| Summary: | Mineral dust has a large impact on regional andglobal climate, depending on its particle size. Especially inthe Atlantic Ocean downwind of the Sahara, the largest dustsource on earth, the effects can be substantial but are poorlyunderstood. This study focuses on seasonal and spatial variationsin particle size of Saharan dust deposition across theAtlantic Ocean, using an array of submarine sediment trapsmoored along a transect at 12? N. We show that the particlesize decreases downwind with increased distance from theSaharan source, due to higher gravitational settling velocitiesof coarse particles in the atmosphere. Modal grain sizes varybetween 4 and 32 μm throughout the different seasons and atfive locations along the transect. This is much coarser thanpreviously suggested and incorporated into climate models.In addition, seasonal changes are prominent, with coarserdust in summer and finer dust in winter and spring. Suchseasonal changes are caused by transport at higher altitudesand at greater wind velocities during summer than in winter.Also, the latitudinal migration of the dust cloud, associatedwith the Intertropical Convergence Zone, causes seasonaldifferences in deposition as the summer dust cloud islocated more to the north and more directly above the sampledtransect. Furthermore, increased precipitation and morefrequent dust storms in summer coincide with coarser dustdeposition. Our findings contribute to understanding Saharandust transport and deposition relevant for the interpretationof sedimentary records for climate reconstructions, as wellas for global and regional models for improved prediction offuture climate.
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