François Dulac

Control of atmospheric export of dust from North Africa by the North Atlantic Oscillation

All year long, massive airborne plumes of desert dust from the Sahara and surrounding regions are exported to the North Atlantic Ocean and the Mediterranean Sea. The mass of African dust transported in the atmosphere is large—about one billion tonnes per year (ref. 3)—and it has been suggested that the wind-blown dusts have a substantial influence on the regional radiative budget. Here we use daily satellite observations of airborne dusts to obtain an 11-year regional-scale analysis of dust transport out of Africa. The substantial seasonal variability over the Atlantic Ocean and Mediterranean Sea can be explained by the synoptic meteorology. Interannual variations in dust transport are similar over both regions, and are well correlated with the climatic variability defined by the North Atlantic Oscillation. This large-scale climatic control on the dust export is effected through changes in precipitation and atmospheric circulation over the regions of dust mobilization and transport. Such natural variability is so large that it is difficult to resolve any anthropogenic influences on atmospheric dust loads, such as those due to desertification or land-use changes. It seems likely that the North Atlantic Oscillation will also affect the distribution—and radiative influence—of other aerosols.

The role of atmospheric deposition in the biogeochemistry of the Mediterranean Sea

Abstract Estimates of atmospheric inputs to the Mediterranean and some coastal areas are reviewed, and uncertainities in these estimates considered. Both the magnitude and the mineralogical composition of atmospheric dust inputs indicate that eolian deposition is an important (50%) or even dominant (>80%) contribution to sediments in the offshore waters of the entire Mediterranean basin. Model data for trace metals and nutrients indicate that the atmosphere delivers more than half the lead and nitrogen, one-third of total phosphorus, and 10% of the zinc entering the entire basin. Measured data in sub-basins, such as the north-western Mediterranean and northern Adriatic indicate an even greater proportions of atmospheric versus riverine inputs. When dissolved fluxes are compared (the form most likely to impinge on surface water biogeochemical cycles), the atmosphere is found to be 5 to 50 times more important than rivers for dissolved zinc and 15 to 30 times more important for lead fluxes. Neglecting co-limitation by other nutrients, new production supported by atmospheric nitrogen deposition ranges from 2–4 g C m−2 yr−1, whereas atmospheric phosphorus deposition appears to support less than 1 g C m−2 yr−1. In spite of the apparently small contribution of atmospheric deposition to overall production in the basin it has been suggested that certain episodic phytoplankton blooms are triggered by atmospheric deposition of N, P or Fe. Future studies are needed to clarify the extent and causal links between these episodic blooms and atmospheric/oceanographic forcing functions. A scientific program aimed at elucidating the possible biogeochemical effects of Saharan outbreaks in the Mediterranean through direct sampling of the ocean and atmosphere before and after such events is therefore highly recommended.

Satellite climatology of African dust transport in the Mediterranean atmosphere

A daily analysis of African dust concentrations in the Mediterranean atmosphere has been made between June 1983 and December 1994 using the International Satellite Cloud Climatology Project (ISCCP-B2) archive of Meteosat visible (VIS) channel images. The ISCCP-B2 archive of Meteosat infrared (IR) images has also been used to determine the frequencies of dust mobilization over the continent, north of 30°N. Despite a large daily variability, climatological results show a clear seasonal cycle with a maximum during the dry season: dust transport begins over the eastern basin in spring and spreads over the western basin in summer. These patterns are shown to be related to both cyclogenesis over North Africa and rainfall over the Mediterranean Sea. Indeed, the frequency of dust mobilization over the continent and of dust outbreaks over the sea are strongly related to the climatology of depressions affecting North Africa. Precipitations appear to be an important factor explaining both the seasonal east-west shift in transport location and the south-north gradients of dust concentrations over the Mediterranean.

