altitudes and favored a rapid westward long-range transport. The AEJ was also anomalously strong, being strengthened by the anticyclonic circulation associated with the anomalous high. Plain Language Summary Dust is an important constituent of the Earth's atmosphere, with a wide range of impacts ranging from human health to effects on climate. In June 2020, massive amounts of dust were lifted from the Sahara, the major dust source region in the world, and transported all the way into the Americas across the tropical Atlantic Ocean. This event was caused by the development of a subtropical high-pressure system over northwest Africa which resulted in sustained strong northeasterlies over the Sahara generating continuous dust emissions for 4 days. Due to the strong low-level convergence along the intertropical discontinuity region, the dust was lifted to roughly 5-6 km above the surface, and then transported westward by the stronger mid-atmospheric winds (>20 m s-1). At Cape Verde and over large swaths of the Atlantic Ocean, the amount of dust suspended in the atmosphere was associated with the largest aerosol optical depths on record.

Observation and modeling of a historic African dust intrusion into the Caribbean Basin and the southern U.S. in June 2020

“…When the NASH became weaker and wandered back to south, the dust outflow region was dominated by the African Easterly Jet (AEJ), which carried the accumulated dust plume rapidly and maintaining its high concentrations from the coastal region toward the Caribbean Basin within four days, resulting in the extraordinary dust loading observed. Our results do not fully agree with what Francis et al (2020) found on the atmospheric drivers of the dust storm. For example, they argued that the development of a subtropical high off the coast of West Africa generated anomalously strong northeasterlies over four days and high dust loading in the eastern tropical Atlantic Ocean.…”

“…We also found that unique synoptic setting associated with anomalous NASH strength and position created the closed atmospheric circulations over West Africa and its adjacent coastal ocean for several days, which trapped the continuously emitted dust in the African coast. In addition, Francis et al (2020) found that the AEJ was much strengthened by the anticyclonic circulation associated with the anomalous sub-tropical high, which favored a rapid westward transport of dust toward the Americas. However, our estimated trans-Atlantic transport speed of 1000 km d -1 is more or less the same as the speed for the summertime dust events during 2003(Huang et al, 2010, suggesting that the strong AEJ in June 2020 was unlikely to be a major factor for the highest-in-record dust detected in the Caribbean Basin.…”

“…Our analysis of the SEVIRI dust images showed that intense haboobs swept through Niger-Mali-Mauritania corridor (south of 20 º N generally) and contributed significantly to the dust event. The dust emissions associated with these haboobs cannot be adequately explained by the large-scale meteorology used in Francis et al (2020), because the reanalysis cannot capture such strong winds accurately (Cowie et al, 2015;Roberts et al, 2017) and their focused dust source region is largely outside the corridor of the intense haboobs identified in the SEVIRI images. We also found that unique synoptic setting associated with anomalous NASH strength and position created the closed atmospheric circulations over West Africa and its adjacent coastal ocean for several days, which trapped the continuously emitted dust in the African coast.…”

“…The dust over the Caribbean Basin during this period has attracted considerable interests from scientific community and media because of its huge extent and massive amount, so-called the "Godzilla" dust plume (https://phys.org/news/2020-06-sahara-blanketscaribbean-air-quality.html), and "a dust plume to remember" (https://earthobservatory.nasa.gov/images/146913/a-dust-plumeto-remember) for its extraordinary characteristics. Francis et al (2020) examined the atmospheric circulation characteristics that drove the formation and transport of this dust storm. In this study, we will use a variety of remote sensing and in situ observations and simulations with the NASA Goddard Earth Observing System (GEOS) model to characterize the gigantic dust plume and assess its impact on the air quality in the southern U.S.…”

Less atmospheric radiative heating due to aspherical dust with coarser size

“…The giant dust is likely important for the radiative effects, as well as for biogeochemical effects in the North Atlantic (Pabortsava et al, 2017;van der Does et al, 2018). In such an extreme case as the "Godzilla" dust storm over North Africa in June 2020 (Francis et al, 2020), the dust loading could be larger than that examined for this study, and our estimates of the warming effects might be conservative during such events.…”