Particle size traces modern Saharan dust transport and deposition across the equatorial North Atlantic (original) (raw)
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Tropical Rains Controlling Deposition of Saharan Dust Across the North Atlantic Ocean
Geophysical Research Letters, 2020
Mineral dust plays an important role in the atmospheric radiation budget as well as in the ocean carbon cycle through fertilization and by ballasting of settling organic matter. However, observational records of open‐ocean dust deposition are sparse. Here, we present the spatial and temporal evolution of Saharan dust deposition over 2 years from marine sediment traps across the North Atlantic, directly below the core of the Saharan dust plume, with highest dust fluxes observed in summer. We combined the observed deposition fluxes with model simulations and satellite observations and argue that dust deposition in the Atlantic is predominantly controlled by summer rains. The dominant depositional pathway changes from wet deposition in summer to dry deposition in winter. Wet deposition has previously been suggested to increase the release of dust‐derived nutrients and their bioavailability, which may be a key contributor to surface‐ocean productivity in remote and oligotrophic parts of...
Compositional changes of present-day transatlantic Saharan dust deposition
Atmospheric Chemistry and Physics Discussions, 2016
Massive amounts of Saharan dust are blown from the African coast across the Atlantic Ocean towards the Americas each year. This dust has, depending on its chemistry, direct and indirect effects on global climate including reflection and absorption of solar radiation as well as transport and deposition of nutrients and metals fertilizing both ocean and land. To determine the temporal and spatial variability of Saharan dust transport and deposition and their marine environmental effects across the equatorial North Atlantic Ocean, we have set up a monitoring experiment using deep-ocean sediment traps as well as land-based dust collectors. The sediment traps were deployed at five ocean sites along a transatlantic transect between northwest Africa and the Caribbean along 12⁰ N, in a down-wind extension of the land-based dust collectors placed at 19⁰ N on the Mauritanian coast in Iwik. In this paper, we lay out the setup of the monitoring experiment and present the particle ...
Marine Geology, 1999
Seasonal lithogenic particle and Al fluxes were obtained from a deep-ocean sediment trap deployment during 1992 and 1993 off NW Africa, and were compared concurrently with atmospheric Al concentrations and two-dimensional backward trajectories of windfields from two barometric levels in the lower and mid troposphere. Marine Al fluxes, lithogenic particle fluxes and grain size distributions in the area were found to be directly linked to airmass pathways and surface mineral Ž found in the sediment traps. The comparison of marine, atmospheric and model derived data used within this study highlights the close temporal coupling between atmospheric dust transport and the deep-ocean particle stock. q
Vertical and areal distribution of Saharan dust over the western equatorial north Atlantic Ocean
Journal of Geophysical Research, 1972
Aerosol measurements were made as a part of the Barbados Oceanographic and Meteorological Experiment (Bomex) during May, June, and July 1969. Maximum dust concentrations occurred between the altitudes of 1.5 km and 3.7 km, a region which we call the Saharan air layer. The average concentration of mineral aerosol within this layer was 61 t•g m-a; in contrast, the average concentration in the low-level air was 22 t•g m -a. These dust concentrations are comparable to those found in continental surface air. Because of the presence of a strong inversion at the base of the Saharan layer, sea. salt was confined to te lower altitudes where the average concentration was 10 tzg m -8. Thus, sea salt appears to be a relatively minor constituent of the trade wind aerosol during much of the year. On the basis of these measurements and of a model describing the movement of Sararan air outbreaks, we estimate that 25 to 37 million tons of dust are transported through the longitude of Barbados each year. This quantity of dust is sufficient to supply all the material required to maintain the present rate of pelagic sedimentation across the entire northern equatorial Atlantic Ocean. The mineral aerosol concentration in sealevel, trade wind air at Barbados, West Indies (13øN, 59øW), has been measured from the fall of 1965 to the present [Delany et al., 1967; Prospero, 1968; Prospero et al., 1970; Carlson and Prospero, 1972]. During the first four years of this program, the air sampling was essentially continuous. These measurements have shown that large quantities of dust are transported across the northern equatorial Atlantic from the deserts of North Africa during the late spring, summer, and early fall. The average surface air concentration of mineral aerosol at Barbados during the dusty season varies from year to year but generally is of the order of 10/•g m -8 of air; the concentration during the rest of the year is more than an order of magnitude less [Prospero, 1968; Carlson and Prospero, 1972]. equatorial North Atlantic and of the relationship of the aerosol distribution to meteorological parameters. The meteorological aspects of this work, a study of the large-scale movement of Sararan air outbreaks over the northern equatorial Atlantic, are presented by us elsewhere [Carlson and Prospero., 1972]
