Tropical Atlantic dust and the zonal circulation
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ORIGINAL PAPER
Tropical Atlantic dust and the zonal circulation Mark R. Jury 1,2
&
Angelie T. Nieves Jiménez 2
Received: 26 April 2020 / Accepted: 9 November 2020 # Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract This study examines the factors driving the variability of Saharan dust over the subtropical Atlantic during summer. Monthly tropospheric dust concentrations from satellite-based model assimilation averaged in the area 10–23 N, 30–65 W, are used to create an index over the period 1980–2017. The seasonal cycle peaks in summer and affects tropical cyclones from July to September. Point-to-field regression analysis is performed, and then composites of dusty and clean seasons are analyzed as maps and sections of meteorological fields. The regression reveals significant differences in oceanic trade winds, (detrended) sea surface temperature, and atmospheric convection. Dusty minus clean composites show a zonal overturning circulation with sinking/rising motions over Africa/western Caribbean and lower easterlies/upper westerlies over the subtropical Atlantic. This circulation spreads Saharan dust and warm dry air in the 1–4 km layer westward at ~ 10 m/s over cooler waters, inhibiting convection. Among the factors driving tropospheric dust variability is the Pacific El Niño Southern Oscillation. Composites of dusty and clean hurricane occurrence show a threefold difference in numbers and a tenfold difference in mean power dissipation index. Analysis of a dust event in August 2009 illustrates how wind shear and cool SST conspire to suppress tropical easterly waves. Supplementary work characterizes the dispersion of a dense Saharan dust plume in June 2020.
1 Introduction Every summer (Jun–Aug) dust plumes from the Sahara Desert are transported across the Atlantic Ocean by the prevailing winds (Engelstaedter and Washington 2007; Jury 2018). Annual dust emissions vary from 400 to 2200 Tg yr−1 (Huneeus et al. 2011) and have characteristic particle sizes < 50 μm (Van der Does et al. 2016). The dust forms a layer 1–4 km deep over the northeast Atlantic and displays multiyear variability (Prospero and Mayol-Bracero 2013). The Sahara Desert is the largest mineral dust source providing about 50% of global emissions (Prospero et al. 2002). Dust plumes often precede or follow African easterly waves (AEW) that pass across the Sahel from July to September (Jones et al. 2003; Barkan et al. 2004; Washington and Todd 2005; Knippertz and Todd 2010; Huang et al. 2010). The dustladen air is transported into the Atlantic by anticyclonic eddies within low level easterly winds (Karyampudi and Carlson 1988; Klose et al. 2010), passing over a cool marine layer * Mark R. Jury [email protected] 1
University of Zululand, Kwadlangezwa 3886, South Africa
2
University of Puerto Rico Mayagüez, Mayagüez, PR 00681, USA
capped by an inversion near 2 km elevation. Cuesta et al. (2009) describe the processes involved in the dispersion of dust plumes over west Africa: diurnal mixing, topographic effects, and transient wi
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