Particle size traces modern Saharan dust transport and deposition across the equatorial North Atlantic

Michèlle Van Der Does,Geert-Jan A Brummer,Laura F Korte,Jan-Berend W Stuut,Chris I Munday

doi:10.5194/acp-16-13697-2016

Abstract

Abstract. Mineral dust has a large impact on regional and global climate, depending on its particle size. Especially in the Atlantic Ocean downwind of the Sahara, the largest dust source on earth, the effects can be substantial but are poorly understood. This study focuses on seasonal and spatial variations in particle size of Saharan dust deposition across the Atlantic Ocean, using an array of submarine sediment traps moored along a transect at 12° N. We show that the particle size decreases downwind with increased distance from the Saharan source, due to higher gravitational settling velocities of coarse particles in the atmosphere. Modal grain sizes vary between 4 and 32 µm throughout the different seasons and at five locations along the transect. This is much coarser than previously suggested and incorporated into climate models. In addition, seasonal changes are prominent, with coarser dust in summer and finer dust in winter and spring. Such seasonal changes are caused by transport at higher altitudes and at greater wind velocities during summer than in winter. Also, the latitudinal migration of the dust cloud, associated with the Intertropical Convergence Zone, causes seasonal differences in deposition as the summer dust cloud is located more to the north and more directly above the sampled transect. Furthermore, increased precipitation and more frequent dust storms in summer coincide with coarser dust deposition. Our findings contribute to understanding Saharan dust transport and deposition relevant for the interpretation of sedimentary records for climate reconstructions, as well as for global and regional models for improved prediction of future climate.

Highlights

Millions of tons of mineral dust are transported from the African continent towards the Atlantic Ocean every year, with several direct and indirect effects on global climate
Such coarse particles are generally not incorporated into climate models (Kok, 2011).The underestimation of the coarse size fraction in climate models may have its origin in the sampling of dust of specific size classes, e.g., PM10 and PM2.5, which form the basis of the guidelines from the World Health Organization (WHO, 2006) on fine-grained particles
We have shown seasonal and spatial changes in Saharan mineral dust transport and deposition across the Atlantic Ocean by means of sediment-trap sampling between October 2012 and November 2013, and spatial changes in the seafloor sediments at the same stations

Summary

Introduction

Millions of tons of mineral dust are transported from the African continent towards the Atlantic Ocean every year, with several direct and indirect effects on global climate. Atmospheric mineral dust affects the atmosphere’s radiation budget by scattering and absorbing incoming and reflected solar radiation, and changes cloud properties by acting as cloud condensation nuclei (Goudie and Middleton, 2001; Highwood and Ryder, 2014; Shao et al, 2011; Wilcox et al, 2010). Climatic effects are largely determined by particle characteristics, including particle size, particle shape, chemical and mineralogical composition, and cloud cover and the albedo of the underlying surface (Claquin et al, 2003; Goudie and Middleton, 2001, 2006; Highwood and Ryder, 2014; Otto et al, 2007; Shao et al, 2011; Sokolik and Toon, 1999).

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