Origins of African dust transported over the northeastern tropical Atlantic

On the basis of 3 years of daily measurements of mineral dust concentrations at Sal Island (Cape Verde), this study focuses on the origin of the dust transported over the northeastern tropical Atlantic during the dusty season (October to April). Using backward trajectories, we have identified three different sectors of origin of the transported dust, covering the Sahara (north/west and central/south) and the Sahel regions. Time variations of the dust transport from these different areas can be explained by the seasonal shift of the subtropical high-pressure center. A geochemical approach highlights that the calcium amount and the Si/Al ratio of the transported dust differ according to these source regions, in relation to their respective soil compositions.

An additional low layer transport of Sahelian and Saharan dust over the north-eastern Tropical Atlantic

Mineral dust concentrations have been measured from a monitoring aerosol station set up at Sal, Cape Verde Island between December 1991 and December 1994 in order to assess the transport process of African dust over the North-eastern Tropical Atlantic. These measurements indicate a pronounced seasonal pattern, with maximum dust concentrations observed during winter. A meteorological analysis shows that the transport of dust occurs at low altitudes, in the trade winds layer, during this season. Large quantities of dust are carried out of North-western Africa, in particular from the Sahel at these altitudes. Such process could constitute the major supply of atmospheric mineral matter to the surface sea water of the Eastern Atlantic.

An illustration of the transport and deposition of mineral dust onto the eastern Mediterranean

Abstract The analyses of aerosol samples and deposition (wet) measurements during August 1991–December 1992 at Erdemli (36°N, 34°E) located on the Turkish coast of the eastern Mediterranean has shown higher dust concentration and total deposition during transitional seasons (spring and autumn) compared to summer and winter seasons. The data, complemented by three-dimensional (3D) air mass back trajectories and satellite observations suggest that North African and Middle East desert derived dust particles are transported to the region during transitional seasons. Transport events in the last part of March 1992 and early October 1992 are studied through combined analyses of ground based and satellite observations and modelling results. It is shown that dust transport constitutes a large fraction of the annual atmospheric deposition in the eastern Mediterranean, with two deposition events of short duration accounting up to 30% of the total annual flux. Therefore, the dissolved and particulate species associated with dust could be extremely variable in the mixing layer during large deposition events and could easily be missed in a short-term sampling program. The possible impact of large pulses on biological productivity of the sea also warrants consideration.

Characterization of the vertical structure of Saharan dust export to the Mediterranean basin

We present the results of our investigations into the vertical structure of several North African dust plumes exported to the Mediterranean in 1997. Two backscatter lidar systems were operated in the western and eastern parts of the Mediterranean basin during dust events identified using Meteosat visible images. Dust transport soundings have shown that dust particles are trapped and transported inside well-defined layers in the free troposphere. In general, the dust transport appeared to be multilayered, with several distinct layers at different altitudes between 1.5 and 5 km. The analysis of Meteosat IR images, the Total Ozone Mapping Spectrometer aerosol index, and back-trajectories clearly shows that these layers have different origins in Africa. Finally, in addition to the free troposphere transport, the presence of dust particles inside the planetary boundary layer has been assessed and quantified for two particular events with aerosol optical thickness of 0.3–0.4. using simultaneous lidar and Sun photometer measurements. In one case only, significant dust load (dust optical thickness of ∼0.1) occurred in the boundary layer.

Role of aerosol size distribution and source location in a three‐dimensional simulation of a Saharan dust episode tested against satellite‐derived optical thickness