Journal of Geophysical Research, 1997
Size-separated mineral aerosol samples were collected and analyzed to investigate the relationships between the mass-particle size distributions (MSDs) of dust particles and the dust loadings in the atmosphere. The data also were used to assess the changes in the MSDs of dust in relation to transport processes and especially the associated effects on dry deposition. Atmospheric dust concentrations, as indicated by aluminum or scandium, in samples collected from three sites in the remote North Atlantic were higher than those in samples collected during a cruise in the North Pacific on board the R/V Moana Wave. However, the mass median diameters (MMDs) for the North Pacific samples were both larger on average (-3 •m versus-•2 •m aerodynamic equivalent diameter) and more variable than those from the North Atlantic; this difference was attributed to wet conditions and particle aggregation over the North Pacific. In addition, for the ensemble of all samples the geometric standard deviations of the mass-particle size distributions, which are analogous to the sorting values used to characterize sedimentary materials, tended to vary inversely and nonlinearly with the mass median diameters. Model-derived dry deposition velocities for the samples were at most weakly related to either the dust concentrations or the MMDs. However, the dry deposition velocities for two subsets of samples were correlated with the geometric standard deviations of the distributions; this is further evidence that the mass flux of dust via dry deposition can be controlled by a relatively small fraction of aerodynamically large particles.
Journal of Geophysical Research Atmospheres
We present here a 3-year simulation (1990 to 1992) of the atmospheric cycle of Saharan dust over the Atlantic with an off-line three-dimensional transport model. The results of the simulation have been compared with selected relevant measurements. Careful attention has been paid to the spatial and temporal consistency between the observations and the model results. Satellite observations of optical thickness and the model show a closely similar latitudinal shift and change of the aerosol plume extent from month to month over 3 years. This is explained by the dominant role of the large-scale transport, well described by the European Centre for Medium-Range Weather Forecasts winds, a sufficiently consistent description of aerosol physics along with a detailed prognostic source function. A feature not captured perfectly by the model is the winter maximum in observed optical depth, which is south of the satellite observation window. This underestimate in the very southern tropical region in winter suggests that additional aerosol sources become important, such as Sahelian dust and carbonaceous aerosols from biomass burning, not included in our simulation. However, spring and autumn simulated optical thickness is 50% less than that observed, while it is only 30% less in summer and winter. This is found for both the subtropical and the tropical Atlantic Ocean, which points to a general underestimate by the model, not just because of aerosol sources missing in the Sahel region. Another seasonal feature is discussed for Sal Island where measurements suggest that low-level dust transport in winter is replaced by a pronounced high-level Saharan dust layer in summer. The model reproduces this pattern except that there is also significant low level transport in summer, associated mainly with peculiar simulated dust transport events from the western Sahara. On a synoptic scale the frequency of dust outbreaks over the North Atlantic and of major dust deposition events in Spain and a dust vertical profile measured by a lidar over the Azores region are reproduced by the model. Long-term daily monitoring of Saharan dust load over marine areas using Meteosat ISCCP-B2 data, 1, Methodology and preliminary results for 1983-1994 in the western Mediterranean, J. Geophys. Res., 102, 16,947-16,958, 1997a. Moulin, C., F. Dulac, C. E. Lambert, P. Chazette, I. Jankowiak, B. Chatenet, and F. Lavenu, Long-term daily monitoring of Saharan dust load over marine areas using Meteosat ISCCP-B2 data, 2, Accuracy of the method and validation using Sun photometer measurements, J. Geophys. Res., 102, 16,959-16,969, 1997b.