Modeling the atmospheric distribution of mineral aerosol: Comparison with ground measurements and satellite observations for yearly and synoptic timescales over the North Atlantic

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.