An off-line global three-dimensional tracer model based on analyzed wind fields was augmented to simulate the atmospheric transport of mineral dust. The model describes the evolution of the aerosol size distribution and hence allows to compute aerosol number and mass concentrations. In this study we describe the parameterization of the sedimentation process and include a preliminary source formulation but exclude wet deposition. Validation of the model is done during a 16-day period in June-July 1988 with very scarce precipitation. It is based on a comparison of every model grid box with daily satellite-derived optical thickness observations of Saharan dust plumes over the North Atlantic and the Mediterranean. The model reproduces accurately the daily position of the dust plumes over the ocean, with the exception of Atlantic regions remote from the African coast. By systematic analysis of transport and aerosol components we show that the largest uncertainty in reproducing the position of the dust clouds is the correct localization of the source regions. The model simulation is also very sensitive to the inclusion of convection and to an accurate treatment of the sedimentation process. Only the combination of source activation, rapid transport of dust to higher altitudes by convective updraft and long-range transport allows the simulation of the dust plumes position. This study shows that a mineral dust transport model is only constrained if both the source strength and the aerosol size distribution are known. The satellite observation of optical thickness over the Mediterranean and assumptions about the size distribution indicate that the dust emission flux was of the order of 17×106 t for the 16-day period under investigation. The simulations suggest that a major aerosol mode initially around 2.5 μm with a standard deviation of 2.0 plays the dominant role in long-range transport of mineral dust.

Assessment of the African airborne dust mass over the western Mediterranean Sea using Meteosat data

The mass of African dust present over the western Mediterranean during a transport episode from northwestern Africa, which occurred in early July 1985, is estimated using a desert aerosol model, an Earth-atmosphere radiative transfer model and Meteosat visible channel data from 4 days running. Dust pixels are selected from Meteosat images, and their aerosol optical thickness is retrieved. A proportionality factor between aerosol optical thickness and atmospheric columnar aerosol loading is computed and applied to the dust pixels. The total mass of atmospheric particles over the basin is obtained by interpolation and spatial integration. The maximum aerosol optical thickness is 1.8. The maximum aerosol columnar loading is evaluated to be 2.3 g m−2. The integrated mass of particles present at a given time is estimated to raise up to about 0.6 × 1012 g at the maximum and the total mass of dust exported from Africa to be of the order of 1012 g. The method is carefully evaluated and uncertainties are discussed, with particular emphasis on the relationship between atmospheric dust mass and aerosol optical depth. The overall uncertainty on the total mass is roughly a factor ±3. In the absence of clouds it appears that the major uncertainty results from the lack of knowledge of the actual mass-size distribution of suspended dust particles, pointing out the lack of relevant data on particles larger than 10 μm in diameter. A simple calculation based on results from both computations and simultaneous field measurements yields a net transfer velocity of particles from the dust layer of approximately 1 cm s−1.

Long‐term daily monitoring of Saharan dust load over ocean using Meteosat ISCCP‐B2 data: 1. Methodology and preliminary results for 1983–1994 in the Mediterranean

In this paper we describe a method to perform an accurate long-term monitoring of the optical thickness and mass column density of airborne desert dust over the Atlantic and the Mediterranean using Meteosat wideband solar (visible (VIS) plus near infrared) sensor. The dust load is retrieved using aerosol models and an Earth-atmosphere radiative transfer model. The method focuses on multiyear (from 1983 to 1994) daily retrieval of the atmospheric dust load using Meteosat low-resolution images prepared for the International Satellite Cloud Climatology Project (B2 format). We account for the variable calibrations, radiometric sensitivities, and spectral bands of the successive sensors (Meteosat 2 to Meteosat 5) as well as for the presence of marine background and stratospheric aerosols. We discuss the sensitivity of the method to different factors, and its accuracy is assessed in a companion paper. The results obtained include the daily geographical distribution of the dust load and the temporal variation of the dust load over marine areas. We illustrate and briefly discuss the results for the western Mediterranean and particularly for the Dynamique des Flux Atmospheriques en Mdditerrande (DYFAMED) marine station in the Ligurian Sea. The dust transport mainly takes place during summer in this area. More than half a million metric tons of suspended dusts are occasionally observed over the western Mediterranean, and we observed an average of 16 dust events per year. At DYFAMED station the 11.5-year mean dust optical thickness is 0.11, with annual means ranging from 0.055 in 1985 to 0.19 in 1992.