Desert Aerosol Transport in the Mediterranean Region as Inferred from the TOMS Aerosol Index

P. L. Israelevich, Z. Levin, J. H. Joseph, and E. GanorDepartment of Geophysics and Planetary Sciences, Tel Aviv University, Ramat Aviv, Israel

http://www.tau.ac.il/geophysics/MEIDEX/home.htm

Saharan dust is transported from its sources over land and sea to regions like Europe, the Middle East and across the Atlantic to areas as far away as Mexico City. This transport occurs in spectacular storm-like events, when clouds of the desert aerosol particles take the shape of giant plumes that can span the North-South extent of the Mediterranean or cover large regions west of Africa. A number of studies in the last twenty years have shown the importance of these aerosols to the global and regional energy balance, to weather forecasting and to rain formation These, however, represent results from limited data and are based on localized statistics. In order to study Mediterranean weather and climate it is important to obtain spatial-temporal resolution of the dust transport.

Here we present results of a study of the desert dust transport to the Mediterranean basin using the TOMS aerosol index (AI). We analyzed the 2D distribution of the positive AI for each pixel of the North African desert and its neighboring areas as a proxy indicator for desert dust, using a continuity equation formulation. Using the analysis of the divergence field of the 2D TOMS AI, the following main sources of the UV-

absorbing aerosols can be inferred (see 1 and 2):

• Region A - the Chad basin and the wadi deltas leading out of mountainous areas of Ahaggar and Tibesti. • Region B - the Eljouf basin, supplied with silt and clays through the wadis descending from the neighboring mountains.

• Regions C and D - two smaller local maxima centered at latitude N330 and longitude E70, and at latitude N250 and longitude E180, respectively.

• Aerosol sources in the Northern Sudan and in the Arabian Peninsula.

• Regions E (at latitude N50 and longitude W20) and F (latitude S70 and longitude W120) located near Central Africa's Atlantic coast are associated with biomass burning.

•During the period April - August, the whole region above Northern Africa is almost permanently loaded with significant amounts of desert dust. •Region A is the more stable source, with maximum activity in April, •Region B shows a more variable dust supply with the maximum in July. •The main sources, A and B, have the same surface properties as to propensity of dust formation. The difference in time variation of two regions is due to the

meteorological conditions. (see 3 and 4)

Heat lows, called Sharav cyclones, form along the Mediterranean coastal zones of North Africa, primarily from March to June and travel West to East. These mobilize the available desert dust and transport it eastward and northward along the Mediterranean basin.

Identifiable dust plumes appear first in the Western sector of the sea, and then move eastward with the speed of ~7-8 degrees/day, corresponding to the average motion of the Sharav cyclone.

In spring, this motion continues at least up to the eastern coast of the sea.

In summer, the dust plume does not penetrate longitudes

to the east of ~E150. (see 5 and 6)

The monthly distributions of the AI have maxima in April for the Eastern, in June for the Central, and in July for Western Mediterranean. These distributions are in good agreement with the results of Moulin et al. [1998].

The distribution of the aerosol index along latitude 35.5o during the year 1999.

The distribution of the dispersion of the aerosol index = [(AI-<AI>)2]1/2 shows that source A is more stationary than B. Biomass burning regions E and F appear to be the most variable.

Daily values of the aerosol index integrated over the whole region of interest and over regions A and B. In spring, the dust sources are more powerful than the sinks, and the dust is permanently available from a reservoir in the atmosphere as soon as the appropriate synoptic conditions arise.

The 2D distribution of the average value of positive AI for each pixel for the period from August, 1996 to April, 2000 (top panel). Main source regions of the UV-absorbing aerosols can be inferred, and these regions are marked in the bottom panel.

Top panel represents the distribution of the highest observed AI. The largest values occur along the main trajectories of the dust plumes. Bottom panel shows schematically the main routes of the desert aerosol transport.